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[dinosaur] Evolution of bone ornamentation in Pseudosuchia + resting metabolic rate of therapsid Moghreberia

Ben Creisler

Two papers in the Biological Journal of the Linnean Society that have not been mentioned on the DML and are now officially published:

F. Clarac,  V. De Buffrénil,  C. Brochu  &  J. Cubo (2017)
The evolution of bone ornamentation in Pseudosuchia: morphological constraints versus ecological adaptation.
Biological Journal of the Linnean Society 121 (2): 395-408. 
DOI: https://doi.org/10.1093/biolinnean/blw034

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Although frequent in vertebrates (e.g. crocodylians, stem-tetrapods, turtles), the adaptive significance of bone ornamentation, that is the honeycomb-like pattern of pits and ridges that occur on the surface of dermal bones, remains poorly understood. In order to help assess the evolutionary history and ecological correlates of this character, ornamentation was quantified in 69 extant and extinct Pseudosuchia (taxa more closely related to crocodiles than to birds). This variable was related to the dominant habitat (terrestrial, amphibious, pelagic) of these taxa within a phylogenetic framework covering more than 250 Myr of evolution. The phylogenetic analyses reveal a significant correlation between the degree of bone ornamentation on the skull roof with lifestyle (terrestrial, amphibious, pelagic). A straightforward adaptive interpretation of these results is to be avoided because skull morphology has recently been shown to strongly influence local development of bone ornamentation in Crocodylia. Indeed, ornamentation in long-snouted amphibious forms scores low or nil values on the skull roof while scoring very high values on osteoderms. Our results also show that amphibious forms, whether marine or fluvial, have a high degree of ornamentation, whereas terrestrial and pelagic forms are either not ornamented or have a low level of ornamentation. It is hypothesized that the high development of ornamentation among semi-aquatic pseudosuchians has been positively selected because it improves basking efficiency in semi-aquatic ambush (i.e. poorly active) predators. This process would have occurred at the Triassic-Jurassic boundary.


Chloe Olivier,  Alexandra Houssaye,  Nour-Eddine Jalil &  Jorge Cubo (2017)
First palaeohistological inference of resting metabolic rate in an extinct synapsid, Moghreberia nmachouensis (Therapsida: Anomodontia).
Biological Journal of the Linnean Society 121 (2): 409-419. 
DOI: https://doi.org/10.1093/biolinnean/blw044

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The independent acquisition of endothermy in synapsids and diapsids are major events in vertebrate evolution since they were the driving force of a suite of correlated changes in anatomical, physiological, behavioural and ecological traits. While avian endothermy is assumed to have occurred at the archosauriform node, the acquisition of mammalian endothermy is poorly constrained both temporally and phylogenetically. Among the many unequivocal anatomical correlates of endothermy in synapsids, the presence of insulative pelage or respiratory turbinates only allows discrete inferences of presence/absence of endothermy. The analysis of bone histology allows richer palaeobiological inferences. We described the osteohistology and growth patterns of Moghreberia nmachouensis and two related taxa (Lystrosaurus and Oudenodon) for comparative purposes. Our observations suggest increasing growth rates from Moghreberia [the presence of incipient fibrolamellar bone (FLB) in humerus and femur], to Lystrosaurus (the presence of well-developed FLB in the femur but the presence of incipient FLB in the humerus), to Oudenodon (the presence of well-developed FLB in humerus and femur). However, qualitative histology does not allow reliable inferences about the occurrence of endothermy. We performed the first quantitative inferences of resting metabolic rates on fossil synapsids (M. nmachouensis as a model and Lystrosaurus and Oudenodon for comparative purposes) using quantitative histology (size, shape and density of osteocyte lacunae) combined with phylogenetic eigenvector maps. Our inferences are consistent with our qualitative histological observations: the mass-independent resting metabolic rate inferred for M. nmachouensis (2.58 mLO2 h−1 g−0.67) is lower than the value inferred for Lystrosaurus (3.80 mLO2 h−1 g−0.67), which is lower than that inferred for Oudenodon (4.58 mLO2 h−1 g−0.67). Optimization of these inferences onto a phylogenetic tree of tetrapods using the parsimony method allowed us to better constrain the temporal (more than 260 Myr ago) and phylogenetic (Neotherapsida) frames of the acquisition of mammalian endothermy.