Omomyid Primates Research Articles

An omomyid primate from the Pontide microcontinent of north-central Anatolia: Implications for sweepstakes dispersal of terrestrial mammals during the Eocene

A new genus and species of omomyid primate is described from the middle Eocene (Lutetian) Lülük Member of the Uzunçarşidere Formation, Orhaniye Basin, north-central Anatolia, Turkey. This is the first Eocene primate to be reported from the vast area between Switzerland and Pakistan. The new taxon is currently represented by a single dentary fragment, limiting the scope of morphological comparisons that can be made with related taxa. Nevertheless, its dentition differs fundamentally from that of contemporary European microchoerids. The new taxon most closely resembles North American middle Eocene omomyines such as Mytonius hopsoni, and it is therefore interpreted as a member of the Asian/North American omomyine radiation. Its occurrence on the Pontide microcontinent must have resulted from sweepstakes dispersal across the intervening Tethyan barrier that separated the Pontides from adjacent parts of Eurasia during the Lutetian. Sweepstakes dispersal by various terrestrial mammal clades, especially rodents and primates, was facilitated by Eocene greenhouse climatic conditions, which promoted extreme precipitation events and frequent flooding of major river drainages.

Modules and Mosaics in the Evolution of the Tetonius – Pseudotetonius Dentition

The Tetonius-Pseudotetonius (T-P) transition is an often-cited example of phyletic gradualism, but rates of evolution and the roles of neutral and adaptive processes across this lineage remain unclear. Linking Tetonius and Pseudotetonius, two omomyid primates, are a series of stratigraphic and morphologic intermediates revealing trends suggestive of a functional and developmental reorganization of the dentition. Notable changes involved the P3, which reduced size and became indistinguishable from the canine and I2, and the P4, which became a robust tall-cusped tooth comparable to a molar. We test two hypotheses: (1) neutral evolution can explain the observed phenotypic differences in the lineage, and (2) P4 lost developmental association with P3 and became integrated with the molars. First, we calculate the rate of evolutionary differentiation, based on the ratio between inter- and intra-species variation in length and width of P3, P4, M1, and M2 teeth, between lineage segments and over the entire lineage. We test the second hypothesis by comparing bivariate correlations between teeth within individual lineage segments. As the lineage evolved, correlations between P3 and the molars diminished, whereas correlations between P4 and the molars increased. We found evidence of varying degrees of stabilizing selection in the lengths and widths of all cheek teeth and evidence of neutral evolution in the width of P4. These results support a trend towards P4 becoming integrated into the molar morphogenetic field, and demonstrates that morphological rates of evolution, and consequently the degree of selective pressures, vary through time and between teeth.

The rodents from Santpedor-2 (Ebro Basin, NE Spain) confirm the Oligocene age of the latest primates from the Paleogene of Europe

This paper provides a revised and updated description and identification of the rodents from Santpedor-2 (northeastern Spain). The age of this locality is particularly relevant, because it is one of the few European sites assigned to the early Oligocene that has yielded primate remains. However, the rodent assemblage from this site has never been described in detail, and therefore the Oligocene age of this locality remained tentative. Despite the scarcity of the material, the rodents from Santpedor-2 are characteristic enough either to be identified as already known species, or at least compared with close species. Eight different rodent taxa were recognized. Among them, the theridomyid Ectropomys exiguus, which is recorded only in late Eocene localities, and the glirid Glamys devoogdi, which is known in both latest Eocene and early Oligocene localities. The other taxa are found only in Oligocene localities, and consist of the theridomyid Paratheridomys margaritae and members of two families that arrived in Europe at the beginning of the Oligocene: eomyids (Eomys sp.) and cricetids (Eucricetodon atavus, cf. Heterocricetodon sp., cf. Pseudocricetodon sp.). Finally, another theridomyid (Issiodoromyinae) seems to correspond to a new species and new genus, the main features of which correspond to early Oligocene evolutionary grades. This rodent assemblage from Santpedor-2 is assigned to the early Oligocene, mainly on the basis of the presence of immigrant rodents that reached Europe in relation with the Grande Coupure (cricetids and eomyids). This dating is notably supported by the presence of P. margaritae together with Glamys devoogdi, both being found in Hoogbutsel (early Rupelian, MP21). Only one rodent (E. exiguus) represents an Eocene relict, as is the case of Pseudoloris, the only primate recorded in this locality. Therefore, this study supports the Oligocene age of Santpedor-2 and then the persistence of omomyid primates in the Iberian Peninsula (at least locally) after the Grande Coupure.

