Introduction
Studies of fossil morphology and systematics provide key data for most areas of paleontological investigation. Descriptions of new taxa and systematic revisions provide the framework on which additional analyses of evolution, biogeography, and diversity can be developed (e.g., Lieberman, Reference Lieberman2000; Cracraft, Reference Cracraft2001; Friis et al., Reference Friis, Pedersen and Crane2010; Bauer and Stigall, Reference Bauer and Stigall2014). During the past 25 years, new tools, e.g., phylogenetic methods and advanced computational programs (e.g., Huelsenbeck and Ronquist, Reference Huelsenbeck and Ronquist2001; Swofford, Reference Swofford2002; Maddison and Maddison, Reference Maddison and Maddison2003), have emerged to provide expanded opportunities for systematic analyses and create potential for ever more detailed understanding of the history of life. The adoption of these tools has varied substantially among specialists of various clades; detailed phylogenetic and multivariate analyses are common in vertebrate paleontology, but comparatively less common within invertebrate systematics. Specifically, most systematic revisions of brachiopod genera have been conducted based on detailed qualitative or quantitative analyses of overall morphology (e.g., Popov et al., Reference Popov, Cocks and Nikitin2002; Cocks, Reference Cocks2005; Thomsen et al., Reference Thomsen, Jin and Harper2006), but have lacked explicit phylogenetic frameworks to examine the evolutionary history of a clade. In this study, we examine whether a combination of phylogenetic and morphometric techniques can improve on prior inferences of evolutionary history among a set of species previously assigned to the Late Ordovician strophomenid genera Eochonetes Reed, Reference Reed1917 and Thaerodonta Wang, Reference Wang1949. We examine and contrast potential inferences derived from specimen-based morphometric analyses, taxon-based phylogenetic analyses, and the possible extension of these results for addressing evolutionary, biogeographic, and ecological patterns within these species.
Species assigned to Thaerodonta were distributed across the palaeocontinent of Laurentia during the Late Ordovician, whereas species assigned to Eochonetes were concentrated within the peri-Laurentian terrains of Scoto-Appalachia and Anticosti Island. The taxonomic history of these genera is complex, reflecting conflicting opinions about whether or not the included species comprise a single lineage or multiple distinct lineages. Eochonetes was originally considered closely related to Chonetes Fischer de Waldheim, Reference Fischer de Waldheim.1830 and Sowerbyella Jones, Reference Jones1928, but has generally been accepted as a distinct genus since 1965. Following its initial description, the genus Thaerodonta has been synonymized with three different genera, most notably with Eochonetes (Cocks and Rong, Reference Cocks and Rong1989, Reference Cocks and Rong2000; Cocks, Reference Cocks2005, Reference Cocks2013), and rerecognized as a valid taxon on eight separate occasions over the last century (Table 1; Howe, Reference Howe1965; Havliček, Reference Havliček1967; Howe, Reference Howe1972; Amsden, Reference Amsden1974; Rõõmusoks, Reference Rõõmusoks1981; Jin et al., Reference Jin, Caldwell and Norford1997; Jin and Zhan, Reference Jin and Zhan2001; Candela, Reference Candela2003).
Table 1 Taxonomic status changes of the genera Eochonetes and Thaerodonta over the past century.
The primary difference between Eochonetes and Thaerodonta is the presence of ventral hinge-line canals visible on internal molds of Eochonetes but not on any individuals recognized as Thaerodonta (Cocks and Rong, Reference Cocks and Rong1989). The ventral tubules, or canals, of Eochonetes do not penetrate the exterior of the shell, and therefore, do not appear to extend into external spines as in true chonetids (Reed, Reference Reed1917; Jones, Reference Jones1928). In the majority of Eochonetes specimens, the canals are not preserved (Cocks and Rong, Reference Cocks and Rong1989). However, few Thaerodonta specimens are preserved as molds, and thus taphonomic biases likely indicate that the lack of recognition of this character among Thaerodonta species is not particularly meaningful. All other morphological characters of Eochonetes closely resemble Thaerodonta, including the hinge-line denticulation of dorsal denticles and ventral fossettes (Cocks and Rong, Reference Cocks and Rong1989).
Although the relationship between Thaerodonta and Eochonetes has previously been assessed based on character data, these comparisons have lacked an explicit phylogenetic framework that could help clarify the evolutionary positions of species assigned to these genera. In this study, we examine whether Thaerodonta and Eochonetes are distinct evolutionary lineages first by assessing species validity through multivariate morphometric analysis and then by generating a species-level phylogenetic hypothesis through parsimony analysis. Results of these two morphological analyses are then used to provide the basis for a comprehensive, phylogenetically informed taxonomic revision of species previously assigned to Eochonetes and Thaerodonta.
Materials and methods
Interspecific and intraspecific variation is widespread within species of Thaerodonta and Eochonetes (Howe, Reference Howe1972; Jin and Zhan, Reference Jin and Zhan2001). Therefore, character-based morphometric analysis was employed to establish species boundaries and validate specific assignments. The multivariate analyses provide a basis to interpret generic and species relationships in the context of morphological space, but only incorporate a subset of all potentially informative characters. Subsequent species-level phylogenetic analysis employed morphometrically constrained species as the focal taxa and incorporated additional character data to reconstruct evolutionary relationships. The combined results of the two analyses provide the basis for the systematic revision.
Taxa analyzed
Twenty-seven operational taxonomic units (OTUs) were analyzed from museum collections, including 20 validly described species and seven OTUs in open nomenclature (Table 2). Morphological data were collected for 397 specimens including all available type and numerous nontype specimens (Supplemental Data 1, 2). Nontype specimens were included to better characterize the degree of intraspecific variation and because type specimens were not always available or able to provide sufficient character data for analysis. Nontype specimens that were chosen for measurement were based on: prior specific assignment, presence of dorsal denticles, and geographic occurrence in North America, the British Isles, or Estonia (Fig. 1; Table 3). Only specimens interpreted as adults were included within the study. Maturity was determined based on the presence of a well-developed bema and muscle scars. When identified, juvenile specimens typically exhibited the same outline shape as the adults but were of smaller size with less-pronounced internal features. Individuals of notably small size for the species or that lacked well-developed internal features were excluded from analysis.
Figure 1 Palaeogeographic reconstruction of Laurentia and Peri-Iapetan terranes during the Late Ordovician. Structural elements of exposed land, e.g., the Transcontinental Arch, Taconic Highlands, Laurentia Parautochthon, and Baltic Highlands, are outlined, shaded in brown, and labeled. Geographic areas discussed herein are in shaded numbered polygons; these include: (1) Western Midcontinent (Texas and Oklahoma basins); (2) North of the Transcontinental Arch (Bighorn, Williston, and Hudson Bay basins); (3) Eastern Midcontinent (Iowa and Illinois basins); (4) Central Basin (Nashville Dome); (5) Cincinnati Basin; (6) Appalachian Basin; (7) Anticosti Island; (8) Scoto-Appalachia (Scotland and Ireland); and (9) Baltic Basin. Modified from Cocks and Torsvik (Reference Cocks and Torsvik2011) and Torsvik and Cocks (Reference Torsvik and Cocks2013).
Table 2 Distribution of OTUs among analyses within the study. *=outgroup.
Table 3 Geographic, stratigraphic, and temporal distribution for species examined.
Species listed in Table 7 are considered to be valid taxa. The original descriptions for most of these species are sufficient for diagnoses and further discussion is not necessary. Character state distribution data in Table 6 and Supplemental Data 4 and 5 can be combined with previously published descriptions to provide enhanced diagnoses. For a complete list of specimens examined, see Supplemental Data 1.
Repositories and institutional abbreviations
Repositories of specimens are indicated by the following abbreviations: AMNH, American Museum of Natural History; BGS, British Geological Survey; FMNH, Field Museum of Natural History; GLAHM, Hunterian Museum, Glasgow, Scotland; GSC, Geological Survey of Canada; NHM, Natural History Museum, London; OUIP, Ohio University Invertebrate Paleontology Collections; SNMOH, Sam Noble Museum of Oklahoma History; SUI, University of Iowa Paleontological Repository; TUG, University of Tartu, Museum of Geology; USNM, United States National Museum of Natural History; YPM, Yale Peabody Museum.
