E. Christopher Kirk

Neocortical expansion is a key innovation of crown primates, although fossil evidence for how the neocortex has evolved remains elusive. We studied neocortical evolution using virtual brain endocasts from extant and Eocene-Miocene fossil primates. We found that neocortex size increased in parallel across crown primate clades, with rapid increases in tarsiers and anthropoids. Although the frontal lobe is commonly described as having expanded disproportionately in primates, we show that it evolved gradually, following a common pattern for allometric scaling. By contrast, nonfrontal neocortical regions underwent rapid increases relative to brain size in haplorhines. Neocortical expansion occurred in concert with increases in optic foramen area, suggesting that visual sensory inputs are functionally tied to modular evolution of the primate neocortex.

Objectives: This study examines brain evolution in early fossil anthropoids using virtual reconstructions of endocranial morphology.Materials and Methods: We estimated the size of the brain and the proportions of its components using new estimates of body mass and a broad sample of virtual endocasts of extant primates and Oligocene–mid‐Miocene anthropoids.Results: Brain size relative to body mass and brain proportions relative to brain size were evaluated. Compared with body mass, (1) Brain size enlargement has occurred convergently in many anthropoid lineages. (2) Tarsiers and anthropoids evolved smaller olfactory bulbs and larger neocortices than strepsirrhines. (3) Enlargement of tarsier and anthropoid non‐frontal regions (occipital, parietal, and temporal lobes), not of the frontal lobe, accounts for larger neocortex size. Considering brain proportions relative to brain size yields similar findings: (1) Tarsiers and extant and fossil anthropoids have smaller olfactory bulbs and larger neocortices than strepsirrhines. (2) Tarsier and anthropoid neocortical expansion occurred principally in regions where visual signals are processed. (3) Correspondingly, species with relatively more visual input also have larger non‐frontal neocortex.Discussion: Shifts in brain proportions found in the stem anthropoid Simonsius, and stem platyrrhines and catarrhines were established before brain enlargement. The neocortex was enlarged, with disproportionate expansion of regions where visual signals are processed, emphasizing the heightened importance of the haplorhine visual system. Depending on the phylogenetic position of omomyoid haplorhines, large shifts in neocorticalization either occurred separately in stem tarsiids and anthropoids or once in the stem lineage leading to tarsiids and anthropoids.

Objectives: Here we describe a well‐preserved primate frontal bone (TMM 46513‐1) recovered from Middle Eocene exposures of the Devil's Graveyard Formation in the Tornillo Basin of Texas. We also discuss the significance of frontal bone morphology for understanding early haplorhine phylogeny.Materials and Methods: TMM 46513‐1 was mechanically prepared, μCT scanned, and compared with a broad array of extant and fossil crown primates. Revised dates for the specimen were provided using ⁴⁰Ar/³⁹Ar geochronology. Dice‐CT scans of Saguinus were used to investigate the contents of bony canals found within the frontal bones of many euarchontans.Results: TMM 46513‐1 resembles Rooneyia viejaensis in several key features, including: (1) the presence of a ventral flange on the lateral process of the frontal that intervenes between the posterosuperior orbit and temporal fossa, (2) the presence of a bony canal system (“orbitofrontal canal”) within the frontal that connects a foramen on the posterior aspect of the lateral process (“lateral process foramen”) with the superomedial orbit and the anterior cranial fossa, (3) a metopic suture that is unfused at its rostral end but obliterated at its caudal end, and (4) an anterior cranial fossa that overlaps the posteromedial portions of the orbits.Conclusions: The lateral segment of the orbitofrontal canal and substantial ventral expansion of the lateral process are features restricted to the Haplorhini among crown primates. The morphological features shared by TMM 46513‐1 and Rooneyia suggest that they may be stem haplorhines, but are less consistent with the hypothesis that Rooneyia is a stem anthropoid.

Dominance and leverage are both possible causes of social inequality. If sexual dimorphism influences patterns of intersexual dominance, we predicted that highly dimorphic species are constrained to exhibit male-biased power (MP), but species with low sexual dimorphism are free to demonstrate a broader range of intersexual power relationships. If market effects influence intersexual leverage, we predicted that females have more power when group composition is more male-biased and estrus is asynchronous. We analyzed data on intersexual power, sexual dimorphism, expected estrous overlap, and sex ratio for 79 extant primate species using phylogenetic logistic regression and ancestral state reconstructions. Although MP is more common, every major primate clade includes non-MP species. MP was associated with greater body mass and canine length dimorphism and with female-biased sex-ratios. Low estrous overlap was associated with non-MP. Although MP was reconstructed as likely ancestral for anthropoids, the last common ancestor of this clade probably did not exhibit high sexual dimorphism. The last common ancestor of catarrhines was probably highly dimorphic, potentially constraining intersexual power relationships. Non-MP probably evolved multiple times in primates and may be less common because multiple traits are linked to MP while fewer traits are associated with female-biased power or equality.

