Introduction
With only 22 described species grouped in three genera (Háva and Löbl, Reference Háva and Löbl2005; Lawrence and Leschen, Reference Lawrence and Leschen2010; Peck, Reference Peck2010), the family Jacobsoniidae is a small group of polyphagan beetles comprising small-to-minute species (0.65–2.5 mm long). The family was provisionally included in Bostrichiformia by Lawrence and Newton (Reference Lawrence and Newton1995) and Philips et al. (Reference Philips, Ivie and Giersch2002) and is currently placed in Derodontiformia (Lawrence et al., Reference Lawrence, Beutel and Leschen2010), although it shares characters with staphyliniforms (Crowson, Reference Crowson1959, Reference Crowson1960; Lawrence et al., Reference Lawrence, Ślipiński, Seago, Thayer, Newton and Marvaldi2011). Löbl and Burkhardt (Reference Löbl and Burckhardt1988) and Háva and Löbl (Reference Háva and Löbl2005) catalogued all extant species of the world and only one species has been described since then (Peck, Reference Peck2010). Jacobsoniidae includes three extant genera: Saphophagus Sharp with one species (S. minutus Sharp) restricted to New Zealand; Sarothrias Grouvelle with 13 species in the Indo-Australian region; and Derolathrus Sharp, with eight described species known from Sri Lanka, North America (Florida) and mainly tropical islands (e.g., Barbados, Canary Islands, Guadeloupe, Fiji, Hawai’i, Madeira, Mauritius, Virgin Islands).
Derolathrus, like remaining jacobsoniid genera, are poorly represented in collections and little about their biology is known (Háva and Löbl, Reference Háva and Löbl2005). There is unstudied material and undescribed species from Africa, Australia, the Neotropics, New Caledonia, and the Solomon Islands (Arnett et al., Reference Arnett and Thomas2000; Lawrence and Leschen, Reference Lawrence and Leschen2010; Peck, Reference Peck2006, Reference Peck2010; Philips et al., Reference Philips, Ivie and Giersch2002; Leschen, personal observation). Derolathrus species are tiny (from about 0.65 mm to about 1.00 mm) and require detailed study, especially of their cuticular microsculpture. Some species may be more widespread than thought; for example, Derolathrus atomus Sharp was described from Hawaii and was recently recorded and considered accidentally introduced to the West Indies by Peck (Reference Peck2010). Derolathrus species have been collected by mass collecting techniques (in flight intercept traps and sifting leaf litter), and in in microhabitats (under bark, in rotten palm wood, from fungal fruiting bodies, in bat guano, and from bird nests; Lawrence and Leschen, Reference Lawrence and Leschen2010; Löbl and Burckhardt, Reference Löbl and Burckhardt1988; Philips et al., Reference Philips, Ivie and Giersch2002). To date, no fossils belonging to Jacobsoniidae have been recorded. Therefore, our discovery of a new Derolathrus species from the middle Eocene is of significance for understanding the origin and early evolution of the family.
Material and methods
The newly described species is known from two individuals preserved in two pieces of Baltic amber. The two specimens are in the collection of Carsten Gröhn (Glinde, Germany) and will be deposited at the Geologisch-Paläontologisches Institut und Museum der Universität Hamburg, Germany (GPIH). The pieces were not embedded in artificial resin. The precise age of Baltic amber is not determined (Weitschat and Wichard, Reference Weitschat and Wichard2010). Due to transportation and extensive re-deposition, no amber-bearing deposits can be considered as the primary depositional site. We follow Ritzkowsky (Reference Ritzkowski1997) who considered Baltic amber to be of Bartonian-Lutetian-Ypresian (middle Eocene) age. Micro-CT was performed at the Zoological Institute and Museum of the Ernst-Moritz-Arndt University of Greifswald. The specimens were scanned using an XRadia XCT-200 (Carl Zeiss) equipped with switchable scintillator-objective lens units. Each amber piece was mounted on a holder and centered. A series of 1600 projections were performed using 10× magnification with the X-ray source setting at 40 kV and 8 W for 10s acquisition time at each interval. Tomography projections were reconstructed with the reconstruction software provided by XRadia. The generated image stacks (TIFF format) have a system based on 1.9 µm pixel size. Images were cropped to 792×360 px (GPIH no. 4570, coll. Gröhn no. 8052) and 980×488 px (GPIH no. 4571, coll. Gröhn no. 8260). Both scans were performed using Binning 2 and subsequently reconstructed using Binning 1 (full resolution) to avoid information loss. Volume renderings were generated using Amira 5.4 (FEI). Recent material was made available from the following collections: Florida State Collection of Arthropods, Gainesville, Florida (Paul Skelley); Museum d’Histoire Naturelle, Geneva, Switzerland (Guilio Cuccodoro); and the New Zealand Arthropod Collection, Auckland.
