The Phylogenetic Affinities of the Enigmatic Mammalian Clade Gondwanatheria

New record of a Mesozoic gondwanatherian mammaliaform from Southern Patagonia

Gondwanatheria is an enigmatic mammaliaform clade distributed in the Cretaceous and Paleogene of South America, Africa, Madagascar, India and Antarctica. The Mesozoic record in South America is restricted to the Latest Cretaceous of Río Negro and Chubut provinces, Argentina and Magallanes Region of southern Chile. The aim of the present contribution is to describe a new specimen of gondwanatherian mammaliaforms from beds belonging to the Maastrichtian Chorrillo Formation, cropping out 30 km SW of El Calafate, Santa Cruz Province, Argentina. It is represented by a single molariform referable to the species Magallanodon baikashkenke with which it shares a unique combination of characters. Analysis of the unique combination of characters exhibited by Magallanodon shed doubts on the monophyly of Ferugliotheriidae and suggest that South American taxa may be closely related to each other. The wide geographical distribution and occurrence of gondwanatherians on geological units of diverse origins suggest that they were capable of facing disparate environmental conditions.

Shape disparity in the blade-like premolars of multituberculate mammals: functional constraints and the evolution of herbivory

Cimolodontan multituberculates were a diverse and long-lived group of mammals characterized by large, blade-like lower fourth premolars (p4). Blade-like (plagiaulacoid) dentitions have evolved numerous times in distantly related mammalian lineages. Here we investigate how p4-shape disparity changed through time in the Cimolodonta. We address two hypotheses: (H1) blade-like dentitions constrain the ability of plagiaulacoid mammals to evolve novel dental morphologies, (H2) cimolodontan dental evolution proceeded gradually along a morphocline during the Late Cretaceous. We quantify functionally important aspects of p4 shape, including ratios reflecting height (H:L), symmetry (L1:L), and mesial-face height (H1:H), in a large sample of cimolodontans spanning the mid-Cretaceous through early Paleogene of North America (ca. 100–35 Mya). Our results do not support the morphocline hypothesis (H2) and, instead, show that cimolodontans evolved a wide range of p4 shapes by the mid-Cretaceous, and that p4-shape disparity remained stable through the Late Cretaceous. We hypothesize that the two-stage cimolodontan chewing cycle (slicing-crushing then grinding) imposed functional constraints on p4 morphology. After the Cretaceous-Paleogene boundary, p4-shape disparity increased sharply, driven by the appearance of the Taeniolabidoidea, Microcosmodontidae, and Eucosmodontidae, in the early Paleocene. We contend that the slicing-crushing functions of the p4 became less important in those taxa, relaxing functional constraints on p4 morphology. Cimolodontans that retained both the slicing-crushing and grinding function of the p4 had a more limited range of p4 morphologies, and probably were largely restricted to animal-dominated omnivory. Taxa that shifted the initial slicing-crushing function from the p4 to the incisors had fewer functional constraints on p4 morphology, and were able to increase their molar grinding capacity to exploit plant-dominated omnivory and herbivory. That the p4 was reduced in herbivorous taxa rather than modified into a broader, multi-cusped tooth lends support to the morphological constraint hypothesis (H1), and this relationship between p4 morphology and function suggests that retaining a large, blade-like p4 might have limited the range of herbivorous diets cimolodontans could exploit. These findings highlight the ecological and evolutionary limitations that specialized dentitions can impose on mammals by restricting their morphological and, in turn, functional diversification.

Mesozoic mammals of China: implications for phylogeny and early evolution of mammals

All Mesozoic mammaliaforms reported from China are briefly documented herein. These forms can be divided into at least five major assemblages: Lufeng, Yanliao (Daohugou), Jehol, Fuxin and Bayan Mandahu, ranging from the Early Jurassic to the Late Cretaceous periods. Although the temporal and geographic distributions of these mammaliaforms are not dense, the records do reveal a pattern that is generally consistent with patterns that have been recognized globally. The initial stage of mammalian evolution was represented by stem mammaliaforms or primitive ‘triconodonts’ from the Lufeng. This was followed by the Middle-Late Jurassic Yanliao episode that showed a high diversity and disparity of mammaliaforms in which terrestrial, swimming, arboreal and gliding species were present. The disparity, at least in molar morphology and types of locomotion, decreased but the diversity persisted into the Cretaceous, a period that was dominated by eutriconodontans, multituberculates and trechnotherians. The superb specimens from nearly all major groups of Mesozoic mammals in China provided a great amount of information that contributed to our understanding on some major issues in phylogeny and the early evolution of mammals, such as divergences of mammals and the evolution of the mammalian middle ear. A hypothesis on the transformation of the allotherian tooth pattern is proposed as an example to illustrate the potential for future studies of mammalian evolution.

