Mary T. Silcox

New Paleocene skeletons and the relationship of plesiadapiforms to crown-clade primates

Plesiadapiforms are central to studies of the origin and evolution of primates and other euarchontan mammals (tree shrews and flying lemurs). We report results from a comprehensive cladistic analysis using cranial, postcranial, and dental evidence including data from recently discovered Paleocene plesiadapiform skeletons (Ignacius clarkforkensis sp. nov.; Dryomomys szalayi, gen. et sp. nov.), and the most plesiomorphic extant tree shrew, Ptilocercus lowii. Our results, based on the fossil record, unambiguously place plesiadapiforms with Euprimates and indicate that the divergence of Primates (sensu lato) from other euarchontans likely occurred before or just after the Cretaceous/Tertiary boundary (65 Mya), notably later than logistical model and molecular estimates. Anatomical features associated with specialized pedal grasping (including a nail on the hallux) and a petrosal bulla likely evolved in the common ancestor of Plesiadapoidea and Euprimates (Euprimateformes) by 62 Mya in either Asia or North America. Our results are consistent with those from recent molecular analyses that group Dermoptera with Scandentia. We find no evidence to support the hypothesis that any plesiadapiforms were mitten-gliders or closely related to Dermoptera.

The primate semicircular canal system and locomotion.

The semicircular canal system of vertebrates helps coordinate body movements, including stabilization of gaze during locomotion. Quantitative phylogenetically informed analysis of the radius of curvature of the three semicircular canals in 91 extant and recently extinct primate species and 119 other mammalian taxa provide support for the hypothesis that canal size varies in relation to the jerkiness of head motion during locomotion. Primate and other mammalian species studied here that are agile and have fast, jerky locomotion have significantly larger canals relative to body mass than those that move more cautiously.

Virtual endocast of Ignacius graybullianus (Paromomyidae, Primates) and brain evolution in early primates

Extant primates are distinctive among mammals in having relatively large brains. As stem primates, Paleogene plesiadapiforms provide direct information relevant to the earliest stages in the evolution of this characteristic. Here we describe a virtual endocast reconstructed from ultra high resolution X-ray computed tomography data for the paromomyid plesiadapiform Ignacius graybullianus (USNM 421608) from the early Eocene of Wyoming. This represents the most complete endocast known for a stem primate, allowing for an unprecedented study of both size and fine details of anatomy. Relative to fossil and extant euprimates, I. graybullianus had large olfactory lobes, but less caudal development of the cerebrum and a poorly demarcated temporal lobe, suggesting more emphasis on olfaction and a less well developed visual system. Although its brain was small compared to those of extant primates, the encephalization quotient of I. graybullianus is higher than that calculated for Paleocene Plesiadapis cookei and overlaps the lower portion of the range documented for fossil euprimates. Comparison to the basal gliroid Rhombomylus suggests that early primates exhibited some expansion of the cerebrum compared to their ancestors. The relatively small brain size of I. graybullianus, an arboreal frugivore, implies that neither arboreality nor frugivory was primarily responsible for the expanded brains of modern primates. However, the contrasts in features related to the visual system between I. graybullianus and fossil and extant euprimates suggest that improvements to these portions of the brain contributed to increases in brain size within Euprimates.

Endocasts of Microsyops (Microsyopidae, Primates) and the evolution of the brain in primitive primates

We describe a virtual endocast produced from ultra high resolution X-ray computed tomography (CT) data for the microsyopid, Microsyops annectens (middle Eocene, Wyoming). It is the most complete and least distorted endocast known for a plesiadapiform primate and because of the relatively basal position of Microsyopidae, has particular importance to reconstructing primitive characteristics for Primates. Cranial capacity is estimated at 5.9 cm(3), yielding encephalization quotients (EQ) of 0.26-0.39 (Jerisons equation) and 0.32-0.52 (Eisenbergs equation), depending on the body mass estimate. Even the lowest EQ estimate for M. annectens is higher than that for Plesiadapis cookei, while the range of estimates overlaps with that of Ignacius graybullianus and with the lower end of the range of estimates for fossil euprimates. As in other plesiadapiforms, the olfactory bulbs of M. annectens are large. The cerebrum does not extend onto the cerebellum or form a ventrally protruding temporal lobe with a clear temporal pole, suggesting less development of the visual sense and a greater emphasis on olfaction than in euprimates. Contrasts between the virtual endocast of M. annectens, and both a natural endocast of the same species and a partial endocast from the earlier-occurring Microsyops sp., cf. Microsyops elegans, suggest that the coverage of the caudal colliculi by the cerebrum evolved within the Microsyops lineage. This implies that microsyopids expanded their cerebra and perhaps evolved an improved visual sense independent of euprimates. With a growing body of data on the morphology of the brain in primitive primates, it is becoming clear that many of the characteristics of the brain common to euprimates evolved after the divergence of stem primates from other euarchontans and likely in parallel in different lineages. These new data suggest a different model for the ancestors of euprimates than has been assumed based on the anatomy of the brain in visually specialized diurnal tree shrews.

