Neil F. Adams , Emily J. Rayfield , Philip G. Cox , Samuel N. Cobb and Ian J. Corfe (2019)
Functional tests of the competitive exclusion hypothesis for multituberculate extinction
Royal Society Open Science 6: 181536
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Multituberculate mammals thrived during the Mesozoic, but their diversity declined from the mid-late Paleocene onwards, becoming extinct in the late Eocene. The radiation of superficially similar, eutherian rodents has been linked to multituberculate extinction through competitive exclusion. However, characteristics providing rodents with a supposed competitive advantage are currently unknown and comparative functional tests between the two groups are lacking. Here, a multifaceted approach to craniomandibular biomechanics was taken to test the hypothesis that superior skull function made rodents more effective competitors. Digital models of the skulls of four extant rodents and the Upper Cretaceous multituberculate Kryptobaatar were constructed and used (i) in finite-element analysis to study feeding-induced stresses, (ii) to calculate metrics of bite force production and (iii) to determine mechanical resistances to bending and torsional forces. Rodents exhibit higher craniomandibular stresses and lower resistances to bending and torsion than the multituberculate, apparently refuting the competitive exclusion hypothesis. However, rodents optimize bite force production at the expense of higher skull stress and we argue that this is likely to have been more functionally and selectively important. Our results therefore provide the first functional lines of evidence for potential reasons behind the decline of multituberculates in the changing environments of the Paleogene.
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Gabriela Fontanarrosa, Fernando Abdala, Susanna KÃmmell & Robert Gess (2019)
ÂThe manus of Tetracynodon (Therapsida: Therocephalia) provides evidence for survival strategies following the Permo-Triassic extinction.
Journal of Vertebrate Paleontology 38(4): 1-13
We present a comprehensive qualitative and quantitative study of the manus of a new therocephalian specimen referable to Tetracynodon from the Early Triassic of South Africa. We examined 18 specimens, representing at least 12 genera, including basal therocephalians (Lycosuchidae and Scylacosauridae) and eutherocephalians (Akidnognathidae, Whaitsioidea and Baurioidea). A temporal range of 23 million years through the Permo-Triassic (Wordian to Anisian) was surveyed. A principal component analysis of the therocephalian manus indicates that (1) metacarpals II, III, and IV acted as a module of which the medial and lateral elements (metacarpals I and V) were independent from each other; and (2) the proximal carpals, ulnare, and radiale lengths show contrasting variation in their measurements (e.g., groups with longer ulnares tend to have shorter radiales). The manus of Permian and Triassic taxa occupy separate regions in morphospace. This segregation pattern suggests selection for a manus with slender, elongated second to fourth metacarpals during and after the Permo-Triassic mass extinction. We show a heterogeneous condition of the fifth distal carpal bone. Although usually interpreted as fused to the fourth distal carpal or absent, the fifth distal carpal was present as a cartilaginous element in Tetracynodon. Strong and proportionally long unguals in relation to length of digits III and IV, wide and stocky basal phalanges, and short non-ungual phalanges strongly suggest that Tetracynodon was a scratch-digger. This supports the proposition that burrowing was an important behavioral strategy of terrestrial taxa during and after the Permo-Triassic mass extinction.
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Juliana Sterli, Marcelo S. de la Fuente & Guillermo W. Rougier (2019) [2018]
New remains of Condorchelys antiqua (Testudinata) from the Early-Middle Jurassic of Patagonia: anatomy, phylogeny, and paedomorphosis in the early evolution of turtles.
Journal of Vertebrate Paleontology 38(4) e1480112: 1-17
New cranial and postcranial remains of the Early-Middle Jurassic turtle, Condorchelys antiqua, are described here in detail, providing new insights into the early evolution of turtles. Unconstrained and constrained cladistic analyses in addition to newly developed total-evidence Bayesian analysis were performed to explore large-scale turtle relationships and evolutionary trends. All the analyses show a similar resolution at the base of the tree, recovering several species of small-sized, fresh water turtles of the EarlyâMiddle Jurassic at the base of the tree following the most basal, large-sized, terrestrial turtles from the Late Triassic. The calibration of the cladistic analyses and the tip-dating analysis provided similar results in the main nodes Testudines, Pan-Cryptodira, Cryptodira, Pan-Pleurodira, and Pleurodira, corroborating that the Jurassic is a key period for turtle evolution. The significant reduction in size in EarlyâMiddle Jurassic stem turtles and the combination of certain characters (e.g., presence of fontanelles, loss of bones, loss of scutes) shown by those taxa suggests heterochronic changes, paedomorphosis in particular, at the base of the turtle tree. These morphological novelties could have trigged, or facilitated, the occupation of the aquatic niche as seen in Jurassic stem turtles.
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