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The end‐Cretaceous mass extinction allowed placental mammals to diversify ecologically and taxonomically as they filled ecological niches once occupied by non‐avian dinosaurs and more basal mammals. Little is known, however, about how the neurosensory systems of mammals changed after the extinction, and what role these systems played in mammalian diversification. We here use high‐resolution computed tomography (CT) scanning to describe the endocranial and inner ear endocasts of two species,Chriacus pelvidensandChriacus baldwini, which belong to a cluster of ‘archaic’ placental mammals called ‘arctocyonid condylarths’ that thrived during theca. 10 million years after the extinction (the Paleocene Epoch), but whose relationships to extant placentals are poorly understood. The endocasts provide new insight into the paleobiology of the long‐mysterious ‘arctocyonids’, and suggest thatChriacuswas an animal with anencephalization quotient (EQ)range of 0.12–0.41, which probably relied more on its sense of smell than vision, because the olfactory bulbs are proportionally large but the neocortex and petrosal lobules are less developed. Agility scores, estimated from the dimensions of the semicircular canals of the inner ear, indicate thatChriacuswas slow to moderately agile, and its hearing capabilities, estimated from cochlear dimensions, suggest similarities with the extant aardvark.Chriacusshares many brain features with other Paleocene mammals, such as a small lissencephalic brain, large olfactory bulbs and small petrosal lobules, which are likely plesiomorphic for Placentalia. The inner ear ofChriacusalso shares derived characteristics of the elliptical and spherical recesses with extinct species that belong to Euungulata, the extant placental group that includes artiodactyls and perissodactyls. This lends key evidence to the hypothesized close relationship betweenChriacusand the extant ungulate groups, and demonstrates that neurosensory features can provide important insight into both the paleobiology and relationships of early placental mammals.

 

While most mammals have whiskers, some tactile specialists—mainly small, nocturnal, and arboreal species—can actively move their whiskers in a symmetrical, cyclic movement called whisking. Whisking enables mammals to rapidly, tactually scan their environment to efficiently guide locomotion and foraging in complex habitats. The muscle architecture that enables whisking is preserved from marsupials to primates, prompting researchers to suggest that a common ancestor might have had moveable whiskers. Studying the evolution of whisker touch sensing is difficult, and we suggest that measuring an aspect of skull morphology that correlates with whisking would enable comparisons between extinct and extant mammals. We find that whisking mammals have larger infraorbital foramen (IOF) areas, which indicates larger infraorbital nerves and an increase in sensory acuity. While this relationship is quite variable and IOF area cannot be used to solely predict the presence of whisking, whisking mammals all have large IOF areas. Generally, this pattern holds true regardless of an animal's substrate preferences or activity patterns. Data from fossil mammals and ancestral character state reconstruction and tracing techniques for extant mammals suggest that whisking is not the ancestral state for therian mammals. Instead, whisking appears to have evolved independently as many as seven times across the clades Marsupialia, Afrosoricida, Eulipotyphla, and Rodentia, with Xenarthra the only placental superordinal clade lacking whisking species. However, the term whisking only captures symmetrical and rhythmic movements of the whiskers, rather than all possible whisker movements, and early mammals may still have had moveable whiskers. Anat Rec, 2018. © 2018 American Association for Anatomy.

 

Mammals underwent a profound diversification after the end‐Cretaceous mass extinction, with placentals rapidly expanding in body size and diversity to fill new niches vacated by dinosaurs. Little is known, however, about the brains and senses of these earliest placentals, and how neurosensory features may have promoted their survival and diversification. We here use computed tomography (CT) to describe the brain, inner ear, sinuses, and endocranial nerves and vessels ofCarsioptychus coarctatus, a periptychid “condylarth” that was among the first placentals to blossom during the few million years after the extinction, in the Paleocene.Carsioptychushas a generally primitive brain and inner ear that is similar to the inferred ancestral eutherian/placental condition. Notable “primitive” features include the large, anteriorly expanded, and conjoined olfactory bulbs, proportionally small neocortex, lissencephalic cerebrum, and large hindbrain compared to the cerebrum. An encephalization quotient (EQ) cannot be confidently calculated because of specimen crushing but was likely very small, and comparisons with other extinct placentals reveal that many Paleocene “archaic” mammals had EQ values below the hallmark threshold of modern placentals but within the zone of nonmammalian cynodonts, indicative of small brains and low intelligence.Carsioptychusdid, however, have a “conventional” hearing range for a placental, but was not particularly agile, with semicircular canal dimensions similar to modern pigs. This information fleshes out the biology of a keystone Paleocene “archaic” placental, but more comparative work is needed to test hypotheses of how neurosensory evolution was related to the placental radiation. Anat Rec, 302:306–324, 2019. © 2018 Wiley Periodicals, Inc.