Stratigraphy, mammalian paleontology, paleoecology, and age correlation of the Wasatch Formation, Fossil Butte National Monument, Wyoming

AbstractFieldwork conducted in the Wasatch Formation in and around Fossil Butte has yielded a diverse assemblage of early Eocene vertebrates. Fossil vertebrates are distributed through three discrete stratigraphic intervals within the uppermost 180 m of the main body of the Wasatch Formation underlying the Green River Formation. These assemblages were derived primarily from fluvial overbank mudstone units overprinted with variably well-developed paleosols. The lowest (20 m) and highest (60 m) sections are characterized by less mature and more hydromorphic paleosols, whereas the middle section (100 m) is typified by more mature paleosols and more abundant channel sandstones.The combined assemblages contain at least 46 species of mammals. Faunal characteristics include high abundances of equid perissodactyls and a relatively high abundance and diversity of notharctines primates, an apparent absence of omomyid primates, relatively high rodent diversity, and relatively diverse and abundant artiodactyls. One new genus (Eoictopsnew genus) and three new species (Eoictops novacekinew species,Palaeosinopa lacusnew species, and ?Notoparamys blochinew species) are included in the Fossil Butte assemblage. Also recorded are late occurrences of two hyopsodontid condylarths and an early occurrence of a rare phenacodontid condylarth. The relatively high abundances of equids and notharctines suggest that vertebrate samples were derived from relatively open paleohabitats that included forested areas along water courses.All three assemblages contain characteristic Lysitean (Wasatchian biochron Wa-6) elements, but the occurrence of the palaeotheriid perissodactylLambdotheriumin the uppermost horizon indicates a Lostcabinian (Wa-7) age for at least the top of the Wasatch Formation. The overlying predominantly fish-bearing Fossil Butte Member of the Green River Formation also containsLambdotheriumand is therefore Wa-7 in age as well.

A New Omomyid Primate from the Earliest Eocene of Southern England: First Phase of Microchoerine Evolution

A second species of the microchoerine omomyid genus Melaneremia, M. schrevei sp. nov. is described. It has been collected from the upper shelly clay unit of the Woolwich Formation, earliest Ypresian, Eocene, of Croydon, Greater London, UK. Phylogenetic analysis shows M. schrevei to be the most primitive member of the main clade of the Microchoerinae and demonstrates the initial dental evolution that separated this European subfamily from other omomyids. Calibration of the Woolwich upper shelly clay unit to the later part of the Paleocene—Eocene Thermal Maximum shows that speciation leading to the Microchoerinae took place within 170 ky of the beginning of the Eocene. Tentative identification of M. schrevei in the Conglomerat de Meudon of the Paris Basin suggests close time correlation with the upper part of the Woolwich Formation.

Skull of the Eocene primate Omomys carteri from western North America

ABSTRACT The description of a skull of the omomyid primate Omomys carteri from the middle Eocene Bridger Formation of southwestern Wyoming provides important new information relevant to our understanding of Eocene primate systematics and evolution. More specifically, Omomys carteri exhibits a long and wide nasal region and lacks the forward orientation of the orbits found in other omomyid primates. Furthermore, other features such as an unreduced cranial arterial system, primitive auditory bulla, expanded internal choanae, and an unreduced canine indicate that the cranium of Omomys carteri is primitive. Phylogenetic analysis of 52 cranial, 194 dental, and 56 postcranial characters, including the new characteristics of Omomys carteri discussed here, suggests that omomyiforms belong to a monophyletic tarsiiform clade exclusive of anthropoids. Anthropoids are still viewed as a sister group to tarsiiforms, forming a haplorhine clade, with an uncertain position for the problematic fossil primate Rooneyia. The ...

