Introduction
Today, the genus Cittarium (Mollusca: Gastropoda: Trochoidea) occurs exclusively in the eastern Tropical America. It is represented by a single species: Cittarium pica (Linnaeus, 1758), which is a characteristic element of the Recent fauna of the greater West Indies, from the Bahamas, in the north, through the northern shores of Venezuela, in the south. The species is common and of economic importance, being the third most commercially significant invertebrate in the region supplanted only by Panulirus argus, the spiny lobster, and Lobatus gigas, the queen conch (Schmidt et al., Reference Schmidt, Wolff and Vargas2002; Robertson, Reference Robertson2003). It has been reviewed in depth by Robertson (Reference Robertson2003).
In the Tropical American region, up to now, the genus Cittarium was known from fossil record only from Pleistocene deposits of Bermuda (Clench and Abbott, Reference Clench and Abbott1943). Furthermore, extensive field work by two of the authors (B.L., C.M.S.) in the past two decades in the Caribbean Neogene of Venezuela, Panama, Costa Rica, and the Dominican Republic has yielded specimens of C. pica solely from Upper Pleistocene deposits of Venezuela (Boca Chica: El Manglillo Formation) and not in the very rich Lower Pleistocene assemblages of Costa Rica (Moin Formation), Panamá (Isla Colon and Escudo de Veraguas Formations) and Venezuela (Cumaná and Caigüire Formations). Cittarium pica has a large, solid shell and therefore its absence in the abundant and diverse Caribbean Neogene assemblages is conspicuous.
Surprisingly, Lozouet (Reference Lozouet2002) recorded the genus from the Upper Oligocene, Chattian, of the SW Atlantic coast of France. The species described, Cittarium maestratii Lozouet, Reference Lozouet2002, is indeed closely similar to the Recent C. pica, and led that author to suggest that the genus originated in the Old World and subsequently expanded its geographical distribution to Tropical America.
As Lozouet (Reference Lozouet2002) pointed out, the presence of Cittarium in the Upper Oligocene of Europe leaves a gap of about twenty million years in the fossil record of the genus, which was difficult to explain. What is more, Lozouet underlined the fact that the apparent absence of representatives of the genus Cittarium in the pre-Pleistocene fossil record of the American Tertiary was surprising.
Therefore, the discovery herein reported and discussed of a new species of Cittarium—C. praepica n. sp.—in the Upper Miocene Cercado Formation of the Dominican Republic is a significant find and an important addition to the geological history and the biogeography of the genus.
Stratigraphic setting
The Cittarium material herein described and discussed was collected during fieldwork on the Rio Gurabo area in the Mao region of northern Dominican Republic. The outcrop where it originates from is located in the Arroyo La Cabra, approximately 50 m upstream from its confluence with Rio Gurabo, which is about 5.5 km south of the bridge over the Gurabo on the Los Quemados to La Caoba DR-18 road, with the following geographic coordinates: N19° 27' 29.41'' W71° 11' 10.11'' (Fig. 1). This location corresponds to the NMB locality number 15913 of Saunders et al. (Reference Saunders, Jung and Biju-Duval1986, text-figure 4), which comprises a stretch approximately 60 m long on the right bank of the arroyo, upstream from the Gurabo River.
Figure 1 Geographic location of the Arroyo La Cabra study site, Mao region, Dominican Republic, in which the specimens of Cittarium praepica n. sp. were collected.
The specimens were collected from a fine grey sandstone bed containing an abundant and diversified molluscan assemblage (mostly gastropods and a few bivalves) about one meter above the riverbed. This bed is part of the Cercado Formation (Saunders et al. Reference Saunders, Jung and Biju-Duval1986, text-figure 4), which is correlated with the upper part of the calcareous nannofossil zone NN11 of Martini (1971) and the Globorotalia humerosa foraminiferal zone, Upper Miocene, Messinian (chronostratigraphy of the Cercado Formation based on Saunders et al., Reference Saunders, Jung and Biju-Duval1986, table 4, and the geologic time scale of Gradstein et al., Reference Gradstein, Ogg and Smith2004, fig. 21.1, with additions from McNeill et al., Reference McNeill, Klaus, Evans and Budd2008 and Waller, Reference Waller2011).
In the middle part of the Cercado, where the specimens were collected, sands and silty sands strongly predominate with minor pebbly layers and lenses. Tests of Amphistegina macroforams are common in these beds, as well as Thalassinoides burrows. The dip is less than 10° toward the north (Saunders et al. Reference Saunders, Jung and Biju-Duval1986).
Material and methods
The material described here was collected in 2014 during fieldwork conducted by the authors on the Rio Gurabo area in the Mao region of northern Dominican Republic and is now deposited in the Naturhistorisches Museum Wien, Austria (NHMW). Subsequently, the Panama Paleontology Collections in the Naturhistorisches Museum Basel, Switzerland were investigated, but no further specimens were found.
Abbreviations
MNHN, Muséum National d’Histoire Naturelle, Paris, France; NHMW, Naturhistorisches Museum Wien, Austria; RMNH, Rijksmuseum van Natuurlijke Historie, now merged with Naturalis Biodiversity Center, Leiden, The Netherlands.