Characters analyzed
Measurements were collected for a series of length, width, angles, and amounts or counts of specific features on the interior and exterior surfaces of specimen valves. These included both qualitative and quantitative characters reflecting external and internal attributes of both valves (Fig. 2). Characters were organized in groups pertaining to external and internal features of ventral or dorsal valves. Similar characters have been used in previous morphometric analyses of Ordovician brachiopods (e.g., Sohrabi and Jin, Reference Sohrabi and Jin2013; Sproat and Jin, Reference Sproat and Jin2013) and to generate phylogenetic hypotheses in other groups of articulated brachiopods (e.g., Leighton and Maples, Reference Leighton and Maples2002; Stigall Rode, Reference Stigall Rode2005; Wright and Stigall, Reference Wright and Stigall2013, Reference Wright and Stigall2014). They also form the basis for many specific diagnoses within the focal genera (e.g., Wang, Reference Wang1949; Howe, Reference Howe1965; Macomber, Reference Macomber1970; Amsden, Reference Amsden1974).
Figure 2 Examples of locations of linear and angular measurements used in the analyses on representative specimens. (1) Ventral interior of Thaerodonta magna, USNM 145048. (2) Ventral interior of Eochonetes glabra, GSC 113889. (3) Dorsal interior of Thaerodonta aspera, SUI 1886. (4) Dorsal interior of Thaerodonta clarksvillensis, USNM 88274. 1=maximum width; 2=shell length; 4=angle between hinge and cardinal extremity; 5=delthyrium size; 7=ventral interarea angle; 9=dental plate length; 11=adductor scar length; 12=adductor scar width; 13=diductor scar length; 14=diductor scar width; 18=primary teeth size; 20=adductor scar length; 21=adductor scar width; 22=angle between muscle scar and hinge; 23=socket width; 26=angle between brachial process and hinge.
Both qualitative characters (based on character states interpreted as homologous) and quantitative characters (based on measurements of specific organisms; Wiley and Lieberman, Reference Wiley and Lieberman2011) were included in this analysis. Both character types have been successfully utilized in previous morphometric and phylogenetic analyses of brachiopods and other clades (e.g., Stigall Rode, Reference Stigall Rode2005; Hunt, Reference Hunt2007; Hopkins, Reference Hopkins2011; Sohrabi and Jin, Reference Sohrabi and Jin2013; Sproat and Jin, Reference Sproat and Jin2013; Wright and Stigall, Reference Wright and Stigall2013, Reference Wright and Stigall2014). Recent analyses by Hopkins (Reference Hopkins2011) demonstrated that inclusion of continuous characters within phylogenetic analyses significantly increases phylogenetic resolution.
Continuous values were used directly for morphometric analysis. For phylogenetic analysis, continuous characters were standardized by dividing the measurement by the maximum width of the valve (typically the hinge line) of the specimen. For complete valves, denticles were counted for each cardinal extremity and averaged for the individual. In specimens in which only part of the hinge line was preserved, denticles were counted on one cardinal extremity for each individual. The angle between the cardinal extremity and the hinge line were treated similarly.
Morphometric analysis
Each specimen for which all characters described could be measured was included in the morphometric analysis. This constrained the data set to 56 specimens within 14 OTUs for dorsal valve analysis and 41 specimens within 13 OTUs for ventral valve analysis (Table 2; Supplemental Data 3, 4). The suite of characters captures overall morphology within the genera and provides a robust framework for multivariate analysis. Similar characters have been used to distinguish Thaerodonta and related genera as well as to delineate species within Eochonetes and Thaerodonta (e.g., Wang, Reference Wang1949; Macomber, Reference Macomber1970; Cocks, Reference Cocks2013).
Principal components analysis (PCA) was used to: (1) examine the separation of the genera Thaerodonta and Eochonetes based on the species previously assigned to each genus; and (2) test individual species boundaries prior to phylogenetic analysis. PCA applies a linear transformation to the original data set to produce an uncorrelated set of variables while minimizing residual variance (Dunteman, Reference Dunteman1989). The correlation algorithm normalizes the characters in order to create even weighting for variables measured in different units (Hammer et al., Reference Hammer, Harper and Ryan2001; Sohrabi and Jin, Reference Sohrabi and Jin2013). This technique is intended for use with continuous character data (James and McCulloch, Reference James and McCulloch1990; Etter, Reference Etter1999), and it is commonly used in analyses of morphometric character data, i.e., analysis of fossil brachiopods (e.g., Sohrabi and Jin, Reference Sohrabi and Jin2013) or fossil or modern humans (White et al., Reference White, Asfaw, Degusta, Gilbert, Richards, Suwa and Howell2003). Analyses were conducted using JMP Pro 11 (SAS Institute Inc., 2009).
To visualize how each type of character influenced the output, two variations of morphometric analyses were conducted: (1) the data set was analyzed in full as described above; and (2) analyses were repeated with characters considered to have potential to skew the results (i.e., length and width) removed. Minor differences occurred among these treatments, as noted below, but the primary results of (1) were repeated in the alternate treatment. Thus, discussion below will focus on analysis (1) unless otherwise indicated.
Phylogenetic analysis
Eighteen OTUs were included in the phylogenetic analysis, including 16 ingroup and 2 outgroup taxa. Two species of Sowerbyella, S. rugosa Meek, Reference Meek1873 and S. socialis Cooper, Reference Cooper1956 were coded for outgroup comparison because Sowerbyella has been considered to be the sister group or ancestral clade from which Eochonetes and Thaerodonta evolved (Howe, Reference Howe1972; Cocks and Rong, Reference Cocks and Rong1989; Sloan, Reference Sloan2005). Based on results of the morphometric analysis and preliminary phylogenetic reconstructions, T. saxea Sardeson, Reference Sardeson1892 and T. recedens Sardeson, Reference Sardeson1892 were combined as a single OTU for phylogenetic analysis. All other OTUs listed in Table 1 were coded for potential inclusion within the phylogenetic analysis. However, only those listed in Table 2 were found to convey phylogenetic signal, as described below.
Inclusion of numerous taxa with very sparse character data can cause topological instability or incorrect placement of taxa within the output cladograms (e.g., Gauthier, Reference Gauthier1986; Novacek, Reference Novacek1992; Wilkinson and Benton, Reference Wilkinson and Benton1995; Gao and Norell, Reference Gao and Norell1998; Wiens, Reference Wiens2003; Wiley and Lieberman, Reference Wiley and Lieberman2011). However, certain taxa with a substantial number of missing characters can still influence tree topology in positive ways (Wiens, Reference Wiens1998, Reference Wiens2003, Reference Wiens2006), thus the inclusion of some taxa with missing character data is preferable to excluding all species with incomplete character sets out of hand because that approach can result in inadequate data about the evolution of the clade (=missing taxa). Following Wiens (Reference Wiens2006), taxa with missing character data were retained in this analysis if they provided phylogenetic structure.
To determine whether taxa provided phylogenetic signal, multiple analyses were conducted that alternately inserted and removed OTUs with missing data during repeated analyses to examine changes in tree topology to test the hypothesis that these species retained potential to drive tree topology. The minimum number of characters required to positively impact tree topology in this dataset was ~10 characters (one-third of the full suite). The taxa excluded from the final phylogenetic analyses (Table 2) did not further resolve the tree but rather caused tree topology to collapse. Morphological data for excluded OTUs were typically severely limited due to lack of preserved valve interiors.
Thirty morphological characters were utilized in the phylogenetic analysis (Table 4). Continuous characters were normalized and converted into discrete data for phylogenetic analysis following the protocol of Wright and Stigall (Reference Wright and Stigall2013). Continuous measurements were unordered and plotted to determine natural breaks (e.g., change in slope or discontinuities) of variation. The natural breaks in the data were utilized to create bins of discrete states via gap coding (see Mickevich and Johnson, Reference Mickevich and Johnson1976; Archie, Reference Archie1985; Thiele, Reference Thiele1993; Swiderski et al., Reference Swiderski, Zelditch and Fink1998). The mean and standard deviation values were calculated for each bin and used to define character state ranges following Morton and Kincaid (Reference Morton and Kincaid1995) and Wright and Stigall (Reference Wright and Stigall2013, Reference Wright and Stigall2014). After separation of character states, the statistical significance of the prospective character ranges was assessed using one-way analysis of variance (ANOVA). Classes of morphological characters were found to be highly unequivocal (P<0.001) with nonoverlapping mean values at the 95% confidence interval. The character state range was calculated as the mean of each state ±1 standard deviation (Table 5). OTUs were coded as exhibiting a single character state if all measurements for specimens of that OTU fell within the range for that character state. Alternately, the taxon was coded as polymorphic for the character if the specimen data were distributed within two or more of the defined character states (Table 6; Supplemental Data 5).