Additional postcranial remains of omomyid primates from the Uinta Formation, Utah and implications for the locomotor behavior of large-bodied omomyids

Most omomyids are relatively small bodied (e.g. <500 g), but beginning in the middle Eocene, some omomyids began to grow larger. The largest omomyids occur in the late middle Eocene during the Uintan NALMA, reaching an estimated body mass over 1 kg. The hind limb skeleton of small omomyids is relatively well known, and is generally thought to show active arboreal quadrupedal and leaping adaptations. New postcranial specimens of previously unknown elements from the larger Uintan omomyids, Ourayia (two species), Chipetaia lamporea, and Mytonius hopsoni have recently been recovered from the Uinta Formation, Utah, and from the Mission Valley Formation, California, and they provide additional information concerning their locomotor behavior. The new specimens include several distal tibiae, partial calcanei, a complete talus and a proximal first metatarsal of Chipetaia; distal femora, distal tibiae, cuboids, and partial calcanei of Ourayia uintensis; a complete calcaneus of Ourayia sp.; and a partial calcaneus and talus of Mytonius. Metric analysis of these elements, together with qualitative observations of non-metric traits, indicate that Ourayia and Chipetaia show equal or greater development of traits associated with leaping behavior (including elongation of the calcaneus, navicular and cuboid) than do smaller omomyids from North America. The elements of Mytonius, although fragmentary, lack some leaping features that are well-developed in Ourayia and Chipetaia, suggesting that Mytonius may have relied more on arboreal quadrupedal locomotion than on leaping.

Paleocommunity reconstruction and accumulation of micromammalian remains (late Eocene, southern England)

A taphonomic study was conducted on disarticulated micromammalian bones and isolated teeth from a late Eocene (Priabonian) assemblage in the Headon Hill Formation (Solent Group, Hampshire Basin, Isle of Wight, United Kingdom). The aim was to understand accumulating mechanisms and additional postmortem agents that influenced and potentially biased the assemblage. Skeletal elements (N 5 4296) belong to three marsupials, two glirid and two theridomyid rodents, two chiropterans, three nyctitheres, two omomyid primates, one pantolestid, and one apatothere. Surface modifications (i.e., etching, breakage, splitting, and impact marks) occur on bone, enamel, and dentine. Qualitative and quantitative analyses of surface modifications of each individual taxon resulted in recognition of several taphonomic agents. Predation of two theridomyid and at least two marsupial species by the mammalian carnivore Cynodictis cf. lacustris (also present in the fauna) is indicated by high proportions of etched remains, broken bones, and puncture marks. Variations within the assemblage in proportions of modified specimens or numbers of individuals are interpreted as reflecting differences in cause of death, habits, locomotor adaptations, and predatorprey relationships. The rodents, marsupials, and probably the nyctitheres are judged to have been living close to the site of deposition, justifying their previous use for isotope-based paleoclimate studies and demonstrating their participation in the local paleocommunity.

Hemiacodon engardae, a new species of omomyid primate from the earliest Uintan Turtle Bluff Member of the Bridger Formation, southwestern Wyoming, USA

In this paper, we describe a new species of Hemiacodon known only from University of Colorado Museum Loc. 92189 (Donna's Locality) in the Turtle Bluff Member of the Bridger Formation, Green River Basin, southwestern Wyoming. Donna's locality has yielded a diverse mostly small-bodied mammalian assemblage of Bridgerian and first appearance Uintan mammalian taxa, as well as range-through taxa. Together with H. engardae sp. nov., the faunal assemblage from Donna's Locality and more recently discovered localities in the same stratigraphic interval provides the first conclusive paleontological evidence of an earliest Uintan age (Ui1A biochron) for the Turtle Bluff Member of the Bridger Formation. The new species is represented by a sample of 11 specimens consisting of well-preserved upper and lower premolars and lower molars. H. engardae is distinct from H. gracilis on the basis of overall larger size as well as a combination of features of the premolars and molars related to a greater development of shearing crests. This suggests that H. engardae may have incorporated more foliage into its diet than the Bridgerian species, H. gracilis.

A new clade of omomyid primates from the European Paleogene

ABSTRACT A new genus, Vectipithex, is erected in the subfamily Microchoerinae, family Omomyidae, for four species, of which one, the type species is new: V. smithorum sp. nov. ‘Protoadapis’ ulmensis is referred to Vectipithex and transferred from the family Adapidae. Two species of Nannopithex, ‘N.’ quaylei and ‘N.’ raabi are also referred to the new genus. Vectipithex has a geographic distribution in north and west Europe and a stratigraphic distribution from the early Middle Eocene (Lutetian) to latest Eocene (late Priabonian). Cladistic analysis shows that Vectipithex is sister group to a clade composed of Microchoerus, Necrolemur, Nannopithex filholi, and ‘N.’ zuccolae and that poorly known ‘N.’ humilidens is sister group to those two. The Microchoerinae are shown to have undergone a phase of rapid diversification in the first few million years of the Eocene.