THE DIET OF WORMS: AN ANALYSIS OF MOLE DENTAL MICROWEAR

Abstract We compared microwear from shearing facets of lower molars from Parascalops breweri (the hairy-tailed mole) and Scapanus orarius (the coast mole) with that from other small mammal species including a tenrec, a hedgehog, 3 primates, and 2 bats. The 2 mole species exhibit a distinct microwear pattern that is characterized by many short, narrow scratches, and relatively few pits. Although the molars of the streaked tenrec (Hemicentetes nigriceps) differ profoundly in morphology from those of moles, they show a very similar pattern of microwear on their shearing facets. This common pattern (missing in the rest of the comparison sample) is likely a product of the importance of earthworms in the diets of both H. nigriceps and the moles and is plausibly explained by the interaction between teeth and soil from the inside and outside of earthworms. These results may be useful in interpreting microwear patterns in fossil mammals.

New discoveries on the middle ear anatomy of Ignacius graybullianus (Paromomyidae, Primates) from ultra high resolution X-ray computed tomography.

A skull of Ignacius graybullianus (USNM 421608) was studied using ultra high resolution X-ray computed tomography (uhrCT). The anatomy of the middle ear in this specimen was previously studied through partial removal of the auditory bulla on one side. The data now available allow for examination of the others unprepared ear, which is more completely preserved, as well as adding to the information available about the previously studied ear. Analysis of the relationships between the bones making up the auditory bulla confirms previous assertions that it is formed from the entotympanic, and not from the petrosal, basioccipital, or basisphenoid. Contrary to previous reconstructions of the middle ear anatomy in all known plesiadapiforms, this specimen exhibits a bony canal for the promontorial artery and/or internal carotid nerves running across the lateral extreme of the promontorium. The identification of this structure is confirmed by the clear presence of a lumen, and its origination at a posterior carotid foramen (pcf) in a position that corresponds to that identified in previous studies of the paromomyid basicranium (Am. J. Phys. Anthropol., 36 (1972) 59, Am. J. Phys. Anthropol., 89 (1992) 477). Remnants of this canal are present bilaterally in USNM 421608, which additionally supports its identification. The presence of bony canals for branches of the internal carotid artery and the internal carotid nerves is a feature seen in scandentians and euprimates that is missing in dermopterans. The unusual lateral route followed by the internal carotid nerves is a primitive euprimate feature missing in all other archontans. As such, this evidence is consistent with a close euprimate-paromomyid relationship, and the inclusion of the latter in the order Primates. The discovery of this feature in paromomyids after almost 30 years of study of the ear region of this family acts as a cautionary note to the interpretation of the middle ear in damaged specimens.

Primate Taxonomy, Plesiadapiforms, and Approaches to Primate Origins

In biology, there is currently a debate being waged about the basic principles of doing taxonomy (e.g., Benton, 2000; Cantino and De Queiroz, 2000; De Queiroz, 1994, 1997; De Queiroz and Gauthier 1990, 1992, 1994; Lee, 1996; Liden and Oxelman, 1996; Liden et al., 1997; Moore, 1998; Nixon and Carpenter, 2000; Pennisi, 1996; Schander and Thollesson, 1995). This debate stems from the common opinion that taxonomy should reflect evolution in some manner, combined with a disagreement about the practical details of how to do this. Although some authors have provided suggestions for making the Linnean system of taxonomy work within the context of a cladistic approach to phylogeny reconstruction (e.g., McKenna and Bell, 1997; Nixon and Carpenter, 2000; Wiley, 1981), others have advocated scrapping the entire Linnean system (De Queiroz, 1994; De Queiroz and Gauthier, 1990, 1992, 1994; Griffiths, 1976), culminating in the dissemination

