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Journal of Paleontology 89(5):762–767
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 meso- coxal 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 (2010) 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, 2010). Another possible jacobsoniid synapomorphy (Lawrence and Leschen, 2010) 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, 1960) 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 5–5–5in 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, 1995). These Eocene fossils can be attributed to the extant genus Derolathrus by the following combination of characters: body minute (less than 1mm 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, 2010). Members of Derolathrus have been collected in various
parts of the world, but are particularly common in warmer areas (Lawrence and Leschen, 2010; Peck, 2010) with the only cool- temperate populations of the genus occurring in New Zealand (ranging from the northern portion of the North Island south- ward 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 pre- sent in Baltic amber, including Embiodea, and warm-adapted lineages of other groups (e.g., Mantodea, Phasmatodea, etc.; Archibald and Farrell, 2003). The widespread distributions of at least two species of
Derolathrus have been attributed to either natural dispersal or accidental introductions (Peck, 2010). 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, 2013). Instead of flapping their wings, these “featherwinged” insects may be “rowing” their wings, which is an adaptation to flight at low Reynolds numbers (Walker, 2002). 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 morpho- logical 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 Foun- dation 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.
References
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Arnett, R.H. Jr., and Thomas, M.C., 2000, Family 78. Jacobsoniidae, in Arnett, R.H., Jr., ed., American Insects, a handbook of the insects of America north of Mexico, Second Edition. Boca Raton, CRC Press, p. 446–447.
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