Table 4 Characters utilized for phylogenetic analysis. Other than for character 1, measurements (in mm) were standardized by maximum width of the specimen. See Table 6 for statistical separation of continuous characters.
Table 5 Statistical separation of quantitative morphological characters. All characters measured in millimeters. Continuous characters other than character 1 were standardized by the maximum width of the specimen. SD=standard deviation.
Table 6 Character state distribution for OTUs included in the phylogenetic analysis of Thaerodonta and Eochonetes. Character states indicated as W=(0 and 1 and 2); X=(0 and 1); Y=(1 and 2); Z=(0 and 2). Acronyms indicating location of species in open nomenclature: TX=Texas; ID=Idaho; WY=Wyoming. ?=missing data; *=outgroup (Sowerbyella).
A cladistic approach using maximum parsimony was employed to reconstruct phylogenetic relationships. Studies comparing the model-based approaches with parsimony have reported conflicting results regarding which methodology is more successful (Pol and Siddall, Reference Pol and Siddall2001; Kolaczkowski and Thornton, Reference Kolaczkowski and Thorton2004; Wright and Hillis, Reference Wright and Hillis2014; Xu and Pol, Reference Xu and Pol2014). Many of these studies utilized large datasets with a vast amount of character data (e.g., Wright and Hillis, Reference Wright and Hillis2014; Xu and Pol, Reference Xu and Pol2014). Studies that have explicitly addressed the efficacy of parsimony versus model-based approaches have established that maximum parsimony is equally successful in general (Rindal and Brower, Reference Rindal and Brower2011) and could be more powerful in particular for analyses of fossil taxa (Spencer and Wilberg, Reference Spencer and Wilberg2013). Our study has a much smaller data set, and there is little information, at present, for small data sets on model-based approaches versus parsimony. Parsimony-uninformative characters, including autapomorphies, were removed from the analysis but would be beneficial in a Bayesian approach. Analyses were conducted in PAUP*4.0b10 (Swofford, Reference Swofford2002) using the branch and bound search method. All characters were equally weighted and unordered. Taxa exhibiting multiple character states were treated as polymorphic. Accelerated transformation (ACCTRAN) was utilized to optimize and analyze characters in MacClade 4.06 (Maddison and Maddison, Reference Maddison and Maddison2003).
Morphometric analysis
Generic discrimination
When analyzed as a group, specimens previously attributed to Thaerodonta constitute a large region of morphospace in the PCA plot. Specimens attributed to Eochonetes occupy a comparatively smaller region of morphospace (Fig. 3). These two populations overlap substantially in multivariate space.
Figure 3 Results of generic (1–2) and species (3–4) separation in morphospace for dorsal (left column) and ventral (right column) valves. AL=adductor length; AW=adductor width; C=cardinal process angle (dorsal valve); D=denticle count (dorsal valve); DL=diductor length (ventral valve); DP=dental plate length (ventral valve); DW=diductor width (ventral valve); HW=angle between hinge line and cardinal extremity (dorsal valve); M=muscle scar angle to hinge line; square=single occurrence of a species on the plot; circle=multiple data points.
Species discrimination
Multivariate analyses of individual OTUs demonstrate considerable overlap among taxa in morphospace (Fig. 3). In most cases, OTUs representing taxa in open nomenclature plot separately from the formal species with which they had been previously aligned. The restriction of some OTUs to single data points within only the ventral or dorsal analyses limits the extent of potential interpretation for some OTUs. Nevertheless, several clear patterns emerge from the PCA plots.
Ventral valve analysis shows many of the North American species clustering near each other on the right side of the plot, whereas the two Eochonetes species from Scoto-Appalachia (i.e., E. advena Reed, Reference Reed1917 and E. celticus Mitchell, Reference Mitchell1977) overlap on the left side of the plot. When length and width are removed from the set of included characters, the occupied species area in morphospace shifts only slightly. There is less separation among North American Thaerodonta species, other than T. johnsonella Amsden, Reference Amsden1974, which remains entirely isolated on the far left. Thaerodonta saxea is the only species occupying a morphospace field entirely overlapping that of other species; this could indicate a lack of differing morphological features distinguishing T. saxea from other taxa, notably T. recedens.
Dorsal valve analysis results in a similar grouping of North American species on the right side of the x-axis, but with greater overall separation than the ventral valve analysis. Removing length and width from the characters changed the orientation of Thaerodonta magna Howe, Reference Howe1965, T. mucronata Howe, Reference Howe1965, and T. aspera Wang, Reference Wang1949. These three species shifted from their original location to the far right, closer to the large grouping. The other species remained in their original locations. Again, the morphospace of T. saxea completely overlaps that of other North American species. Thaerodonta mucronata also displays a high degree of overlap with the other North American species.
Morphometric discussion
Eochonetes does not significantly differ from Thaerodonta in morphospace based on the character data utilized. Thus, the two genera are interpreted as lacking significant differences among the general morphological attributes analyzed in the multivariate analyses. This lack of distinction supports the previously hypothesized (e.g., Cocks and Rong, Reference Cocks and Rong1989) synonymy of these two genera, which will be explored more fully below in discussion of the phylogenetic analysis.
Most of the OTUs analyzed exhibited differentiation in morphospace. Taking into account the plots of both valves, species that exhibited a moderate to high degree of separation within morphospace are interpreted as valid, discrete taxa. The high degree of overlap exhibited between or among certain species indicates that a close examination of their validity should be tested in a phylogenetic framework. For example, the consistent overlapping occupation of morphospace between T. saxea and T. recedens indicates high morphological similarity and suggests that additional scrutiny of the distinction of these species using more character data is warranted. Howe (Reference Howe1972) also noted that it is difficult to confidently separate members of these two species when examining large populations.
Implications
Reconstructing species boundaries of extinct organisms is challenging (see discussion by Allmon and Yaccobucci, Reference Allmon and Yaccobucciin press). Both ecophenotypic and taphonomic biases can impact species recognition in the focal clade. These challenges can be exacerbated in taxa, e.g., brachiopods, that exhibit differential growth based on environmental conditions (Rudwick, Reference Rudwick1970) or in instances where preservation of key diagnostic characters is strongly controlled by taphonomy (e.g., many Cambrian arthropods). Within Eochonetes, the recognition of hinge-line tubules is highly influenced by preservation style, and variations in valve morphology have been previously attributed to environmental influences (e.g., Howe, Reference Howe1965).
The results of the morphometric analyses presented here suggest caution in over interpreting results based on single characters. Indeed, our analyses demonstrate that different subsets of the morphological data analyzed provide distinct and complementary information about species identity. In this dataset, species that occupied overlapping morphospace of one valve often exhibited differentiation in the other, e.g., compare the positions of Thaerodonta clarksvillensis (Foerste, Reference Foerste1912) and T. recedens, or the directional vectors in Figure 3.1 and 3.2. Furthermore, the multivariate analyses were repeated with overall length and width included, overall length and width excluded, and with the addition of discrete data (e.g., counts) to determine how the resulting plot was affected. Removing overall size reduced the loading for PC1, but had limited influence on the position of data in morphospace. Moreover, experimentation with including or excluding different types of data (i.e., measurements, angles, or counts) showed that the nonmeasurement data has a greater effect on the resulting plot. Thus, incorporating data from both valves and also exploring the impact of various data partitions was critical to understanding the morphospace occupation of the species.
The morphological analysis was a valuable precursor to the subsequent phylogenetic analysis because it provided a framework to experiment with various character types and test species boundaries in morphospace. Because morphometric analysis examines overall similarity within a subset of all possible character data, multivariate techniques do not provide a direct hypothesis of evolutionary relationships the way that synapomorphy-based phylogenetic analysis does. However, these methods provide an outstanding platform on which to test hypotheses of morphological or ecological discrimination of species, which can resolve certain sets of research questions and is a necessary foundation for phylogenetic analysis.