The cranium of Adalatherium hui, as represented in the holotype and only specimen (UA 9030), is only the second known for any gondwanatherian mammal, the other being that of the sudamericid Vintana sertichi. Both Adalatherium and Vintana were recovered from the Upper Cretaceous (Maastrichtian) Maevarano Formation of northwestern Madagascar. UA 9030 is the most complete specimen of a gondwanatherian yet known and includes, in addition to the cranium, both lower jaws and a complete postcranial skeleton. Aside from Adalatherium and Vintana, gondwanatherians are otherwise represented only by isolated teeth and lower jaw fragments, belonging to eight monotypic genera from Late Cretaceous and Paleogene horizons of Madagascar, the Indian subcontinent, Africa, South America, and the Antarctic Peninsula. Although the anterior part of the cranium is very well preserved in UA 9030, the posterior part is not. Nonetheless, comparable parts of the crania of Adalatherium and Vintana indicate some level of common ancestry through possession of several synapomorphies, primarily related to the bony composition, articular relationships, and features of the snout region. Overprinted on this shared morphology are a host of autapomorphic features in each genus, some unique among mammaliaforms and some convergent upon therian mammals. The cranium of Adalatherium is compared with the crania of other mammaliamorphs, particularly those of allotherians or purported allotherians (i.e., haramiyidans, euharamiyidans, multituberculates, Cifelliodon, and Megaconus). Particular emphasis is placed on several recently described forms: the enigmatic Cifelliodon from the Early Cretaceous of Utah and several new taxa of euharamiyidans from the Late Jurassic of China.

The cochlear canal of Adalatherium hui, from the Late Cretaceous of Madagascar, exhibits a combination of features previously unknown among mammaliaforms. The cochlear canal of Adalatherium is short and ‘C’-shaped (curved through at least 210°). A presumptive lagenar nerve canal extends from the apex of the cochlea to the internal acoustic meatus. In conjunction with an apical expansion of the cochlear canal, this morphology suggests that Adalatherium retained a lagenar macula. Adalatherium is derived among mammaliaforms in possessing primary and secondary osseous laminae, a cribriform plate, and a distinct cochlear ganglion canal. Strikingly, Adalatherium shows a single-layered primary osseous lamina that lacks internal, radially oriented canaliculi for auditory nerve fibers. In this respect, the primary lamina of Adalatherium is structurally different from the double-layered primary lamina containing habenulae perforatae of extant therians. Furthermore, Adalatherium resembles the gondwanatherian Vintana in possessing a secondary canal running parallel to the cochlear ganglion canal. Although its exact role and homology are uncertain, the morphology of this secondary canal and its tributary canaliculi is most consistent with a vascular function. The inner ear of Adalatherium augments a growing list of characters (or character combinations) in which gondwanatherians are not only different from the inferred plesiomorphic condition for mammaliaforms but also distinct from the derived characters of therians. Collectively, the new morphological and phylogenetic data support homoplastic transformation of such structures as the cribriform plate and primary osseous lamina in mammals and independent loss of the lagenar macula within gondwanatherians.

The fossil record of mammaliaforms (mammals and their closest relatives) of the Mesozoic era from the southern supercontinent Gondwana is far less extensive than that from its northern counterpart, Laurasia^1,2. Among Mesozoic mammaliaforms, Gondwanatheria is one of the most poorly known clades, previously represented by only a single cranium and isolated jaws and teeth^1,2,3,4,5. As a result, the anatomy, palaeobiology and phylogenetic relationships of gondwanatherians remain unclear. Here we report the discovery of an articulated and very well-preserved skeleton of a gondwanatherian of the latest age (72.1–66 million years ago) of the Cretaceous period from Madagascar that we assign to a new genus and species, Adalatherium hui. To our knowledge, the specimen is the most complete skeleton of a Gondwanan Mesozoic mammaliaform that has been found, and includes the only postcranial material and ascending ramus of the dentary known for any gondwanatherian. A phylogenetic analysis including the new taxon recovers Gondwanatheria as the sister group to Multituberculata. The skeleton, which represents one of the largest of the Gondwanan Mesozoic mammaliaforms, is particularly notable for exhibiting many unique features in combination with features that are convergent on those of therian mammals. This uniqueness is consistent with a lineage history for A. hui of isolation on Madagascar for more than 20 million years.