 

The Picrodontidae from the middle Palaeocene of North America are enigmatic placental mammals that were allied with various mammalian groups but are generally now considered to have close affinities to paromomyid and palaechthonid plesiadapiforms based on proposed dental synapomorphies. The picrodontid fossil record consists entirely of dental and gnathic remains except for one partial cranium of Zanycteris paleocenus (AMNH 17180). Here, we use µCT technology to unveil previously undocumented morphology in AMNH 17180, describe and compare the basicranial morphology of a picrodontid for the first time, and incorporate these new data into cladistic analyses. The basicranial morphology of Z. paleocenus is distinct from plesiadapiforms and shares similarities with the Palaeogene Apatemyidae and Nyctitheriidae. Results of cladistic analyses incorporating these novel data suggest picrodontids are not stem primates nor euarchontan mammals and that the proposed dental synapomorphies uniting picrodontids with plesiadapiforms and, by extension, primates evolved independently. Results highlight the need to scrutinize proposed synapomorphies of highly autapomorphic taxa with limited fossil records. 

After successfully diversifying during the Paleocene, the descendants of the first wave of mammals that survived the end‐Cretaceous mass extinction waned throughout the Eocene. Competition with modern crown clades and intense climate fluctuations may have been part of the factors leading to the extinction of these archaic groups. Why these taxa went extinct has rarely been studied from the perspective of the nervous system. Here, we describe the first virtual endocasts for the archaic order Tillodontia. Three species from the middle Eocene of North America were analyzed: Trogosus hillsii, Trogosus grangeri, and Trogosus castoridens. We made morphological comparisons with the plaster endocast of another tillodont,Tillodon fodiens, as well as groups potentially related to Tillodontia: Pantodonta, Arctocyonidae, and Cimolesta. Trogosus shows very little inter‐specific variation with the only potential difference being related to the fusion of the optic canal and sphenorbital fissure. Many ancestral features are displayed by Trogosus, including an exposed midbrain, small neocortex, orbitotemporal canal ventral to rhinal fissure, and a broad circular fissure. Potential characteristics that could unite Tillodontia with Pantodonta, and Arctocyonidae are the posterior position of cranial nerve V3 exit in relation to the cerebrum and the low degree of development of the subarcuate fossa. The presence of large olfactory bulbs and a relatively small neocortex are consistent with a terrestrial lifestyle. A relatively small neocortex may have put Trogosus at risk when competing with artiodactyls for potentially similar resources and avoiding predation from archaic carnivorans, both of which are known to have had larger relative brain and neocortex sizes in the Eocene. These factors may have possibly exacerbated the extinction of Tillodontia, which showed highly specialized morphologies despite the increase in climate fluctuations throughout the Eocene, before disappearing during the middle Eocene. 

The placental order Dermoptera, which includes two extant species, the Philippine and Sunda flying lemurs, Cynocephalus volans and Galeopterus variegatus, respectively, is generally held to be the sister group of Primates. Yet, little has been reported on their cranial anatomy. Here, the anatomy of the ear region is described and illustrated for a juvenile and adult C. volans based on CT scans. The inclusion of a juvenile is essential as nearly all cranial sutures are fused in the adult. Soft tissues are reconstructed based on sectioned histological pre- and postnatal specimens previously reported by the author. Numerous unusual features are identified, including: a small parasphenoid beneath the basisphenoid, a tensor tympani fossa on the epitympanic wing of the squamosal, a cavum supracochleare for the geniculate ganglion of the facial nerve that is not enclosed in the petrosal bone, a secondary facial foramen between the petrosal and squamosal, a secondary posttemporal foramen leading to the primary one, a subarcuate fossa that is floored in part by a large contribution from the squamosal, a body of the incus larger than the head of the malleus, and a crus longum of the incus that lacks an osseous connection to the lenticular process. Documentation of the anatomy of the Philippine flying lemur ear region is an essential first step in morphological phylogenetic analyses where features of the basicranium are widely sampled. 

The individual bones of the adult cranium of the gray short-tailed opossum, Monodelphis domestica (Wagner, 1842) are described and illustrated in multiple views based on CT scans. The author previously reported on the outer bony surfaces of the skull of Monodelphis Burnett, 1830, and the current contribution is a companion piece, paying particular attention to the inner bony surfaces (within the endocranium and nasal cavity) and the facets between individual cranial elements, including the ethmo- and frontoturbinals. Comments are provided on the internal nasal floor skeleton, which in M. domestica includes a fused conglomerate formed by the medial palatine processes of the premaxillae, the vomer, the ethmoid, the presphenoid, and the orbitosphenoids. This conglomerate includes horizontal shelves just dorsal to the hard palate, and occurs widely in marsupials but is currently unknown in monotremes and placentals. 

After the end-Cretaceous extinction, placental mammals quickly diversified, occupied key ecological niches and increased in size, but this last was not true of other therians. The uniquely extended gestation of placental young may have factored into their success and size increase, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping9,10 by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30–75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals. 

Placental mammals had a smaller brain-to-body-size ratio after the dinosaur extinction but later developed the largest vertebrate brains.