Systematic paleontology
Order Coleoptera Linnaeus, Reference Linnaeus1758
Family Jacobsoniidae Heller, Reference Heller1926
Subfamily Derolathrinae Sen Gupta, Reference Sen Gupta1979
Genus Derolathrus Sharp, Reference Sharp1908
Derolathrus groehni new species
Figure 1 Derolathrus groehni n. sp., holotype, dorsolateral view, with exposed hind wings shown. Scale bar represents 200 µm.
Figure 2 Micro-CT scan of Derolathrus groehni n. sp., holotype: (1) dorsal view, (2) ventral view.
Figure 3 Derolathrus groehni n. sp., paratype: (1) dorsal view, under normal light; (2) ventral view, under normal light; (3) dorsal view, micro-CT scan; (4) ventral view, micro-CT scan. Scale bars represent 200 µm in (1) and (2).
Material
Holotype, one completely preserved adult; GPIH no. 4570, coll. Gröhn no. 8052. Paratype, GPIH no. 4571, coll. Gröhn no. 8260.
Diagnosis
Rugose microsculpture present on dorsal surfaces of head and pronotum, weaker microsculpture present on elytra; head with four aligned rows of small, non-setiferous punctures, temples well developed, vertex with a rim and lacking a basal fovea; eyes large, each eye with about 15 ommatidia; club 1-segmented with distinct sensory area about half the length of the club; pronotum about 1.4 times as long as wide and weakly constricted at base, disc with fine, dense, non-setiferous punctures and an elongate longitudinal median groove; elytra with nearly complete sutural striae, discal striae absent; hind wings well developed; abdominal ventrite I distinctly longer than ventrite II.
Occurrence
Baltic amber, middle Eocene (ca. 37–54.5 Ma; Ritzkowsky, Reference Ritzkowski1997).
Description
Body 0.90 mm long. Shape subcylindrical, dorsoventrally compressed, narrowly elongate (Fig. 1). Body color dark brown; surface mostly shiny with dorsal and ventral surfaces punctate and rugose microsculpture present on dorsal surfaces of head and pronotum.
Head oval, slightly deflexed, 0.18 mm long, broadest across eyes, 0.15 mm wide, distinctly constricted behind eyes with well-developed temples; dorsally covered with four longitudinally aligned rows of small non-setiferous punctures (Fig. 4.1); ridge on vertex, basal fovea absent. Labrum small, deflexed. Mandibles small. Maxillary palps with apical palpomere aciculate, penultimate palpomere larger, swollen. Gular sutures not visible. Eyes large, laterally protruding; their diameter slightly longer than the distance from their anterior margin to antennal insertion; consisting of about 15 ommatidia. Antenna with 10 antennomeres; basal antennomere and antennal insertion concealed from above; antennomere 2 elongate, distinctly wider than antennomere 3, about two times as along as antennomere 3; antennomeres 3–8 moniliform; club 1-segmented with distinct sensory area about half the length of the club and demarked by a visible rim.
