K
ENNETHD. R
OSE11Center for Functional Anatomy & Evolution, Johns Hopkins University School of Medicine, Baltimore, Maryland 21093, USA;
kdrose@jhmi.edu.
The Messel Pit Fossil Site has produced an extraordinary vertebrate fauna of 132 species, including at least 44 species of mammals in 31 genera (Morlo et al., 2004). Virtually all of the mam-malian species are represented by complete or nearly complete articulated skeletons, providing a unique window on the anatomical structure and paleobiology of Eocene mammals. No other site in the world has provided such an extensive and diverse database of Eocene mammal skeletons. The penecontempora-neous coals of Geiseltal near Halle have produced some specimens comparable to those from Messel (e.g., Thalmann, 1994; Erfurt, 2000), but skeletons are less numerous and the Geiseltal fauna is much less well documented.
Similar preservation to that at Messel is known outside of Europe from the late Wasatchian Fossil Butte Member of the Green River Formation in southwestern Wyoming, but only a small number of mammalian taxa and skeletons are known (eight species, most of which are represented by only one or two specimens; e.g., Koenigwald et al., 2005a; Rose and Koenigswald, 2005; Simmons et al., 2008;
Grande, in press). Nevertheless, at least half of the Green River taxa have close relatives at Messel, prompting comparisons. The late early-middle Eocene Bridger Formation of Wyoming, approximately coeval with Messel, has produced a large and diverse
mammalian fauna, with relatively complete skeletal material known for many genera (e.g., Matthew, 1909; Gregory, 1920; Simpson, 1931; Gunnell and Gingerich, 1993). Here, too, many genera are closely related to Messel taxa. The Willwood Formation of the Bighorn Basin, Wyoming, spanning nearly all of the Wasatchian, has produced the largest and most di-verse samples of early Eocene mammals from any-where. The close faunal similarity to early Eocene European mammal faunas has long been known (e.g., McKenna, 1975). As McKenna noted, the North Atlantic connection between North America and Eu-rope was disrupted by the end of the early Eocene;
consequently, one might expect the resemblance between Willwood and Messel faunas to be as great as, or greater than, that between Bridger and Messel faunas. Nearly half of all Messel genera have closely related counterparts from the Willwood and Bridger formations.
Eocene Asian faunas generally show less simi-larity to the Messel fauna, probably because the Obik Sea and Turgai Strait presented a formidable barrier to dispersal between Europe and Asia. Even so, at least a quarter of Messel genera have close relatives in the recently discovered early Eocene Vastan local fauna from Gujarat, India (e.g., Rose et al., 2006), indicating that some degree of faunal exchange between India and western Europe took place in the
early or middle Eocene. Surprisingly, the Vastan fauna shows closer resemblance to contempora-neous western European and North American faunas than it does to those from Asia (e.g., Rana et al., 2008; Kumar et al., 2010).
The diversity of mammals at Messel, belonging to 13 different orders, allows broad comparisons with close relatives from other faunas. For many Eocene taxa the Messel fossils provide the best or only view of the complete animal (e.g., Macrocranion, various rodents, marsupials). In other cases Messel skele-tons offer important insight into variation within genera or families (e.g., apatemyids, artiodactyls, leptictidans, pantolestids), or provide critical stages for understanding evolutionary transitions (Pholidota, Euprimates). I review here the role of certain Messel taxa in our understanding of other important Eocene faunas.
Bats are by far the most common mammals at Messel, but they are rare in most other Eocene deposits, most species being represented by only jaws or isolated teeth. Skeletons of related bats are known from the Fossil Butte Member (e.g., Simmons and Geisler, 1998; Simmons et al., 2008). The most diverse early Eocene bat fauna, consisting of jaws, teeth, and isolated bones, unexpectedly comes from Vastan and includes two genera in common with Messel and several other closely related taxa (Smith et al., 2007). Knowledge of the paleoecology of Mes-sel bats (e.g., Habersetzer and Storch, 1989) enhances our understanding of these early bat as-semblages.
The two genera of marsupials from Messel show that the herpetotheriid Amphiperatherium was more e e ial han he a bo eall adap ed Peradectes (Koenigswald and Storch, 1992; Kurz, 2001). This ecomorphological distinction has allowed tentative allocation of isolated tarsals from the Willwood For-mation to Herpetotherium and Peradectes. Similarly, complete skeletons of the amphilemurid eulipo-typhlan Macrocranion (e.g., Storch, 1993) have enabled identification of very similar isolated ele-ments from Willwood quarry samples in which Macrocranion i common. One of Me el mo curious denizens was the leptictidan Leptictidium, whose three species are characterized by elongate hind limbs and very short forelimbs (e.g., Storch and Lister, 1985). Dentally, and in many postcranial aspects (but not the separate tibia and fibula), Leptictidium is more derived than any leptictid. But all Eocene and Oligocene leptictids had a fused tibia-fibula and longer metatarsals than in Leptictidium (Rose, 1999a). Moreover, the tibia and fibula are already fused in the oldest known Paleocene leptictids, indicating that Leptictidium must have diverged from leptictids not later than the early Paleocene (Rose, 2006).