Phylogenetic analysis
Results
Parsimony analysis yielded three most parsimonious trees with lengths of 217 steps (Fig. 4). The consistency and retention indices are 0.866 and 0.642, respectively, for these trees, which exceed the values derived from equivalently sized randomized data sets at the α=0.05 level (Klassen et al., Reference Klassen, Mooi and Locke1991). Support was further assessed by calculating the g1 statistic, a metric for assessing internal consistency and decisiveness within the dataset by examining the degree of skewness in the distribution of tree lengths (Hillis and Huelsenbeck, Reference Hillis and Huelsenbeck1992). The g1 statistic for 10,000 random trees generated via heuristic search from the data matrix is ˗0.442. This is significantly higher than within a random data set and significant at the α=0.01 level (Hillis and Huelsenbeck, Reference Hillis and Huelsenbeck1992), which signifies substantial phylogenetic structure within the data.
Figure 4 Strict consensus tree generated via the branch and bound method utilizing data from Table 6 in PAUP*4.0b10 (Swofford, Reference Swofford2002). Tree length is 217 steps. The retention index is 0.642, and the consistency index is 0.866. Character states for nodes were optimized using MacClade 3.04 (Maddison and Maddison, Reference Maddison and Maddison2003) utilizing ACCTRAN optimization. Node numbers are circled on the cladogram; character and character state follows node number in the following description. Character states that change unambiguously at each node: Node 1, 11(0), 25(1), 30(0); Node 2, 12(1); Node 3, 19(1); Node 4, 6(1), 22(1); Node 5, 17(2), 25(2), 27(1); Node 6, 23(0), 29(0), Node 7, 7(1), 16(2); Node 8, 9(1); Node 9, 10(1), 12(1); Node 10, 4(0), 11(1), 18(1); Node 11, 13(1), 21(1), 26(2); Node 12, 9(1); Node 13, 17(1).
Recognition of clades
Recovered tree topology (Fig. 4) indicates that species previously assigned to Eochonetes and Thaerodonta do not form distinct evolutionary lineages. Specifically, species referred to as Eochonetes appear at three isolated positions within a larger clade dominated by Thaerodonta species. Thus, neither Eochonetes nor Thaerodonta as previously defined is monophyletic. However, species previously referred to the two genera do form a clade when combined together, thus Eochonetes and Thaerodonta together form a single monophyletic genus. The name Eochonetes has priority and therefore is the correct taxonomic unit for this genus. We therefore support transferring all OTUs above node 1 in Figure 4 to Eochonetes and designating Thaerodonta as a junior subjective synonym of Eochonetes as indicated in Figure 5. Two species formerly assigned to Thaerodonta, T. saunjaensis Rõõmusoks, Reference Rõõmusoks1981 from Estonia and Thaerodonta sp. from Wyoming optimize within the outgroup species and are excluded from the revised Eochonetes clade. A complete list of species previously assigned and the taxonomic status recognized herein is presented in Table 7.
Figure 5 Cladogram from Figure 4 relabeled with revised taxonomic names and scaled against the Late Ordovician timescale. Observed ranges indicated by solid lines; ghost lineages indicated by dashed lines. Time scale modified from Cohen et al. (Reference Cohen, Finney, Gibbard and Fan2014). Stratigraphy correlated from sources listed in Supplemental Data 6.
Table 7 Original taxonomic assignments of operational taxonomic units and the revised interpretations herein. Species now excluded from Eochonetes are listed at the bottom of the table. Asterisks denote taxa requiring systematic revision.
Monophyly of the ingroup (the revised Eochonetes clade) is supported by the presence of accessory teeth, the presence of denticles on the dorsal valve (Fig. 6.1), and the lack of raised muscle scars. Maximum shell width, length, and height are variable throughout the ingroup and many species exhibit polymorphic characters. The maximum width typically occurs at the hinge line; although one or more individuals of a few species (i.e., E. celticus, E. advena, E. dignata Wang, Reference Wang1949, and E. saxea) exhibit hinge lengths less than the maximum width. In some individuals, the commissure is slightly raised in the center rather than lying flat, but well-developed fold and sulcus structures are absent at the species level. Delthyrial width varies among species, but a pseudodeltidial covering is consistently present. The bema is always present among ingroup species, although the height of the bema, compared to the median ridges, is variable between and within species. Primary teeth are consistently present, but size (Fig. 6.2) varies among species.
Figure 6 Cladograms depicting character evolution shown through change in color of the branches as well as character shifts at ancestral nodes. (1) Denticles. (2) Socket size. (3) Dorsal lateral septa. (4) Accessory sockets/teeth.
The basal Eochonetes clade is composed of E. clarksvillensis, E. celticus, and E. advena (Figs. 5, 7). Monophyly of this clade is supported by wide ventral adductor scars and the appearance of hinge-line denticles (Fig. 6.1). The sister relationship of E. celticus and E. advena is supported by the presence of ventral tubules along the hinge line.
Figure 7 (1–4) Eochonetes clarksvillensis (Foerste, Reference Foerste1912); (1) dorsal exterior (USNM 87151 7C); (2) ventral interior (OUIP 1547); (3) dorsal interior (USNM 88274); (4) dorsal interior (USNM 87151 7B). (5–8) Eochonetes advena Reed, Reference Reed1917; (5) dorsal exterior (GLAHM L-3190); (6) ventral interior (GLAHM L-2719[6]); (7) ventral interior (GLAHM L-1806[20]); (8) dorsal interior (GLAHM L-2884[4]). (9–11) Eochonetes celticus Mitchell, Reference Mitchell1977; (9) ventral exterior (BGS NIL 5312); (10) ventral interior (BGS NIL 9114); (11) ventral interior (BGS GU 983). (12–15) Eochonetes maearum n. sp.; (12) ventral exterior (USNM 124834); (13) dorsal exterior (USNM 124838); (14) dorsal exterior (FMNH PE 11070); (15) dorsal interior (FMNH PE 11072). (16–19) Eochonetes johnsonella (Amsden, Reference Amsden1974); (16) dorsal exterior (SNMOH 6679); (17) ventral interior (SNMOH 6680); (18) dorsal interior (SNMOH 6678); (19) dorsal interior (SNMOH 6679). (20–22) Eochonetes voldemortus n. sp.; (20) ventral exterior (USNM 133257); (21) dorsal exterior (USNM 133257); (22) dorsal interior (USNM 133258).
Monophyly of the remaining Eochonetes species is supported by an increased dorsal muscle scar angle as well as high costal density. Each node of this pectinate portion of the tree is supported by 100% agreement in the strict consensus tree and is supported by character evidence. Monophyly of E. johnsonella (Fig. 8) plus crownward species is supported by the synapomorphies of increased fossette/denticle count and better-developed lateral ridges in the dorsal muscle scar field (Fig. 6.3). Monophyly of species including E. voldemortus n. sp. (Fig. 7) crownward is supported by a well-developed bema and decreased socket size.
Figure 8 (1–4) Eochonetes dignata (Wang, Reference Wang1949); (1) ventral exterior (USNM 560301); (2) dorsal exterior (USNM 560301); (3) ventral interior (SUI 1888a); (4) dorsal interior (SUI 1888b). (5–11) Eochonetes glabra (Dewing, Reference Dewing1999); (5) ventral exterior (GSC 113897); (6) dorsal exterior (GSC 113900); (7) dorsal exterior (GSC 113900b); (8) ventral interior (GSC 113900a); (9) ventral interior (GSC 113910); (10) dorsal interior (GSC 113908a); (11) dorsal interior (GSC 113908b). (12–14) Eochonetes mucronata (Howe, Reference Howe1965); (12) ventral exterior (USNM 145038); (13) ventral interior (USNM 145038); (14) dorsal interior (USNM 145039k). (15–18) Eochonetes aspera (Wang, Reference Wang1949); (15) ventral exterior (SUI 1885); (16) dorsal exterior (SUI 1885); (17) ventral interior (SUI 1886a); (18) dorsal interior (SUI 1886b). (19–21) Eochonetes magna (Howe, Reference Howe1965); (19) dorsal exterior (USNM 145047); (20) ventral interior (USNM 145048); (21) dorsal interior (USNM 145049).