A NEW MICROCHOERINE OMOMYID (PRIMATES, MAMMALIA) FROM THE ENGLISH EARLY EOCENE AND ITS PALAEOBIOGEOGRAPHICAL IMPLICATIONS

Abstract: A new genus and species of omomyid primate, Melaneremia bryanti, is described from the Early Eocene Blackheath Beds of Abbey Wood, London, UK. It shares unique derived characters with the European subfamily Microchoerinae and is its most primitive member. It is nevertheless more derived than the primitive omomyid Teilhardina belgica from the beginning of the European Eocene. Cladistic analysis shows that the Microchoerinae are sister group to a clade comprising subfamilies Omomyinae and Anaptomorphinae, but excluding Teilhardina belgica and T. asiatica, which are stem omomyids. The Mammalian Dispersal Event (MDE), which marks the beginning of the Eocene (55·8 Ma), saw the dispersal of primates, perissodactyls and artiodactyls into the Northern Hemisphere. At this time similar species of Teilhardina lived in Europe, Asia and North America. The Abbey Wood microchoerine lived about 1 million years later. It co‐occurs with non‐primate species identical or very similar to those that lived in North America. The latter were ground‐dwellers, whereas the microchoerine and others that show distinct differences from North American relatives were tree‐dwellers. Land‐bridges connected North America and Europe via Greenland at the beginning of the Eocene, but 2 million years later these had been severed by submarine rifting. North American species at Abbey Wood indicate that a land connection still remained at c. 55 Ma. However, the forest belt that must have been continuous during the MDE to allow tree‐dwellers to disperse between the continents is likely by this time to have been disrupted, perhaps by volcanic eruption.

Chronology of primate discoveries in Myanmar: influences on the anthropoid origins debate.

The history of primate paleontology in Asia is long and complex, beginning with the first discoveries of fossil primates on the Indian subcontinent in the early 1830's. The first Eocene mammals from Asia were collected in Myanmar and described in 1916, while the first primates, Pondaungia and Amphipithecus, were described in 1927 and 1937, respectively, both from the Pondaung Formation in Myanmar. For the next 60 years, these two Pondaung taxa remained as the only known Eocene primates from Myanmar and one of the few records of Eocene primates from all of Asia. Taxonomically, Pondaungia and Amphipithecus were linked with a number of different groups, including archaic, hoofed ungulates (condylarths), adapiform primates, omomyid primates, and anthropoids. While no consensus existed, Pondaungia and Amphipithecus were most commonly compared with anthropoids. Beginning in the late 1990s, new primates were discovered in Myanmar, including smaller-bodied forms such as Bahinia and Myanmarpithecus. Also, new and better specimens of the larger-bodied Pondaungia and Amphipithecus began to appear, including the first cranial and postcranial fragments. Evaluations based on these new specimens, especially the postcrania, indicate that the two larger-bodied Myanmar taxa are adapiform primates that show their closest affinities to North American notharctines. The smaller-bodied taxa remain enigmatic, but may share their closest affinities with North American and Asian omomyid primates and Asian Tarsius. None of the known Asian primate taxa appear closely related to African anthropoids, which suggests that true anthropoids did not reach Asia until the latest Oligocene or earliest Miocene. These facts make an Asian origin for Anthropoidea unlikely. Additional and earlier evidence from both Asia and Africa is required before the ultimate origin of anthropoids can be determined. It appears possible that true anthropoids were an ancient radiation that may have been part of a Gondwanan (southern hemisphere) community that is, at present, poorly sampled and little understood.