Evolution of locomotion in Anthropoidea: the semicircular canal evidence

Our understanding of locomotor evolution in anthropoid primates has been limited to those taxa for which good postcranial fossil material and appropriate modern analogues are available. We report the results of an analysis of semicircular canal size variation in 16 fossil anthropoid species dating from the Late Eocene to the Late Miocene, and use these data to reconstruct evolutionary changes in locomotor adaptations in anthropoid primates over the last 35 Ma. Phylogenetically informed regression analyses of semicircular canal size reveal three important aspects of anthropoid locomotor evolution: (i) the earliest anthropoid primates engaged in relatively slow locomotor behaviours, suggesting that this was the basal anthropoid pattern; (ii) platyrrhines from the Miocene of South America were relatively agile compared with earlier anthropoids; and (iii) while the last common ancestor of cercopithecoids and hominoids likely was relatively slow like earlier stem catarrhines, the results suggest that the basal crown catarrhine may have been a relatively agile animal. The latter scenario would indicate that hominoids of the later Miocene secondarily derived their relatively slow locomotor repertoires.

Quantification of neocortical ratios in stem primates

Extant euprimates (=crown primates) have a characteristically expanded neocortical region of the brain relative to that of other mammals, but the timing of that expansion in their evolutionary history is poorly resolved. Examination of anatomical landmarks on fossil endocasts of Eocene euprimates suggests that significant neocortical expansion relative to contemporaneous mammals was already underway. Here, we provide quantitative estimates of neocorticalization in stem primates (plesiadapiforms) relevant to the question of whether relative neocortical expansion was uniquely characteristic of the crown primate radiation. Ratios of neocortex to endocast surface areas were calculated for plesiadapiforms using measurements from virtual endocasts of the paromomyid Ignacius graybullianus (early Eocene, Wyoming) and the microsyopid Microsyops annectens (middle Eocene, Wyoming). These data are similar to a published estimate for the plesiadapid, Plesiadapis tricuspidens, but contrast with those calculated for early Tertiary euprimates in being within the 95% confidence intervals for archaic mammals generally. Interpretation of these values is complicated by the paucity of sampled endocasts for older stem primates and euarchontogliran outgroups, as well as by a combination of effects related to temporal trends, allometry, and taxon-unique specializations. Regardless, these results are consistent with the hypothesis that a shift in brain organization occurred in the first euprimates, likely in association with elaborations to the visual system.

Cranial Dimensions as Estimators of Body Mass and Locomotor Habits in Extant and Fossil Rodents

ABSTRACT Estimating body mass and locomotor habits of fossil rodents is challenging for taxa without available postcranial material. Although equations exist to estimate body mass from dental dimensions based on extant rodents, the applicability of such equations is doubtful given that modern rodents have evolved highly specialized teeth, whereas fossil taxa often exhibit a much less derived condition. For the present study, 11 cranial variables from a sample of 203 extant rodents of known body mass representing a range of taxonomic groups were assessed using reduced major axis (RMA) regression. The results show a strong correlation between body mass and each of the 11 cranial variables. The best estimators for body mass are skull length and cheek-tooth area, whereas the least reliable measures are palate length, and foramen magnum, as well as braincase and occipital condyle dimensions. We estimated body mass for specimens of five fossil Ischyromyidae rodents for which body mass had never been estimated (Paramys copei, P. delicatus, Reithroparamys delicatissimus, Rapamys atramontis, and Ischyromys typus). Principal components and canonical variates analyses based on 10 cranial dimensions for 103 members of Sciuromorpha demonstrate that a relationship exists between locomotor habits and cranial variables in this suborder. In these analyses, early ischyromyids are all placed in the terrestrial group with Aplodontia, Marmota, Cynomys, and Spermophilus. This contradicts previous hypotheses regarding early rodent locomotion, which suggested that they were arboreal or more generalized in their habits.