Monophyly of the species from Eochonetes dignata (Fig. 8) crownward is supported by the synapomorphies of an increased number of accessory teeth (Fig. 6.4) and a wide ventral interarea angle. The sister relationship of E. dignata and E. glabra Dewing, Reference Dewing1999 (Fig. 8) is supported by long dental plates. Monophyly of E. mucronata (Fig. 8) through E. recedens (Fig. 9) is supported by character evidence including wide ventral adductor muscle scars and a wide angle between the ventral muscle scars and the hinge line. Monophyly of E. aspera (Fig. 8) and more derived species is supported by increased strength of the ventral lateral ridges, increased ventral adductor scar length, and a narrower angle between the cardinal extremity and the hinge line.
Figure 9 (1, 4) Eochonetes minerva n. sp.; (1) ventral interior (USNM 145043); (4) dorsal interior (USNM 145045). (2–3, 5–6) Eochonetes vaurealensis (Dewing, Reference Dewing1999); (2) ventral exterior (GSC 113879); (3) dorsal exterior (GSC 11379); (5) ventral interior (GSC 113889); (6) dorsal interior (GSC 11893). (7–13) Eochonetes recedens (Sardeson, Reference Sardeson1892); (7) ventral exterior (USNM 24223a); (8) dorsal exterior (USNM 24223a); (9) dorsal exterior (SUI 1883, previously Thaerodonta saxea); (10) ventral interior (SUI 1881); (11) ventral interior (SUI 1884, previously T. saxea); (12) dorsal interior (SUI 1882); (13) dorsal interior (USNM 24691k).
Monophyly of the species Eochonetes magna (Fig. 8) plus crownward species is supported by increased dorsal adductor scar width, increased ventral diductor scar length, and a large cardinal process angle. Increased dental plate length is the synapomorphy supporting the monophyly of E. minerva n. sp., E. recedens, and E. vaurealensis Dewing, Reference Dewing1999 (Fig. 9). The sister group relationship of E. recedens and E. vaurealensis is supported by decreased fossette numbers.
Evolutionary and ecological implications
The most notable character suites within a phylogenetic context include the development of hinge-line dentition and musculature within the clade (Fig. 7). Notably, both musculature and dentition become more robustly developed during the evolution of the clade. Increased robustness in these features has often been considered to be indicative of adaptation to higher energy depositional environments in other brachiopod taxa (Cocks, Reference Cocks1970; Hurst, Reference Hurst1975), and Eochonetes species appear reflective of that overall trend.
Species of closely related genera, e.g., Sowerbyella and Eoplectodonta, lack developed hinge-line denticulation; the development of denticles and accessory teeth begins with the evolution of the Eochonetes clade. Eochonetes primarily radiated into shallower water environments than those occupied by Sowerbyella species, e.g., S. sericea (Sowerby, Reference Sowerby1839) (early Katian) occurred in high abundance in the shallow-shelf benthic assemblages (Cocks, Reference Cocks2013). Notably, the contemporaneous E. advena, which is part of the basalmost Eochonetes clade, occupied a similar deepwater environment that was dominated by siltstone and mudstone deposition (Donovan et al., Reference Donovan, Lewis and Harper2002). Eochonetes advena exhibits the synapomorphic fossettes, but limited additional hinge line development. Conversely, E. recedens, which occurs abundantly in the storm-influenced shelf environment of the Maquoketa Formation, Elgin Member (Sloan, Reference Sloan2005), exhibits well-developed hinge-line denticulation including one or two accessory teeth and numerous denticles concentrated in the center of the hinge line. Other species with well-developed denticulation include E. magna (Fig. 8.19–8.21) and E. mucronata (Fig. 8.12–8.14) from the Aleman Formation of Texas, which is composed of intercalated, thin-bedded carbonate and cherty dolostone deposited in a midramp location (Howe, Reference Howe1959; Pope, Reference Pope2004). Both species have accessory teeth, numerous (10–20) denticles along their hinge line, deeply incised muscle scars with prominent ridges, and attain the maximum size of any Eochonetes species. Overall, the distribution of morphological features among taxa results in a clade-wide pattern in which species present in higher energy environments exhibit enhanced hinge-line denticulation as well as more deeply incised muscle scars than their counterparts found in lower energy environments.
In addition to informing evolutionary and ecological analyses, phylogenetic hypotheses can also provide a framework for biogeographic analysis. A recent phylogenetic biogeographic study by Bauer and Stigall (Reference Bauer and Stigall2014), utilized the topology presented herein to investigate biogeographic evolution with the Eochonetes clade. Their results indicated that Eochonetes likely originated in basins north of the Transcontinental Arch (Fig. 1), exhibited early dispersal into the northern midcontinental region, and later evolved predominately by vicariant speciation (Bauer and Stigall, Reference Bauer and Stigall2014).
Discussion
All analyses within this study support the synonymy of Thaerodonta with Eochonetes. In addition, three new species have been identified and five species previously assigned to Thaerodonta or Eochonetes have been reassigned to other genera. Morphometric analyses aided in morphological delineation of the species and genera, and phylogenetic analyses provided additional insight into evolutionary patterns within the clade. The combination of these methods provided a framework for comparing data types (e.g., discrete versus quantitative characters) and produced a level of insight greater than could be attained by either alone. For taxa classified primarily by discrete characters, single approach analyses might be sufficient. However, combination approaches, such as the one employed herein, have the potential to provide greater analytical power, particularly when working with variable taxa characterized primarily by quantitative features.
Systematic paleontology
Superfamily Plectambonitoidea Jones, Reference Jones1928
Family Sowerbyellidae Öpik, Reference Öpik1930
Subfamily Sowerbyellinae Öpik, Reference Öpik1930
Genus Eochonetes Reed, Reference Reed1917
Type species
Eochonetes advena Reed, Reference Reed1917 from the upper Katian Lady Burn Starfish Beds, Farden Member, South Threave Formation, Girvan, Scotland.
Diagnosis
Cardinal extremities acute to alate, exterior unequally costellate with multiple costae between two stronger costae. Profile concavoconvex. Ventral valve with posterior conical cavities, delthyrial thickening, and hinge-line fossettes. Dorsal valve with hinge-line denticles, thickened notothyrium, median ridge short then bifurcating, and consistently present but variably developed bema.
Occurrence
Katian to Hirnantian age strata of the United States, Canada, Scotland, and Ireland.
Description
Ventral valve moderately convex; interarea long, orthocline to apsacline; fossettes developed along all or part of hinge length. Pseudodeltidum small, convex, apical, often rounded. Teeth usually small; crural fossettes deep; dental plates thick, extending anteriorly into lateral ridges of muscle scar field; accessory teeth well developed. Oblique infilled canals rarely present along hinge line of moldic specimens, not extending exteriorly. Delthyrial cavity divided by strong horizontal thickening between base of dental plates and median septum. Muscle-scar field bilobed anteriorly; median septum sharp, bifurcating anteriorly; adductor scars small, oval to subtriangular, posteriorly located; diductor scars longer, straight, stretching anteriorly.
Dorsal valve gently concave, with short hypercline interarea; denticles prominent, developed along part or all of hinge length, often irregularly spaced; sockets and accessory sockets on either side of cardinal process. Cardinal process short, elevated; chilidial plates strong, attached to brachial process, separated from cardinal process by deep grooves; floor of notothyrium thickened. Muscle scar field separated medially by two high ridges; adductor scars elongate; adductor scars elongate, straight, each separated centrally by lateral ridge of variable height. Bema present, variably developed, encompassing muscle scar field. Emended from Wang (Reference Wang1949).