Figure 4 Enlargements of Derolathrus groehni. (1) Head and pronotum of holotype, showing the longitudinal median pronotal depression; (2) same as (1), showing pronotal depression; (3) left hind wing of holotype, showing setae surrounding its distal and posterior margins; (4) pro- and mesothorax of paratype, showing separated pro- and mesocoxae and well-developed prosternal and mesoventral processes; (5) abdomen of holotype, ventral view, showing elongate abdominal ventrite 1. pg, pronotal groove; v, ventrite. Scale bars represent 100 µm.
Prothorax as wide as head, 0.22 mm long and 0.16 mm wide, about 1.4 times as long as wide, narrower than elytra, widest in anterior third, tapering weakly to hind margin, which is narrower than anterior margin and weakly constricted at base; dorso-lateral edge with complete carina; pronotum covered with a few short setae; disc with shallow longitudinal median groove (Fig. 4.1, 4.2); prosternum broad, weakly punctate; prosternal process long, with rounded apex slightly extending beyond globular. Mesoventrite densely punctate, process broad, apex rounded between mesocoxal cavities (Fig. 4.4). Mesocoxae globular, slightly closer than procoxae. Metaventrite about 3.5 times as long as mesoventrite, slightly convex, without discrimen. Metacoxae widely separated. Trochanters large, trapezoidal, gradually widened to apex. Femora slender and robust. Tibiae gradually widened to apex, apical spurs not visible. First and second tarsomeres short, minute; third tarsomeres elongate, widened to apex, longer than the first two combined. Pretarsal claws long. Mesoscutellum not visible.
Elytra 0.52 mm long, broadest nearly at middle, each 0.11 mm wide, apices rounded; elytral striae present only as nearly complete sutural stria (discal striae absent), originating from pit near inner apical margin; surface with short setae in vague longitudinal rows; elytral epipleura, narrow, long, ending at middle of ventrite 3. Hind wings (Fig. 4.3) large, venation highly reduced; ventral margin and apex with about 35 long hairs.
Abdominal ventrites I and V longer than ventrites II–IV (Fig. 4.5); ventrite I not connate with metaventrite, with suture distinct, about two times as long as ventrite II; ventrites 2–4 almost of same length; surface with a few short setae. Female genitalia with elongate coxites and styli.
Etymology
The specific epithet is a patronym formed from the surname of Carsten Gröhn, recognizing his kind support of our study.
Remarks
Derolathrus groehni is easily distinguished from the micropththalmic (small-eyed) and apterous species D. anophthalmus Franz (Canary Islands) by the well-developed eyes and presence of hind wings. Derolathrus groehni is easily separated from the D. parvulus Rücker (Madeira) and D. troglophilus Sen Gupta (Fiji) by the well-developed eyes with about 15 ommatidia each; from D. atomus (Hawai’i) and D. ceylonicus Sen Gupta (Sri Lanka) by having a well-developed longitudinal median groove (or depression) on the pronotum; from D. sharpi Grouvelle (Guadeloupe) by the absence of prothoracic dorso-lateral ridges and the presence of wrinkled microsculptures on pronotum; and from D. insularis Dajoz (Mauritius) by the presence of a well-developed mesocoxal process and a remarkably long abdominal ventrite I. The new species is very similar to the recently described species D. cavernicolus Peck; they both have relatively large eyes and a median longitudinal depression on the pronotum. However, D. groehni can be distinguished from D. cavernicolus by the shape of the pronotum, the free abdominal ventrite I which is not fused to the metaventrite and details of the punctation (absence of densely scattered large setiferous punctures on head) and form of the median pronotal groove (in D. groehni the groove is longitudinal and straight whereas it is Y-shaped or subtriangular in D. cavernicolus). In addition, Lawrence and Leschen (Reference Lawrence and Leschen2010) mentioned an unnamed Derolathrus species from New Zealand that has a basal fovea on the vertex of the head but also differs from D. groehni by having smaller eyes, having a shorter prothorax that lacks a median longitudinal groove on the pronotum, and a much shorter abdominal ventrite I.
