An Early Cretaceous Tribosphenic Mammal and Metatherian Evolution

Abstract

Derived features of a new boreosphenidan mammal from the Lower Cretaceous Yixian Formation of China suggest that it has a closer relationship to metatherians (including extant marsupials) than to eutherians (including extant placentals). This fossil dates to 125 million years ago and extends the record of marsupial relatives with skeletal remains by 50 million years. It also has many foot structures known only from climbing and tree-living extant mammals, suggesting that early crown therians exploited diverse niches. New data from this fossil support the view that Asia was likely the center for the diversification of the earliest metatherians and eutherians during the Early Cretaceous.

A radiation of arboreal basal eutherian mammals beginning in the Late Cretaceous of India

“…S1) (19,22). Its basal placement in the analyses presented here strengthens the hypothesis that eutherian mammals, or even therian mammals, were ancestrally scansorial or arboreal (38)(39)(40). Arboreality has recently been linked to lower extrinsic mortality and increased longevity across extant mammals (41), and there may have been strong selective pressure for early eutherians to seek refuge in trees to escape from terrestrial predators, such as dinosaurs, or to explore additional trophic niches.…”

“…Character order follows Wible et al (2), with the addition of the seven characters detailed below. These changes result primarily from three factors: (i ) the new specimens are better preserved than the relatively worn and pitted Naskal specimens originally described, and demonstrate that some of the apparent morphology observed in those original specimens was inaccurate because of weathering/erosion and did not reflect original features of this taxon; (ii ) new specimens demonstrate that many features are polymorphic, as with incisor shape (character 5), metacone to paracone size (character 77), metacone and paracone base relationship (character 79), metaconule shape and position (character 89), protocone position and height (characters 95 and 96), paraconid crest shape (character 109), and protoconid height (character 112); or (iii ) new specimens represent elements that were not previously preserved, including mandibular fragments (characters 130-135), molar series (character 62), ultimate upper premolars (characters [40][41][42][43][44][45][46], and humeri (characters 359-363). None of these changes are unexpected with increases in available fossil data, but the polymorphic characters are of particular interest for evaluating taxonomic affiliations of new discoveries and for comparing the amount of variation observed in fossil taxa to that of modern taxa (50).…”

A new Early Cretaceous eutherian mammal from the Sasayama Group, Hyogo, Japan

“…Compared with the angular process of the basal metatherian Deltatheridium, the rod-like angular process of S. kawaii is clearly different from the shelf-like process of Deltatheridium [34]. The angular process of the oldest known definitive eutherian, Juramaia, is apparently not inflected [3], and even the oldest known metatherian, Sinodelphys, also lacks the inflected angular process [35]. The rod-like inflected angular process is possibly an apomorphic character for eutherians, seen only in early members.…”

Arboreality has allowed for the evolution of increased longevity in mammals

“…The nature of these dichotomies may play out in the evolution of longevity. We propose that the lack of significant differences between arboreal and terrestrial euarchontans, and possibly marsupials, may be the result in part of the long history and persistence of arboreality in their evolutionary histories (30)(31)(32)(33)(34)(35)(36)(37)(38). Unlike euarchontans, however, marsupials are not characterized by increased longevity in general (Fig.…”

“…Arboreality is the primitive condition for primates, initially evolved in an ancestral euarchontan (30)(31)(32)(33)(34)(35). Marsupials also probably are primitively arboreal (34,(36)(37)(38), with subsequent and repeated events of terrestriality (38). In contrast, many other eutherian orders and placental mammals in general are likely characterized by terrestrial evolutionary histories, with subsequent events of arboreality derived multiple times in these lineages (34,39).…”