Other species
Chonetes (Eochonetes) advena Reed, Reference Reed1917 from the Lower Quarrel Hill Formation and Lady Burn Formation of the Drummuck Subgroup (upper Katian), Girvan, Scotland; Thaerodonta aspera Wang, Reference Wang1949 from the Elgin Member of Maquoketa Formation (upper Katian), Winneshiek County, Iowa; Eochonetes celticus Mitchell, Reference Mitchell1977 from the Member III of the Bardahessiagh Formation (lower Katian) and Killey Bridge Formation (upper Katian), Pomeroy, County, Tyrone, Northern Ireland; Thaerodonta dignata Wang, Reference Wang1949 from the lower Maquoketa Formation (upper Katian), Clermont County, Iowa; Plectambonites glabra Shaler, 1865 from the Ellis Bay Formation (Hirnantian), Anticosti Island, Québec, Canada; Thaerodonta johnsonella Amsden, Reference Amsden1974 from the Leemon Formation (Hirnantian), Cape Girardeau County, Missouri; Thaerodonta magna Howe, Reference Howe1965 from the Aleman Formation (middle Katian), Hueco Mountains, Hudspath County, Texas; Eochonetes maearum n. sp. from the Bighorn Formation, Rock Creek Beds (upper Katian), Johnson County, Wyoming; Eochonetes minerva n. sp. from the Cutter Formation, El Paso County, and Aleman Formation (middle–upper Katian), Culberson County, Texas; Thaerodonta mucronata Howe, Reference Howe1965 from the Aleman Formation (middle Katian), Hudspath and El Paso Counties, Texas; Leptaena recedens Sardeson, Reference Sardeson1892 from the Maquoketa Formation (middle Katian), Spring Valley, Fillmore County, Minnesota, and Arnheim Formation (upper Katian), Tennessee; Plectambonites rugosaclarksvillensis Foerste, Reference Foerste1912 from the Waynesville and Liberty formations (upper Katian), Butler, Warren, and Clarksville counties, Ohio, and Lewis County, Kentucky; Sowerbyella (Eochonetes) vaurealensis from the Lavache through Homard members of the Vaureal Formation (upper Katian), Anticosti Island, Québec, Canada; Eochonetes voldemortus n. sp. from the Saturday Mountain Formation (upper Katian), South Lemhi Range, Idaho.
Remarks
The previous separation of Thaerodonta and Eochonetes was based primarily on the presence of ventral hinge-line canals in species attributed to Eochonetes but absent in species of Thaerodonta. The differentiation of these genera based on a single character prone to preservation bias is not supported by this analysis. Morphospace analyses detailed above demonstrate that Eochonetes and Thaerodonta are not distinguishable in general shell features (Fig. 3). The phylogenetic hypothesis generated indicates Eochonetes and Thaerodonta are not discrete evolutionary lineages. Species previously referred to Eochonetes share closer relationships to species previously referred to as Thaerodonta than to each other. This indicates that neither of the previous concepts of Eochonetes and Thaerodonta is monophyletic, however, the combined set of species does form a monophyletic lineage with respect to the outgroup taxa. Therefore, these species are transferred into a single monophyletic genus herein. Because the name Eochonetes Reed, Reference Reed1917 was described earlier, it has priority over Thaerodonta Wang, Reference Wang1949 and is the correct generic name for this clade.
Eochonetes was originally erected based on the specimens discovered in the Starfish Beds (upper Katian) of the Drummuck Subgroup in Thraeve Glen, Scotland. Reed (Reference Reed1917) described the ventral hinge-line canals as being characteristic of the superfamily Chonetoidea. The canals were preserved as small rods along the hinge of moldic specimens, but they do not to correspond with external spines and are therefore not homologous with the spines of true chonetids (e.g., Racheboeuf, Reference Racheboeuf2000). All other characteristics, internal and external, of Eochonetes align more appropriately with the family Sowerbyellidae (Jones, Reference Jones1928; Cocks and Rong, Reference Cocks and Rong2000; Cocks, Reference Cocks2013). Results of this investigation support the assignment of Eochonetes to Sowerbyellidae.
Species of Eochonetes are abundant in Katian strata of North America and some have been assigned to Sowerbyella (Fig. 10.1–10.8), which is characterized by similar external and internal structures. Specifically, the semicircular outline, variably costellate exterior surface ornamentation, and the bifurcated median septum in the ventral valve of Eochonetes resemble those of Sowerbyella (Wang, Reference Wang1949). Both Sowerbyella and Eochonetes possess similar ventral and dorsal muscle scar fields and have similar cardinalia (Howe, Reference Howe1972). Eochonetes is distinguished from Sowerbyella on the basis of well-developed hinge-line denticulation (accessory teeth and denticles), a less divergent brachial process, a narrower muscle field, and an increased delthyrial thickening that produces two small conical cavities in the posterior region of the ventral valve.
Figure 10 (1–4) Sowerbyella socialis Cooper, Reference Cooper1956; (1) ventral exterior (USNM 117525a); (2) dorsal exterior (USNM 117525a); (3) ventral interior (USNM 117527a); (4) dorsal interior (USNM 117527b). (5–8) Sowerbyella rugosa Meek, Reference Meek1873; (5) ventral exterior (OUIP 183a); (6) dorsal exterior (OUIP 183b); (7) dorsal interior (OUIP 183e); (8) dorsal interior (OUIP 183f). (9–10) Eoplectodonta moelsi (Rõõmusoks, Reference Rõõmusoks1981); (9) ventral exterior (TUG 1371-8); (10) dorsal exterior (TUG 1371-8). (11–12) Plectodonta convexa (Rõõmusoks, Reference Rõõmusoks1981); (11) ventral exterior (TUG 1971-12); (12) dorsal exterior (TUG 1971-12). (13) Plectodonta nubila (Rõõmusoks, Reference Rõõmusoks1981); ventral exterior (TUG 1371-15). (14–16) Sowerbyella saunjaensis (Rõõmusoks, Reference Rõõmusoks1981); (14) ventral exterior (TUG 1371-11); (15) dorsal exterior (TUG 1371-11); (16) dorsal interior (TUG 1371-19).
‘Thaerodonta’ was previously synonymized with Eoplectodonta Kozlowski, Reference Kozlowski1929 on the basis of similar characters, i.e., hinge-line denticulation and the presence of divergent dorsal lateral ridges (Muir-Wood and Williams, Reference Muir-Wood and Williams1965; Mitchell, Reference Mitchell1977). Howe (Reference Howe1972) questioned this synonymy, because Eochonetes lacks the well-developed medium septum and external oblique rugae, which are considered generic traits of Eoplectodonta. Fundamentally, the problem with the ‘Thaerodonta’-Eoplectodonta synonymy is that the hinge-line denticulation in the two genera is opposite in nature; Eochonetes has dorsal denticles with corresponding ventral fossettes, whereas Eoplectodonta has ventral denticles with corresponding dorsal fossettes (Howe, Reference Howe1972). The primary difference supports independent, rather than homologous, acquisition of hinge-line denticulation in these two lineages.
Rejection of previously assigned species
Five species previously assigned to the Eochonetes clade are herein transferred to other genera (Table 7). Leptaena minnesotensis Sardeson, Reference Sardeson1892 from the Orthisina Bed, Kenyon, and Galena Series Bed, (lower Katian) Berne, Minnesota is transferred to Sowerbyella. This species lacks dorsal denticles and clusters with Sowerbyella in the phylogenetic analysis (Fig. 4) based on features of raised muscle scars and a high angle between the brachial process and hinge line. Thaerodonta saunjaensis Rõõmusoks, Reference Rõõmusoks1981 (Fig. 10.14–10.16) from the Saunja Formation, (middle Katian), Miaremetsa, Estonia is likely assignable to Sowerbyella. The dorsal valve appears to lack denticles, and this taxon grouped within the Sowerbyella outgroup in the phylogenetic analyses (Fig. 4). The lack of denticles suggests that T. saunjaensis could belong to Sowerbyella rather than Eochonetes. However, the scarcity of specimens with preserved interiors renders this assignment tentative. Thaerodonta moelsi Rõõmusoks, Reference Rõõmusoks1981 (Fig. 10.9, 10.10) from the Kõrgessaare Formation, (middle Katian), Kõrgessaare, Estonia is transferred to Eoplectodonta. This species possesses a length to width ratio (=0.71) that falls within the Eochonetes spectrum (=0.39–0.77) but at the high end and well above the mean (0.57, N=333). This species exhibits an anacline ventral interarea and the dorsal interarea has an increased inclination relative to the cardinal area instead of the orthocline to apsacline ventral interarea that characterizes Eochonetes. In addition, the ventral valve lacks the characteristic delthyrial thickening and posterior conical cavities of Eochonetes. Finally, T. moelsi possesses a true median septum within the dorsal valve and has dorsal fossettes with corresponding ventral denticles, which are diagnostic of Eoplectodonta. Thaerodonta convexa Rõõmusoks, Reference Rõõmusoks1981 (Fig. 10.11, 10.12) from the Kõrgessaare Formation, (middle Katian), Paopa, Estonia is transferred to Plectodonta. This species exhibits extreme convexity, attaining approximately double the ventral valve height of the average Eochonetes specimen, and a significantly greater length to width ratio than exhibited by Eochonetes species. The interarea of T. convexa is anacline and the dorsal interarea differs from the orthocline to apsacline ventral interarea of Eochonetes. In addition, T. convexa exhibits increased inclination of the ventral interarea toward the cardinal area. The lack of a true median septum supports the assignment of this species to Plectodonta instead of Eoplectodonta. Thaerodonta nubila Rõõmusoks, Reference Rõõmusoks1981 (Fig. 10.13) from Adila Formation, (Hirnantian), Kaapsalyski Cliff, Estonia is transferred to Plectodonta. This species is more quadrate and has a less-curved beak than either T. convexa and T. moelsi. Compared to Eochonetes, this species has a greater shell height (=6.06 mm), which coincides with the end of the Eochonetes continuum (=0.45–8 mm), but the ventral interarea differs from Eochonetes in its anacline orientation. One individual appears to have 2 or 3 dorsal fossettes, but fossettes were not observed in other disarticulated specimens. The interior of T. nubila closely resembles that of T. convexa; accessory dentition is either poorly preserved or poorly developed and a true median ridge is absent. Thus, this species likely belongs within Plectodonta.