Discussion
These fossil beetles are easily placed in the family Jacobsoniidae based on their distinctive small body size, elongate body shape, and more importantly, the greatly elongated metathorax combined with the relatively short abdomen and extremely slender metanepisterna, which are potential synapomorphies of Jacobsoniidae (Lawrence and Leschen, Reference Lawrence and Leschen2010). Another possible jacobsoniid synapomorphy (Lawrence and Leschen, Reference Lawrence and Leschen2010) is the wing structure with reduced venation and very long fringe hairs (as seen in the holotype of the new species), a feature also present in some staphylinoids (Crowson, Reference Crowson1960) to which jacobsoniids may be related (see Introduction). Among the three known jacobsoniid genera (Derolathrus, Sarothrias, and Saphophagus), the new fossil species can be readily excluded from Saphophagus by the three-segmented tarsi (tarsal formula 5–5–5 in Saphophagus) and abdomen with five free ventrites (basal two ventrites connate in Saphophagus); from Sarothrias by the well separated and broadly open procoxal cavities (contiguous and closed in Sarothrias) and the presence of a distinct antennal club (the club is not distinct in Sarothrias; e.g., Ślipiński and Löbl, Reference Ślipiński and Löbl1995). These Eocene fossils can be attributed to the extant genus Derolathrus by the following combination of characters: body minute (less than 1 mm in length) and narrowly elongate, prothorax elongate and posteriorly narrowed, mesoscutellum not visible, tarsal formula 3–3–3, and with a highly distinctive metaventrite as long as all five visible abdominal ventrites combined (Peck, Reference Peck2010).
Members of Derolathrus have been collected in various parts of the world, but are particularly common in warmer areas (Lawrence and Leschen, Reference Lawrence and Leschen2010; Peck, Reference Peck2010) with the only cool-temperate populations of the genus occurring in New Zealand (ranging from the northern portion of the North Island southward to Codfish Island; Leschen, personal observation). Like most modern Derolathrus species, D. groehni probably lived in a warm climate, since many other thermophilic insects are present in Baltic amber, including Embiodea, and warm-adapted lineages of other groups (e.g., Mantodea, Phasmatodea, etc.; Archibald and Farrell, Reference Archibald and Farrell2003).
The widespread distributions of at least two species of Derolathrus have been attributed to either natural dispersal or accidental introductions (Peck, Reference Peck2010). The hind wings of D. groehni and other flight-capable Derolathrus with a reduced surface area and a fringe of setae are like those that have repeatedly evolved in other minute insects, including many beetles like ptiliid beetles that have a fossil history dating to the Cretaceous (e.g., Shockley and Greenwalt, Reference Shockley and Greenwalt2013). Instead of flapping their wings, these “featherwinged” insects may be “rowing” their wings, which is an adaptation to flight at low Reynolds numbers (Walker, Reference Walker2002). The wing morphology implies that D. groehni was also probably capable of long-distance dispersal and might have been widely distributed in the Eocene. Meanwhile, a detailed comparative study of Derolathrus may help to determine more about the biology of the group, especially with regard to dispersal capability and how these minute beetles are able to colonize oceanic islands.
To date, no fossil representatives of Jacobsoniidae are known. Our discovery of a new Derolathrus species from 37–54.5 million years ago represents the sole fossil record for this small family, providing information about the morphological evolution of the group. It not only extends the geographical distribution of the Recent genus Derolathrus, but also will serve as a valuable calibration reference for future DNA-based phylogenetic studies.
Acknowledgments
We are grateful to four anonymous reviewers, associate editor S. Chatzimanolis, and editor S.J. Hageman for comments about the manuscript. Financial support was provided by the National Basic Research Program of China (2012CB821903), the National Natural Science Foundation of China (91114201, 41222013, and 31172047), and the Outstanding Youth Foundation of Jiangsu Province (BK2012049). R.A.B.L. was supported in part by Core funding for Crown Research Institutes from the Ministry of Business, Innovation and Employment’s Science and Innovation Group.