Two obscure groups now usually considered to be cimolestan derivatives or placed in their own orders
apatemyids and pantolestids are known from exquisite skeletons from both Messel and the Fossil Butte Member (e.g., Koenigswald et al., 2005b). In these examples of closely allied genera, we have the opportunity to examine intrafamilial anatomical diversity at levels of detail rarely possible. Both Heterohyus from Messel and Apatemys from Fossil Butte possess arboreally adapted skeletons with an unusually specialized manus: the central digits are disproportionately long, as in extant Daubentonia and Dactylopsila an adaptation for probing for wood-boring insects (Koenigswald, 1990; Koenigswald et al., 2005a). But in both apatemyids the elongate digits are the second and third, not the third and fourth as in the two unrelated living taxa, and they are distinctly longer in Heterohyus than in Apatemys (Fig. 1). Thus we can infer that apatemyids preyed on wood-boring insects long before Daubentonia did, but in a similar manner, and that Heterohyus was more specialized for this role than was Apatemys. Quite a different niche was occupied by the otter-like
Figure 2. Dichobunoid artiodactyls: A. Diacodexis metsiacus from the Willwood Formation; B. Aumelasia cf. gabineaudi from Messel;
C. Messelobunodon schaeferi from Messel. A and B to same scale.
pantolestids Buxolestes (Messel), Palaeosinopa (Fossil Butte and Willwood), and Pantolestes (Bridger). All show very similar adaptations for semiaquatic and semifossorial habits, and they differ in few features except for the longer tail in Palaeosinopa (Rose and Koenigswald, 2005), under-scoring their close relationship. Skeletons of both Buxolestes and Palaeosinopa even contain fish re-mains in the abdomen.
The dichobunoid artiodactyls Messelobunodon and Aumelasia are very similar in postcranial anatomy to early Eocene Diacodexis (Fig. 2), the oldest known artiodactyl (Franzen, 1983, 1988;
Rose, 1985). All have a rabbit-like, cursorially specialized skeleton with relatively short forelimbs, long hind limbs, and especially long distal segments.
Both Messelobunodon and Diacodexis are plesio-morphic in retaining a femoral third trochanter, lost in nearly all other artiodactyls. Nevertheless, like virtu-ally all other early and middle Eocene artiodactyls, they resemble primitive ruminants postcranially more than they do other artiodactyls, suggesting either that they have a special relationship to ruminants or that small size may have constrained basal artiodactyls to this bauplan. Diacodexis was a widespread Holarctic genus, and recently discovered remains from the early Eocene of India show the same postcranial fea-tures as in North American Diacodexis and Messelobunodon (Kumar et al., 2010).
The adapoid primates from Messel, particularly Europolemur, bear striking similarities to North American Notharctus and Smilodectes (Bridger) and Cantius (Willwood). The Messel primates are dis-cussed by Gingerich (2011).
The last example included here is the basal pholidotan Eomanis, now known from several skele-tons from Messel (Storch, 1978, 2003). Numerous features now inextricably tie Eomanis to later
pholidotans, such as North American late Eocene Patriomanis (e.g., Emry, 2004). At the same time, other traits one in the dentary and several in the postcranial skeleton afford strong evidence that Eomanis, and likely Eurotamandua as well, are closely allied with (and probably derived from) palaeanodonts (e.g., Rose, 1999b; Gaudin et al., 2009). The fossorially adapted skeletons of Palaeanodon (Willwood), Metacheiromys (Bridger), Eomanis, and Eurotamandua share numerous de-rived features of the forelimb, in particular, that are similar in detail and unlike those of other fossorial mammals. Thus Eomanis provides a critical link between metacheiromyid palaeanodonts and later undoubted pholidotans.
By virtue of their exceptional preservation and completeness, the Messel mammals paint an unusually vivid picture of mammalian paleobiology in central Europe during the Lutetian. Equally important, they provide unique insights into the anatomy, paleo-biology, and phylogenetic relationships of related taxa from elsewhere in Laurasia during a critical period in the history of mammals.
ACKNOWLEDGMENTS
I am grateful to the many friends and colleagues who have shared information concerning the mammals mentioned here; Jens Franzen, Lance Grande, Gregg Gunnell, Wighart von Koenigswald, and Gerhard Storch have been especially generous. Images were provided by Wighart von Koenigswald and the Senckenberg Forschungsinstitut und Naturmuseum Frankfurt, Abteilung Paläoanthropologie und Messel-forschung. The U.S. National Science Foundation, National Geographic Society, and the Alexander von Humboldt Stiftung have supported different phases of my research on postcranial skeletons of Eocene mammals.
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