Systematic paleontology
Class Gastropoda Cuvier, 1797
Superfamily Trochoidea Rafinesque, 1815
Family Unassigned
Genus Cittarium Philippi, 1847
Type species
Turbo pica Linnaeus, 1758 by original designation, Recent, Caribbean.
Remarks
Traditionally, the genus Cittarium was allocated to the family Trochidae Rafinesque, 1815 (Hickman and McLean, Reference Hickman and McLean1990; Lozouet, Reference Lozouet2002). Recent studies by Williams et al. (2012) suggested, based on molecular studies, that Cittarium was closely related to the genera Tectus Montfort, 1810 and Rochia Gray, 1857, forming a distinct clade, albeit with only low to moderate support. Although not formally described, Williams's (Reference Williams2012) results suggest a possible familial rank for this clade, pending further studies. We follow Williams (Reference Williams2012) in her conclusion by not assigning Cittarium to any family within Trochoidea.
Cittarium praepica new species
Figure 2 (1–4) Cittarium praepica n. sp., holotype NHMW 2014/0179/0001, height 17.5 mm, width 24.2 mm; (5–7) Cittarium praepica n. sp., paratype NHMW 2014/0179/0002, height 23.6 mm, width 30.7 mm, both from Arroyo La Cabra, right bank, approximately 50 m upstream from its confluence with Río Gurabo, Cercado Formation, Messinian, Upper Miocene; (8–10) Cittarium pica Linnaeus, 1758, RMNH.MOL.14765, height 29.8 mm, width 33.5 mm, Cuba, north coast, Recent; (11, 12) Cittarium maestratii Lozouet, Reference Lozouet2002, holotype MNHN-PL1037A, height 39.0 mm, width 41.0 mm, St-Paul-le`s-Dax (Lestrilles), Landes Dept., France, Chattian, Upper Oligocene.
Types
Holotype NHMW 2014/0179/0001 (ex. BL coll.) (Figs. 2.1–2.4), height 17.5 mm, width 24.2 mm; paratype NHMW 2014/0179/0002 (ex. BL coll.) (Figs. 2.5–2.7), incomplete, height 23.6 mm, width 30.7 mm. Arroyo La Cabra, right bank, approximately 50 m upstream from its confluence with Río Gurabo (Fig. 1), Cercado Formation, Messinian, Upper Miocene.
Diagnosis
Turbiniform shell, with depressed rounded whorls bearing relatively narrow cords below the shoulder; a large, rounded aperture; a thin outer lip; a columella without a tooth; a flattened expanded callus bearing a small funicle; a deep, narrow, rounded umbilicus; and a pigmentation pattern of brownish-violet marbling.
Occurrence
Upper Miocene, Cercado Formation, Dominican Republic.
Description
Shell medium-sized, solid, turbiniform. Protoconch not preserved. Teleoconch with 3.5 depressed, rounded whorls. Suture weakly incised, linear. Early teleoconch whorls eroded, last two whorls with slightly flattened, shallow sutural ramp bearing weak rugae; below shoulder ornament of weak, narrow spiral cords, the shoulder cord being slightly broader and rugose. Very close-set, strongly prosocline, slightly raised growth lines give surface a weakly lamellar appearance. Aperture round, relatively large, strongly inclined toward the axis of coiling; peristome complete. Outer lip thin. Inner lip smooth, with well-delimited, flattened callus on parietal part, forming medial border of umbilicus. Callus bearing a small, poorly delimited funicle. Umbilicus round, narrow, and deep. Surface covered with distinct pigmentation patterns of large brownish-violet marbling pigmentation pattern.
Etymology
From Latin preposition prae, meaning before, in front, referring to the occurrence of the new species geologically younger than the Pleistocene-Recent Cittarium pica. Cittarium is gender neutral.
Materials
Known only from type material.
Remarks
Cittarium praepica n. sp. has the smallest shell of the three congeners known, although it is quite possible the specimens at hand did not reach full maturity. Cittarium pica has a much larger shell, attaining over 130 mm in maximum diameter (Robertson, Reference Robertson2003). The Recent species is well represented in the Naturalis collections in Leiden (the Netherlands), and the differentiation below was based on specimens matched for size (Figs. 2.8–2.10). At all growth stages known, C. praepica has a lower spire than C. pica.
Cittarium praepica differs from both C. pica and C. maestratii (Figs. 2.11, 2.12) in having more depressed whorls, especially the last whorl and in having the spiral cords below the shoulder on the last whorl finer than in either of the shells of the other two species. Regarding general shape and spiral sculpture, C. pica and C. maestratii are more closely similar to each other than they are to C. praepica. The mid-columellar denticle present in both C. pica and C. maestratii, albeit more weakly developed in the Oligocene species, seems to be lacking in C. praepica. The callus bordering the umbilicus medially is strongly developed in C. praepica and the Recent Caribbean C. pica, and bears a small funiculus, whereas the callus is much narrower and without a funiculus in the European Oligocene species. The shape and depth of the umbilicus and the color pattern are remarkably similar in all three species.