Eochonetes maearum new species
1957 Thaerodonta sp.; Reference RossRoss, p. 457, pl. 40, fig. 22.
1970 Thaerodonta aff. T. clarksvillensis; Reference MacomberMacomber, p. 439, pl. 78, fig. 7a–d.
Type specimens
FMNH PE 11072 (holotype), FMNH PE 11070 and 11071 (paratypes), USNM 124834 (paratype) from the Rock Creek Beds of the upper Bighorn Formation, Johnson County, Wyoming.
Diagnosis
Angular to slightly acute cardinal extremities; wide delthyrium covered apically by short rounded pseudodeltidium; ventral interior unknown; denticles numerous (7), present on each cardinal extremity concentrated in center of hinge line; brachial process small, rounded; median septa faint, lacking elevation; lateral ridges and bema faint; bema extending outward from tips of brachial process.
Occurrence
Upper Katian Rock Creek Beds of the upper Bighorn Formation in Johnson County, Wyoming.
Description
Outline semicircular. Hinge line straight; cardinal extremities angular to slightly acute. Ventral interarea apsacline. Delthryium wide, covered apically by short rounded pseudodeltidium. Ventral interior unknown. Dorsal interarea hypercline. Denticles numerous, 7 on each cardinal extremity, concentrated in center of hinge line. Cardinal process small, slightly elevated; chilidial plates steep; brachial process small, rounded, extending anteriorly into median ridges; no true median septa; two ridges do not touch. Sockets wide, shallow. Muscle scar wide, lobe-shaped, separated by two median ridges that extend for two-thirds of valve; adductor scars equal in size with lateral ridges in center; lateral ridges faint, not extending as far as median ridges. Bema faint; anterior extension from brachial process.
Etymology
Named for Betty Mae Bauer and Elsie Mae Shimanek.
Remarks
Concentration of denticles in the center of the hinge line is unique to this species. In other Eochonetes species, denticles begin in the center and extend to the end of the cardinal extremity. The cardinal extremities of this species are angular rather than acute as is more common among Eochonetes species. The external surface of each ventral valve is swollen anteriorly. This swelling includes excess shell material built up along the commissure.
The small size of Eochonetes maearum n. sp. is similar to that of E. dignata. However, the dorsal lateral septa of E. maearum n. sp. are less well developed and the bema is fainter than that of E. dignata. The brachial process is similar in size and shape to that of E. johnsonella. The median ridge separation is also very similar to that of E. johnsonella; the two ridges connect with the brachial process and do not ever touch. Notably, E. maearum n. sp. has centrally located, less-numerous denticles, whereas denticles are pronounced along the entire hinge length of both E. dignata and E. johnsonella.
Macomber (Reference Macomber1970) previously described Eochonetes maearum n. sp. as having an affinity to E. clarksvillensis based on internal and external characters, i.e., shape and size range. However, the dorsal muscle field of E. clarksvillensis is much more deeply incised than that of E. maearum n. sp. The size and shape of the denticles of E. maearum n. sp. are similar to those of E. clarksvillensis, but the denticles of E. maearum n. sp. are more evenly spaced and sized. Those of E. clarksvillensis increase in size with lateral extent.
Eochonetes minerva new species
1965 Thaerodonta cf. T. recedens; Reference HoweHowe, p. 649, pl. 81, figs. 13–17.
Type specimens
USNM 145043 (holotype) from the Aleman Limestone, Montoya Group, west-facing escarpment 3 mi east of Helms West Well, Hudspath County, Texas; USNM 145042 (paratype) from the Baylor Mountains east-facing escarpment, 1.6 mi N50°W of Watson Ranch House, Culberson County, Texas; USNM 145044 (paratype) from the H-2 Hueco Mountains west-facing escarpment, 3 mi east of Helms West Wall, Hudspath County, Texas; USNM 145045 (paratype) from the Franklin Mountains east-facing escarpment, 1.1 mi south of state line, El Paso County, Texas.
Diagnosis
Acute to alate cardinal extremities; ventral median septum extending one-third of shell length, then bifurcating; primary teeth large, one or two accessory teeth present with that adjacent to primary tooth more pronounced than other; primary sockets ae wide followed by two adjacent accessory sockets; denticles beginning immediately after sockets and persisting to cardinal extremity tip; low short median septum on dorsal valve between branched median ridges.
Occurrence
Upper Katian in the Aleman and Cutter formations of the Montoya Group of Texas.
Description
Outline semicircular to subquadrate; cardinal extremities acute to alate. Ventral valve evenly gently convex; interarea apsacline. Delthryium moderately wide; pseudodeltidium very small, rounded. Median septum extending one-third of length then bifurcateing. Adductor muscle scars subcircular, located posteriorly; diductor muscle scars elongate, extending two-thirds of shell length. Primary teeth strong; one or two accessory teeth with first stronger than second; fossettes (5–10) developed few millimeters from accessory teeth and extending to end of cardinal extremity. Dental plates extending anteriorly into strong lateral ridges. Dorsal valve slightly concave; interarea hypercline. Cardinal process stout, elevated posteriorly; brachial process short, rounded. Median septum bifurcating posteriorly; small short ridge appearing briefly anteriorly between two median ridges. Adductor muscle scars weakly impressed, wide, with lateral ridges; lateral ridges beginning posteriorly, elevating, continuing anteriorly. Bema faint but elevated slightly, outlining muscle scar field. Primary sockets wide, with two accessory sockets adjacent; denticles occurring immediately adjacent, extending to tip of cardinal extremity.
Etymology
Named for the Roman goddess of wisdom.
Remarks
Dentition is well-developed but not as pronounced as in the other two Eochonetes species in the southern midcontinental region (E. magna and E. mucronata). The median septum that reappears anteriorly between the two median ridges is unique to this species and was not observed in the other species examined. The ventral valve exterior is worn, whereas the interior of E. minerva n. sp. is similar to that of E. magna but lacks the well-defined accessory teeth of the latter. The cardinalia region of the dorsal valve is characterized by a brief median ridge prior to the bifurcation, which is similar in shape to those of E. magna and E. mucronata. The ventral valve is similar in outline to that of E. recedens, but the muscle scars are more deeply impressed and at more acute angle in E. minerva n. sp. The internal dorsal muscle scars are lightly impressed, which is similar to those of the species of Eochonetes found in the Maquoketa Group of the midcontinental region, but this species differs in the presence of a faint, true median septum occurring between the two median ridges. In morphospace, E. minerva n. sp. plots (Fig. 3) within the E. magna and E. clarksvillensis occupied space for the dorsal valve and adjacent to, but not overlapping, the E. clarksvillensis morphospace field for the ventral valve. In the phylogenetic analyses, E. minerva n. sp. is hypothesized to be in a sister relationship with E. vaurealensis and E. recedens rather than creating a polytomy with E. recedens, which would be expected if the species were synonymous.