Function of pretribosphenic and tribosphenic mammalian molars inferred from 3D animation

Appearance of the tribosphenic molar in the Late Jurassic (160 Ma) is a crucial innovation for food processing in mammalian evolution. This molar type is characterized by a protocone, a talonid basin and a two-phased chewing cycle, all of which are apomorphic. In this functional study on the teeth of Late Jurassic Dryolestes leiriensis and the living marsupial Monodelphis domestica, we demonstrate that pretribosphenic and tribosphenic molars show fundamental differences of food reduction strategies, representing a shift in dental function during the transition of tribosphenic mammals. By using the Occlusal Fingerprint Analyser (OFA), we simulated the chewing motions of the pretribosphenic Dryolestes that represents an evolutionary precursor condition to such tribosphenic mammals as Monodelphis. Animation of chewing path and detection of collisional contacts between virtual models of teeth suggests that Dryolestes differs from the classical two-phased chewing movement of tribosphenidans, due to the narrowing of the interdental space in cervical (crown-root transition) direction, the inclination angle of the hypoflexid groove, and the unicuspid talonid. The pretribosphenic chewing cycle is equivalent to phase I of the tribosphenic chewing cycle, but the former lacks phase II of the tribosphenic chewing. The new approach can analyze the chewing cycle of the jaw by using polygonal 3D models of tooth surfaces, in a way that is complementary to the electromyography and strain gauge studies of muscle function of living animals. The technique allows alignment and scaling of isolated fossil teeth and utilizes the wear facet orientation and striation of the teeth to reconstruct the chewing path of extinct mammals.

A new phylogeny for basal Trechnotheria and Cladotheria and affinities of South American endemic Late Cretaceous mammals

The endemic South American mammals Meridiolestida, considered previously as dryolestoid cladotherians, are found to be non-cladotherian trechnotherians related to spalacotheriid symmetrodontans based on a parsimony analysis of 137 morphological characters among 44 taxa. Spalacotheriidae is the sister taxon to Meridiolestida, and the latter clade is derived from a primitive spalacolestine that migrated to South America from North America at the beginning of the Late Cretaceous. Meridiolestida survived until the early Paleocene (Peligrotherium) and early Miocene (Necrolestes) in South America, and their extinction is probably linked to the increasing competition with metatherian and eutherian tribosphenic mammals. The clade Meridiolestida plus Spalacotheriidae is the sister taxon to Cladotheria and forms a new clade Alethinotheria. Alethinotheria and its sister taxon Zhangheotheria, new clade (Zhangheotheriidae plus basal taxa), comprise Trechnotheria. Cladotheria is divided into Zatheria (plus stem taxa, including Amphitherium) and Dryolestida, including Dryolestidae and a paraphyletic array of basal dryolestidans (formerly classified as "Paurodontidae"). The South American Vincelestes and Groebertherium are basal dryolestidans.

Developmental Patterns in Mesozoic Evolution of Mammal Ears

Complex structures with significant biological function can arise multiple times in evolution by common gene patterning and developmental pathways. The mammalian middle ear, with its significant hearing function, is such a complex structure and a key evolutionary innovation. Newly discovered fossils have now shown that the detachment of the ear from the jaw, an important transformation of the middle ear in early mammals, has major homoplasies; the morphogenesis of these homoplasies is also illuminated by new genetic studies of ear development in extant mammals. By extrapolating the developmental morphogenesis of genetic studies into the early mammal fossil record, evolution of the middle ear in early mammals provides an integrated case study of how development has impacted, mechanistically, the transformation of a major structural complex in evolution.