Robertson (Reference Robertson2003) discussed the differential shell erosion in large gerontic specimens of the Recent C. pica, by which the black, or purplish-brown faded stripes or marks become slightly higher in relief than the surrounding white areas. This differential erosion was ascribed to the action of blue-green algae weakening the outermost and only white shell areas. The larger of the two specimens of C. praepica n. sp., although poorly preserved, clearly shows this type of surface relief (Fig. 2.7), suggesting it might have been a gerontic specimen. The smaller specimen does not show this differential erosion.
Discussion
Presently, the genus Cittarium is represented by a single Tropical American species: Cittarium pica (Linnaeus, 1758), which is a common element of the shallow marine fauna of the West Indies. As Lozouet (Reference Lozouet2002) fittingly stated, the lack until then of known ancestors of Cittarium pica in the pre-Pleistocene deposits of Tropical America was rather surprising.
The occurrence of Cittarium maestratii Lozouet, Reference Lozouet2002 in the Oligocene (Chattian) of France reported by Lozouet (Reference Lozouet2002), the only pre-Pleistocene representative of Cittarium known at the time, created two apparent astounding gaps. The first one, a gap of approximately 25 million years in the geological history of the genus and, the second, a biogeographical gap spanning the entire Atlantic Ocean, and separating the two only Cittarium records known. Cittarium praepica n. sp. from the Miocene (Messinian) of the Dominican Republic, the third member hitherto known of the genus, bridges both gaps, giving us a few more clues on the geological history and the biogeography of Cittarium.
Extreme disjunctions between fossil and Recent occurrences such as these, as noted by Lozouet (Reference Lozouet2002) and shown by Jung and Heitz (Reference Jung and Heitz2001), are not uncommon and may result from a conjugation of both taphonomical artefacts and sampling deficiencies. The fossils of vagile organisms living in habitats associated high hydrodynamic energy such as intertidal and shallow subtidal rocky shores, and therefore more prone to mechanical abrasion and fragmentation, would be harder to find than those of others living in taphonomically more benign environments and would be underrepresented in scientific collections.
In Recent faunas, Cittarium pica is associated with rocky intertidal and shallow subtidal habitats, sheltering in crevices and small depressions (Robertson, Reference Robertson2003). We assume that the Chattian and Messinian representatives of the genus had similar ecological requirements. In the Miocene deposits of the Cercado Formation, in the Dominican Republic, C. praepica was found in a fossiliferous sandy bed, together with an abundant and varied molluscan assemblage, rich in Conus species, Semicassis reclusa (Guppy, Reference Guppy1873) and Ficus gibsonsmithi Beu, Reference Beu2010, among others, as well as bivalves. The molluscan genera represented in the fossiliferous bed, as well as its sandy lithology, are not particularly suggestive of rocky shores, but certainly of shallow subtidal habitats. Nevertheless, a single well-preserved specimen of a new species of the trochoid genus Tegula, also commonly associated with shallow rocky bottoms, was found together with the Cittarium specimen and awaits description. This suggests that some kind of hard substrate or rocky habitat may have existed in the vicinity in Messinian times that could sustain the animals of both genera.
Cittarium praepica is the earliest member of the genus in the Caribbean. This record, dating to about six million years ago, significantly reduces the 25-million-year gap in the fossil record of the genus between its first appearance, represented by the Chattian C. maestratii, and the Late Pleistocene to Recent C. pica. It also provides a Caribbean biogeographical history—an ancestor—for the Recent C. pica, the lack thereof Lozouet (Reference Lozouet2002) characterized as “surprising.” Interestingly enough, the European Oligocene species and the Recent Caribbean C. pica have more closely similar shells in shape and sculpture than the two Caribbean species.
This new Upper Miocene Dominican record supports the general biogeographical model for the genus devised by Lozouet et al. (2002), placing the origin of Cittarium in the northeastern Atlantic with a subsequent immigration to the Caribbean. The new Dominican find constrains the arrival of Cittarium in the Caribbean to the time interval between Late Oligocene (Chattian) and Late Miocene (Messinian), however when exactly it arrived to the New World during that period of time and indeed what was the fate of the genus in Europe after the Late Oligocene and when and why it finally disappeared from the Old World remains a mystery.
Nevertheless, the working hypothesis formulated by Lozouet (Reference Lozouet2002, p. 769) that the dispersal of Cittarium from Europe to tropical America may have occurred as early as the Late Oligocene or the Early Miocene cannot—so far—be confirmed, despite extensive collecting by the authors in numerous lower and middle Miocene Caribbean fossil assemblages, some of which containing many typical hard bottom gastropod genera such as Scutellastra, Tegula, Thais, and Patula (i.e., upper Lower Miocene, upper Burdigalian Cantaure Formation of Venezuela [Vermeij, Reference Vermeij2001; Landau and Vermeij, Reference Landau and Vermeij2010]).
Acknowledgments
We would like to thank Pierre Lozouet, Muséum National d’Histroire Naturelle, Paris, France, for making images of the holotype of Cittarium maestratii available and his permission for reproduction (Figs. 2.11, 2.12); and Suzanne Williams of the NHM UK, London for providing us with the latest information on the systematic placement of the genus Cittarium.