Eochonetes mucronata (Howe, Reference Howe1965)
1965 Thaerodonta mucronata Reference HoweHowe, p. 648, pl. 81, figs. 18–24.
1965 Thaerodonta mucronata scabra Reference HoweHowe, p. 648, pl. 82, figs. 9–11.
Type specimens
USNM 145035 (holotype), USNM 145036–145039 (paratype series) from the Aleman Limestone, Montoya Group, west-facing escarpment 3 mi east of Helms West Well, Hudspath County, Texas; USNM 145040–145041 (paratype series) from the Cutter Limestone, Montoya Group, small ridge, 0.7 mi S55°W of Sugarloaf Mount, El Paso County, Texas.
Diagnosis
Cardinal extremities very acute, mucronate; anterior margin truncate to gently rounded. Exterior roughly lamellose on some specimens. Two accessory teeth (ventral valve) with corresponding sockets (dorsal valve) nearly equal in size to primary teeth; denticles/fossettes numerous (15+), beginnng after accessory teeth, terminating prior to cardinal extremity tip. Teeth curving slightly toward cardinal extremities. Dorsal adductor muscle scars subcircular, persisting for most of shell length, divided by strongly elevated lateral ridges.
Occurrence
Upper Katian in the Aleman and Cutter formations of the Montoya Group of Texas.
Remarks
Howe (Reference Howe1965) differentiated Eochonetes mucronata scabra as a subspecies of E. mucronata on the basis of possessing a rough lamellose exterior. No preserved valve interiors of E. mucronata scabra were recognized from the strata. The rough lamellose exterior has been observed in occasional specimens of other species (e.g., E. magna, E. recedens), and thus it is more likely caused by depositional or taphonomic conditions rather than reflecting a specific and discrete genotype. Consequently, this feature is not considered to be of taxonomic importance, and E. mucronata scabra is synonymized with E. mucronata herein. The teeth and accessory dentition of E. mucronata resemble those of E. magna in the curvature toward the cardinal extremities; however, the overall angles of the ventral muscle scars in E. magna exceeds those of E. mucronata.
Eochonetes recedens (Sardeson, Reference Sardeson1892)
1892 Leptaena recedens Reference SardesonSardeson, p. 330, pl. 4, figs. 29–32.
1892 Leptaena saxea Reference SardesonSardeson, p. 330, pl. 4, figs. 33–35.
1949 Thaerodonta recedens; Reference WangWang, p. 20, pl. 11A, figs. 1–3.
1949 Thaerodonta saxea; Reference WangWang, p. 21, pl. 11B, figs. 1–5.
1988 Thaerodonta recedens; Reference HoweHowe, p. 214, figs. 2.9–2.12, 2.14–2.17, 10, 11.
Syntypes
YPM IP 027652–027654 and 201725 from the Elgin Member, Maquoketa Group, Minnesota.
Diagnosis
Lateral margins acute to gently sloping; anterior margin truncate. Ventral anterior margin occasionally forming broad sulcus. Accessory teeth subdued if present. Cardinal fossettes/denticles originating in center of hinge line; averaging 5–9 per cardinal extremity, evenly spaced along hinge line, typically increasing in size laterally.
Occurrence
Upper Katian of Iowa, Missouri, Minnesota, and Illinois in the Maquoketa Group.
Materials
SUI 1881–1884; USNM 145046, 418151, 418152–418156, and 418158. For a complete list of nonfigured examined specimens examined, see Supplemental Data 1.
Remarks
Wang (Reference Wang1949) suggested that the comparatively larger size, narrower sulcus, stronger accessory teeth, and weaker denticles justified distinguishing Eochonetes saxea from E. recedens. However, Howe (Reference Howe1965) noted that when examining large populations, differences between the two species are not consistently expressed. The morphometric analyses presented herein recovered nearly complete overlap in morphospace occupation of E. saxea and E. recedens in all analyses. No other consistent morphological differences were present in the additional characters coded within the phylogenetic analysis. Due to the high level of morphological similarity exhibited by these two taxa, E. recedens and E. saxea are synonymized herein.
The ventral lateral ridges, which extend from the dental plates, are rounded with a slight bend in them, which is unique to this species (Fig. 9.10, 9.11). Denticle number varies among specimens, but the location is invariant; the denticles consistently originate in the center of the shell and extend laterally prior to terminating ~1–2 mm from the tip of the cardinal extremity.
Eochonetes recedens specimens exhibit subequal shell width and length. Individuals found in the northern midcontinental region have acute cardinal extremities compared to the more alate specimens found in the Central Basin. The shell outline is similar to that of E. aspera, but E. recedens is distinguished by its more angular cardinal extremities. The anterior portion of E. recedens exhibits a truncated to gently rounded shape similar to that of E. johnsonella, but E. recedens can be distinguished by less-divergent dorsal median ridges and a more-elongated ventral muscle scar field.
Eochonetes voldemortus new species
1959 Thaerodonta sp.; Reference RossRoss, p. 458, pl. 55, figs. 40–43.
Type specimens
USNM 133257 (holotype) and USNM 133258 (paratype) from the Saturday Mountain Formation, on ridge between Black Canyon and South Creek, just north of divide, South Lemhi Ridge, Idaho.
Diagnosis
Length commonly less than half the width; lateral margins gently sloping; ventral interior unknown; denticles numerous, 9 or 10 on one cardinal extremity, originating in center of hinge, terminating 1 mm from cardinal extremity tip; bulbous median ridge prior to bifurcation; very faint bema.
Occurrence
Upper Katian Saturday Mountain Formation of Idaho.
Description
Outline semicircular. Hinge line straight; cardinal extremities acute. Ventral interarea apsacline. Delthryium small, covered apically by rounded pseudodeltidium. Ventral interior unknown. Dorsal interarea hypercline. Denticles numerous, 9 or 10 on one cardinal extremity, originating in center of hinge, terminating 1 mm from cardinal extremity tip. Cardinal process stout; chilidial plates steep; brachial process short, pointed; sockets wide, deep. Muscle scar field short, narrow; median ridges extending from central portion of brachial process, one-half shell length; lateral ridges elevated, terminating early. Bema faint, extending directly from tip of brachial process; median ridges extending anterior to extent of bema.
Etymology
Named after the fictional antagonist, Voldemort, of J.K. Rowling’s Harry Potter series.
Remarks
Shell width to length ratio is low (0.48–0.49). Dorsal valve muscle scars are very faint and lateral septa originate near brachial process but do not extend as far as the median ridges. Typically, the muscle scar field of Eochonetes extends for approximately two-thirds of the shell, whereas in this species is approximately one-half. The median ridges split after a very brief bulbous median ridge.
The small size of Eochonetes voldemortus n. sp. is similar to that of E. dignata and E. maearum n. sp., but the width to length ratio is smaller (0.49) than in the other species (0.55–0.65). The bema is the faintest of all the species compared. Rather than having a brief median septum as seen in E. magna and E. mucronata, there is a rounded connection of the two ridges, ~0.5 mm in length.
Acknowledgments
We thank D.I. Hembree, G.C. Nadon, Y. Candela, B. Pratt, and an anonymous reviewer for constructive comments that helped us improve this manuscript. T. Adrain, R. Burkhalter, S. Butts, N. Clark, L.R.M. Cocks, M. Coyne, K. Hollis, Z. Hughes, B. Hussaini, M. Isakar, P. Mayer, P. Shepherd, and R. Swisher provided access to specimens housed in their institutions. This study was supported by NSF (EF-1206750, EAR-0922067 to ALS) and the Yale Peabody Museum’s Schuchert and Dunbar Grants-in-Aid Award, the Dry Dredgers Paleontological Research Award, Geological Society of America Student Research Grant, and an Ohio University Geological Sciences Alumni Grant to JEB. This paper is a contribution to IGCP Project 591, ‘The Early to Middle Paleozoic Revolution.’
Accessibility of supplemental data
Data are available from the Dryad Digital Repository: http://doi.org/10.5061/dryad.62h0b.
