John R. Wible

Diphyodonty (the presence of two generations of teeth) characterizes extant mammals and has long been regarded as pivotal to the evolution of other mammalian traits, including precise occlusion and determinate growth. Nevertheless, our understanding of the pattern of dental replacement in Mesozoic mammals is incomplete. Here, we report a nearly complete skeleton of an Early Cretaceous eutriconodontan that is the only mammal to our knowledge that appears to have multiple generations (polyphyodonty) at some loci in the four tooth types. In the auditory apparatus, its ossified Meckel’s cartilage has slight contact with the mandible, supporting the interpretation that Meckel’s separated from the mandible before it separated from the middle ear ossicles. The new fossil shows adaptations for semiaquatic life, with hands and feet like the platypus that were similarly webbed. Its unusual pattern of dorsoventrally flattened tail bones resembles the extant nutria (coypu), a rodent native to South America. Reappearance of polyphyodonty and molariform replacement in the new fossil and related gobiconodontids is probably associated with their ecomorphological niche and carnivorous diet.

Leptictidae are an extinct early Cenozoic group of mammals that has been challenging to place phylogenetically within Eutheria. They have been considered closely related to various crown placental groups such as Lipotyphla, Macroscelidea, or Euarchontoglires based on their “insectivore-grade” features, or as very early placentals or even non-placental eutherians because of their plesiomorphies. Since the 1920s their brain anatomy has been studied using natural endocasts, and oftentimes they are viewed as a model for the ancestral neurosensory conditions of placentals. Here, we use computed tomography (CT) for the first time to generate digital endocranial casts of two species of the canonical early Oligocene leptictid genus, Leptictis. Both species present similar endocranial anatomies with variation in the number of condyloid canals and the enclosure of the inferior petrosal sinus. Critically, we find differences compared to descriptions of natural endocasts, most notably that the rhinal fissure is more dorsally positioned, suggesting that Leptictis did not have a particularly expanded neocortex compared to Lipotyphla, Afrosoricida, and Macroscelidea. Furthermore, Leptictis was not particularly encephalized, and its brain size relative to body size was within the range of modern insectivorous taxa and Eocene mammals generally. Similar endocranial features in Leptictis and Eoryctes are likely plesiomorphic and do not represent a close phylogenetic relationship, and brain anatomy does not clarify the troublesome relationships of leptictids. The small neocortex and petrosal lobules might reflect fossorial adaptations in Leptictis, but more endocranial data from other Leptictidae will be fundamental to better understand any neurosensory shift that may have occurred within this group.

Ever since the discovery of Macrauchenia patachonica by Charles Darwin in 1834, the affinities of litopterns—a group of extinct South American Native Ungulates (SANUs)—have been elusive. In particular, the interfamilial relationships and timing of the familial diversification within the order Litopterna have not been addressed with adequate taxon and character sampling, and modern phylogenetic methods. We address these issues using a new morphological matrix of 703 dental and mandibular characters, scored for the earliest litopterns alongside early SANUs of other orders (Astrapotheria, Notoungulata, Pyrotheria, and Xenungulata). We also included members of families that have been often included within Litopterna, such as Didolodontidae, Protolipternidae, Indaleciidae, and Notonychopidae, the last two sometimes grouped in the order Notopterna. We conducted maximum parsimony and undated and tip-dated Bayesian phylogenetic analyses. Our results indicate that (1) didolodontids, protolipternids, and kollpaniines should be considered early SANUs, but not particularly closely related to any order, (2) Indaleciidae and Notonychopidae usually form a monophyletic group (Notopterna), and (3) Litopterna is a monophyletic unit composed of four families [Adianthidae, Anisolambdidae (subfamilies Anisolambdinae and Sparnotheriodontinae), Macraucheniidae, Proterotheriidae], and tentatively the notoptern families Indaleciidae and Notonychopidae with a probable origin ~64.0 Mya in the Early Palaeocene.

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.

After the end-Cretaceous extinction, placental mammals quickly diversified¹, occupied key ecological niches^2,3 and increased in size^4,5, but this last was not true of other therians⁶. The uniquely extended gestation of placental young⁷ may have factored into their success and size increase⁸, 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 mapping^9,10 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.

Mammals are the most encephalized vertebrates, with the largest brains relative to body size. Placental mammals have particularly enlarged brains, with expanded neocortices for sensory integration, the origins of which are unclear. We used computed tomography scans of newly discovered Paleocene fossils to show that contrary to the convention that mammal brains have steadily enlarged over time, early placentals initially decreased their relative brain sizes because body mass increased at a faster rate. Later in the Eocene, multiple crown lineages independently acquired highly encephalized brains through marked growth in sensory regions. We argue that the placental radiation initially emphasized increases in body size as extinction survivors filled vacant niches. Brains eventually became larger as ecosystems saturated and competition intensified.

The os paradoxum or dumb-bell-shaped bone is a paired bone occurring in the middle of the specialized bill of the platypus Ornithorhynchus anatinus. It has been variously considered as a neomorph of the platypus, as the homologue of the paired vomer of sauropsids, or as a part of the paired premaxillae. A review of the near 200-year history of this element strongly supports the os paradoxum as a remnant of the medial palatine processes of the premaxillae given its ontogenetic continuity with the premaxillae and association with the vomeronasal organ and cartilage, incisive foramen, and cartilaginous nasal septum. In conjunction with this hypothesis, homologies of the unpaired vomer of extant mammals and the paired vomer of extant sauropsids are also supported. These views are reinforced with observations from CT scans of O. anatinus, the Miocene ornithorhynchid Obdurodon dicksoni, and the extant didelphid marsupial Didelphis marsupialis. At the choanae, Obdurodon has what appears to be a separate parasphenoid bone unknown in extant monotremes.