Omomyid primates (Tarsiiformes) from the Early Middle Eocene at South Pass, Greater Green River Basin, Wyoming

Recent fieldwork in the Gardnerbuttean (earliest Bridgerian) sediments along the northeastern edge of the Green River Basin at South Pass, Wyoming, has yielded a large and diverse sample of omomyid (tarsiiform) primates. This assemblage includes two species each of Artimonius gen. nov., Washakius, and Omomys, one species of Anaptomorphus, Trogolemur and Uintanius, and a new, primitive species of the rare omomyine genus,Utahia. Utahia is known elsewhere only from its type locality in the Uinta Basin and its phylogenetic position is poorly understood. Utahia carina sp. nov. allows for re-evaluation of the affinities of this genus relative to other omomyines. In most characters, such as a lesser degree of molar trigonid compression, more widely open talonid notches, and a lack of molar talonid crenulation, the new species is more primitive than U. kayi. The dental anatomy of U. carina also indicates that Utahia is morphologically intermediate between washakiins and omomyins, although the balance of anatomical features places Utahia as the sister taxon to a broadly defined "Ourayini" clade. Morphological similarity between U. carina, Loveina zephyri, and primitive Washakius suggests that while the omomyin and washakiin clades may have diverged by the middle Wasatchian, substantial morphological distinctions are first evidenced only in the early Bridgerian. This may be due either to a lack of appropriate faunal samples from older sediments, or, more likely, because ecological circumstances in the early Bridgerian favored omomyine diversification and subsequent replacement of previously occurring taxa. This hypothesis is further supported by the stratigraphic co-occurrence of U. carina, W. izetti, and a primitive variant of W. insignis at South Pass, a marginal area. Basin margins have been hypothesized to provide heterogeneous habitats conducive to the production of evolutionary innovation. Basin margin samples have also been cited as evidence that anaptomorphines were relegated to upland refugia as omomyine taxa began to appear in the later part of the early Eocene. Another possible explanation for the unusual co-occurrence of species at South Pass relates to fluctuating lake levels in the Green River Basin, which intermittently would have made lowland environments inhospitable for arboreal fauna. This would have created a situation whereby species which would normally be allopatric become sympatric at South Pass.

Omomyid primates (Tarsiiformes) from the Early Middle Eocene at South Pass, Greater Green River Basin, Wyoming

Omomyid primates (Tarsiiformes) from the Early Middle Eocene at South Pass, Greater Green River Basin, Wyoming

Wasatchian-Bridgerian (Eocene) paleoecology of the western interior of North America: changing paleoenvironments and taxonomic composition of omomyid (Tarsiiformes) primates.

Many changes in mammalian faunas occurred across the early (Wasatchian) to middle (Bridgerian) Eocene boundary as documented in the fossil record from the Western Interior of North America. One of the more striking changes took place within the tarsiiform primate family Omomyidae. In the early Eocene, omomyids were dominated, both in abundance and diversity, by the subfamily Anaptomorphinae. In the middle Eocene, the subfamily Omomyinae dominated in abundance, while both subfamilies were nearly equally diverse. Examination of a series of paleoecological indicators including leaf-margin analysis, cenogram analysis, ecological diversity analysis of trophic structure, the distribution and development of ancient soil horizons (paleosols), and the distribution of lacustrine and fluvial facies in the Bighorn and southern Green River basins of Wyoming reveals factors that may have influenced the composition of omomyid primates. Subtle but important changes occurred in paleoclimates with mean annual temperatures reaching Cenozoic maximums at the end of the Wasatchian into the early Bridgerian. Both land mammal ages were typified by subtropical, closed forested conditions, but the Bridgerian was probably more humid and wetter than the Wasatchian. Paleohabitats most commonly sampled in the Wasatchian of the Bighorn Basin are proximal and distal floodplains, while those of the Bridgerian in the southern Green River Basin are lake margins and proximal floodplains. Changes in paleoclimate may have triggered a wave of omomyine immigration near the end of the Wasatchian with omomyines entering into habitats previously occupied by anaptomorphines. Lake margin and proximal floodplain habitats are those most commonly occupied by omomyines in the Bridgerian with anaptomorphines being more common in basin margin and distal floodplain areas not commonly sampled. Omomyine immigration and sampling of differing paleohabitats are two possible explanations for the changes documented in omomyid diversity and abundance.

Comment on possible cut marks and taphonomic history of Senga 5A in the Western Rift Valley, Zaire

A new specimen of Dyseolemur pacificus Stock, 1934, represents the most complete dentition known for any North American omomyid primate. Crowns are preserved from each tooth position except I2 on either the right or left side, or both. The specimen comes from the upper part of the Santiago Formation near Oceanside, San Diego County, California, and is of middle Eocene (late Uintan) age. The crowns of the anterior teeth are tarsiiform in shape (they are high and pointed, tapering from base to tip). The size relationships among the anterior teeth differ from those of many omomyids in having relatively large canines and small incisors, a pattern that is similar to anthropoids and adapids. Despite this similarity, the crowns of these teeth in Dyseolemur are unmistakably tarsiiform in structure. The implications of these results are discussed in the light of recently published hypothetical phylogenies concerning the origin of anthropoids and the origin of modern Tarsius .

