Microstructural features of the femur in early ophiacodontids: A reappraisal of ancestral habitat use and lifestyle of amniotes

Abstract

a b s t r a c tOphiacodontids have long been considered the basalmost synapsids, and to have retained a fairly aquatic, piscivorous lifestyle typical of stem-amniotes. A restudy of their bone histology and microanatomy shows that Clepsydrops collettii, a Late Carboniferous ophiacodontid, has a thin, compact cortex and lacks a medullary spongiosa, two features that suggest a truly terrestrial lifestyle. The Early Permian Ophiacodon uniformis has a thicker cortex with a few resorption cavities and bone trabeculae surrounding the free medullary cavity. An inference model yields a terrestrial lifestyle for both taxa, though O. uniformis may have been slightly more aquatic (possibly amphibious) than C. collettii. However, an optimization of inferred lifestyle of other early stegocephalians (based on bone microanatomy) suggests that the first amniotes were terrestrial. The potentially amphibious lifestyle of O. uniformis, though not supported by our inference model, would thus be secondary. Histological features of femoral cortices in these two taxa closely resemble those previously described in extant species of large varanids and teids. This similarity, along with other comparative elements, is discussed in reference to the possible growth patterns and life history traits of Clepsydrops and O. uniformis. Mots clés : Clepsydrops OphiacodonMicro-anatomie osseuse Histologie osseuse Inférence de mode de vie Patron de croissance Paléobiologie r é s u m é Les ophiacodontidés ont longtemps été considérés comme les synapsidés les plus basaux et on leur a longtemps attribué le mode de vie aquatique et le régime piscivore typique des premiers amniotes-souches. Cette étude de micro-anatomie et d'histologie osseuse montre que le fémur de Clepsydrops collettii, un taxon du Carbonifère supérieur, possède un cortex compact peu épais et est dépourvu de spongieuse médullaire, deux caractères indicatifs d'un mode de vie franchement terrestre. Ophiacodon uniformis, du Permien inférieur, a un cortex plus épais avec quelques lacunes de résorption, ainsi que des travées osseuses autour de la cavité médullaire. L'usage d'un modèle d'inférence conduit à attribuer un mode de vie terrestre à ces deux taxons, encore que O. uniformis ait pu être un peu plus aquatique (peut-être G Model No. of Pages 13 2 M. Laurin, V. de Buffrénil / C. R. Palevol xxx (2015) xxx-xxx amphibie) que C. collettii. Cependant, l'optimisation des inférences de mode de vie effectuées chez d'autres stégocéphales en référence à la micro-anatomie de leurs os suggère que les premiers amniotes étaient terrestres. D'éventuelles moeurs amphibies chez Ophiacodon (bien qu'elles ne soient pas confirmées par l'emploi de nos modèles d'inférence) seraient donc secondaires. Les caractères histologiques du fémur dans les deux taxons étudiés ici rappellent fortement ceux attribués dans la littérature aux grands varanidés et téidés. Cette similitude, ainsi que d'autres éléments comparatifs, est discutée en référence aux modèles de croissance et aux traits d'histoire de vie qui pourraie...

Was Mesosaurus a Fully Aquatic Reptile?

“…Ophiacodon also resembles the terrestrial type (like Varanus salvator or V. rudicollis) in having the cervical vertebrae longer than the dorsals and the caudals, and the dorsals are also longer than the caudals. This is congruent with the latest bone microanatomical study (Laurin and de Buffrénil, 2016). However, this interpretation should be treated with caution because we did not study the specimens directly and there could be some distortion due to incompleteness of the available specimens plotted, a condition that is reflected in their noisy profile (Figure 11H).…”

“…Similarly, another work that inquired into the lifestyle of Ophiacodon through a study of bone microanatomy (Germain and Laurin, 2005) found evidence supporting a semi-aquatic to terrestrial pattern. However, a more recent study (Laurin and de Buffrénil, 2016) concluded (based on another bone section) that this basal synapsid may have been terrestrial and that this may reflect the early amniote condition because an older basal synapsid (Clepsydrops collettii) exhibited a microanatomical signature typical of terrestrial tetrapods. However, this could also reflect the condition in early synapsids.…”

“…Mesosaurs may occupy a basalmost position in sauropsids, according to some recent phylogenetic (e.g., Laurin and de Buffrénil, 2016;Laurin and Piñeiro, 2017) and anatomical (e.g. Villamil et al, 2015;Piñeiro et al, 2016) studies.…”

Bio-inspired method based on bone architecture to optimize the structure of mechanical workspieces

“…The local strain at the trabeculae connections enables to ensure a better distribution of the contact pressure [12], [13]. This structure meets a variety of specifications and have been adapted to the lifestyle of each living species [14], [15], [16].…”

The discrepancy between morphological and microanatomical patterns of anamniotic stegocephalian postcrania from the Early Permian Briar Creek Bonebed (Texas)

“…Different sampling planes thus may intersect a variable microstructure of the medullary region; the density of trabeculae increases proximally and distally together with decreasing cortical bone thickness. Laurin and Buffrénil (2015) documented the misinterpretation of lifestyle based on the microstructural features of Ophiacodon because the section plane was not precisely mid-diaphyseal. However, the specimens from Briar Creek Bonebed varied not only on the microstructural level, but also represent the different histology (growth pattern, tissue type, vascularization pattern), suggesting that the plane of section cannot explain all of the observed variability.…”