The dentition of Dyseolemur, and comments on the use of the anterior teeth in primate systematics

Abstract A new specimen of Dyseolemur pacificus Stock, 1934, represents the most complete dentition known for any North American omomyid primate. Crowns are preserved from each tooth position except I 2 on either the right or left side, or both. The specimen comes from the upper part of the Santiago Formation near Oceanside, San Diego County, California, and is of middle Eocene (late Uintan) age. The crowns of the anterior teeth are tarsiiform in shape (they are high and pointed, tapering from base to tip). The size relationships among the anterior teeth differ from those of many omomyids in having relatively large canines and small incisors, a pattern that is similar to anthropoids and adapids. Despite this similarity, the crowns of these teeth in Dyseolemur are unmistakably tarsiiform in structure. The implications of these results are discussed in the light of recently published hypothetical phylogenies concerning the origin of anthropoids and the origin of modern Tarsius .

Omomyid primates (Tarsiiformes) from the Bridger Formation, middle Eocene, southern Green River Basin, Wyoming

New specimens of omomyid primates from the Bridger Formation, middle Eocene, of southwestern Wyoming reveal the presence of one new genus and two new species of anaptomorphines and two new genera and species of omomyines. The new genus and species of anaptomorphine, Sphacorhysis burntforkensis, has affinities with Trogolemur and suggests new relationships within the tribe Trogolemurini. The new species of anaptomorphine, Gazinius bowni, shows that this rare genus was not necessarily restricted to basin margin faunas as previously thought. The new genera of omomyines, Wyomomys bridgeri and Ageitodendron matthewi, are both related to the late middle Eocene genus Ourayia, and help to clarify relationships within the Ourayiini, new tribe. Further analysis of new specimens reveals that Hemiacodon and Macrotarsius are closely related and should be included in a tribe, Macrotarsiini, distinct from other omomyids. Omomyines dominate the omomyid primate radiation both in diversity and total specimen number through most of the Bridgerian; anaptomorphines remain a rare, but persistent element of mammalian faunas through the Bridgerian.

New early Eocene anaptomorphine primate (Omomyidae) from the Washakie Basin, Wyoming, with comments on the phylogeny and paleobiology of anaptomorphines

Recent paleontological collecting in the Washakie Basin, southcentral Wyoming, has resulted in the recovery of over 100 specimens of omomyid primates from the lower Eocene Wasatch Formation. Much of what is known about anaptomorphine omomyids is based upon work in the Bighorn and Wind River Basins of Wyoming. This new sample documents greater taxonomic diversity of omomyids during the early Eocene and contributes to our understanding of the phylogeny and adaptations of some of these earliest North American primates. A new middle Wasatchian (Lysitean) anaptomorphine, Anemorhysis savagei, n. sp., is structurally intermediate between Teilhardina americana and other species of Anemorhysis and may be a sister group of other Anemorhysis and Trogolemur. Body size estimates for Anemorhysis, Tetonoides, Trogolemur, and Teilhardina americana indicate that these animals were extremely small, probably less than 50 grams. Analysis of relative shearing potential of lower molars of these taxa indicates that some were primarily insectivorous, some primarily frugivorous, and some may have been more mixed feeders. Anaptomorphines did not develop the extremes of molar specialization for frugivory or insectivory seen in extant prosimians. Incisor enlargement does not appear to be associated with specialization in either fruits or insects but may have been an adaptation for specialized grooming or food manipulation.

Revision of the Wind River faunas, Early Eocene of central Wyoming. Part 12. New species of omomyid Primates (Mammalia: Primates: Omomyidae) and omomyid taxonomic composition across the Early-Middle Eocene boundary

Revision of the Wind River faunas, Early Eocene of central Wyoming. Part 12. New species of omomyid Primates (Mammalia: Primates: Omomyidae) and omomyid taxonomic composition across the Early-Middle Eocene boundary