Anatomy, ontogeny, and ecology of Mesosauridae, the first secondarily aquatic amniotes

Mesosaurier, kleine Reptilien aus dem Unteren Perm, gelten als die frühesten aquatischen Reptilien. Trotz ihrer kurzen Existenz und begrenzten Verbreitung bieten sie wichtige Einblicke in die frühe Amnioten-Evolution. Diese Arbeit fasst vier Artikel zusammen, die Aspekte der Anatomie, Ontogenese und Ökologie von Mesosauridae beleuchten. Kapitel 1 behandelt die Knochenhistologie und Pachyosterosklerose in Mesosaurierknochen, was auf eine vollständig aquatische Lebensweise hinweist. Wir identifizieren auch eine große intraspezifische Variation, die möglicherweise durch unregelmäßige Probenahmeorte beeinflusst wird. Kapitel 2 untersucht die morpho¬logische Variation bei Mesosauriern und deren Umwelt-verteilung basierend auf ihrer Größe. Ontogenetische Veränderungen stehen im Zusammenhang mit dem Wechsel des Ablagerungsmilieus und deuten auf Veränderungen in Ernährung und Lebensraum mit dem Wachstum hin. In Kapitel 3 wird die kaudale Autotomie bei Mesosauriern überprüft und das Vorhandensein kaudaler Bruchebenen bestätigt. Wir schlagen eine stärker von den Extremitäten angetriebene Fortbewegung vor. Unsere Ergebnisse legen nahe, dass kaudale Autotomie ursprünglich in einer breiten Gruppe von Reptilien vorhanden war, anstatt konvergent in verschiedenen Linien entstanden zu sein. Kapitel 4 beschreibt auf Grundlage von Mesosaurier-Exemplaren vier grundlegende axiale Entwicklungs-modelle bei Amnioten und rekonstruiert ihren ursprünglichen Zustand in der Klade. Diese Muster bleiben im Laufe der Zeit relativ stabil, obwohl regionale Einflüsse möglich sind. Diese Erkenntnisse beleuchten wichtige Aspekte der Mesosaurier-Paläontologie und unterstreichen ihre Bedeutung für die Erforschung der Evolution früher Amnioten.The enigmatic mesosaurs were small, reptiles that lived during the Lower Permian period and were the earliest known reptiles to return to living in water. Despite their short existence and restricted geographical distribution, mesosaurs can provide important evidence about the early evolution of amniotes and the colonization of aquatic environments. This thesis regroups four articles published in peer-reviewed journals that investigate aspects of the anatomy, ontogeny, and ecology of Mesosauridae. In Chapter 1, my co-authors and I study bone histology and pachyosterosclerosis in the long bones, vertebrae, and ribs of mesosaurs. Thin sections show a high degree of osteosclerosis in their bones, supporting a fully aquatic lifestyle. We also recover a large intraspecific variation in Mesosauridae, albeit possibly due to irregular sampling locations. In Chapter 2 we examine morphological variation in mesosaurs and the environmental distribution of individuals as a function of size. We highlight ontogenetic changes in mesosaurs associated with a transition across depositional environments, which suggest that mesosaurs underwent diet and life environment change with growth. In Chapter 3, we review the information on caudal autotomy in mesosaurs. We confirm the presence of caudal fracture planes in the clade and propose a more limb-driven propulsion than previously considered. Our results also suggest that caudal autotomy was ancestrally present in a large radiation of reptiles, rather than a feature evolved convergently in multiple lineages. In Chapter 4, based on data in mesosaur specimens, we describe four axial developmental patterns in amniotes and reconstruct their ancestral condition in the clade. We demonstrate that these patterns are relatively stable throughout time, albeit possibly affected by regionalization. These new elements reveal crucial aspects of mesosaur paleontology and substantiate their significance for studying the evolution of early amniotes

Supplementary Table 1

Completeness scores of main skeletal sections. Abbreviations: Bol: Bolosauria; Lan: Lanthanosuchoidea; Mes: Mesosauridae; Mil: Millerosauria; Nyc: Nycteroleteroidea; Par: Pareiasauria; Pro: Procolophonoidea. SCM and CCMa columns are the main values (same as in table 4)

Regionalization, constraints, and the ancestral ossification patterns in the vertebral column of amniotes

The development of the vertebral column has been studied extensively in modern amniotes, yet many aspects of its evolutionary history remain enigmatic. Here we expand the existing data on four major vertebral developmental patterns in amniotes based on exceptionally well-preserved specimens of the early Permian mesosaurid reptile Mesosaurus tenuidens: (i) centrum ossification, (ii) neural arch ossification, (iii) neural arch fusion, and (iv) neurocentral fusion. We retrace the evolutionary history of each pattern and reconstruct the ancestral condition in amniotes. Despite 300 million years of evolutionary history, vertebral development patterns show a surprisingly stability in amniotes since their common ancestor. We propose that this stability may be linked to conservatism in the constraints posed by underlying developmental processes across amniotes. We also point out that birds, mammals, and squamates each show specific trends deviating from the ancestral condition in amniotes, and that they remain rather unchanged within these lineages. The stability of their unique patterns demonstrates a certain homogeneity of vertebral developmental constraints within these lineages, which we suggest might be linked to their specific modes of regionalization. Our research provides a framework for the evolution of axial development in amniotes and a foundation for future studies.Peer Reviewe

Data from: Assessing the completeness of the fossil record: comparison of different methods applied to parareptilian tetrapods (Vertebrata: Sauropsida)

As paleontological studies are generally distorted by gaps and biases in the fossil record, it is important to assess its completeness. Here we address the fossil record of Parareptilia, a Permian–Triassic amniote clade, applying two measures of specimen completeness: the skeletal completeness metric (SCM) and the character completeness metric (CCM). The SCM quantifies how much of the skeletal material of a taxon is preserved, whereas the CCM measures the amount of phylogenetic information available. The latter was implemented using two different approaches. In this study, we compare three completeness metrics. Two CCM implementations show a strong correlation with each other, but only the second implementation of the CCM correlates significantly with the SCM, possibly due to character selection in phylogenetic data sets. There is no correlation between diversity of parareptiles and their completeness, implying that the observed fluctuations in diversity are not driven by the completeness of the fossils. The mean completeness of parareptiles through time is consistently high compared with previously studied tetrapod clades, suggesting that most parareptile taxa are based on reasonably complete specimens. Clade-specific differences reveal no link between body size and completeness. However, the analyses confirm the impact of ecology, with aquatic mesosaurids being better preserved than terrestrial taxa

Supplementary Table 2

Results of the three completeness metrics, assigned stratigraphical age, completeness model and clade for the 76 parareptile genera considered in this study. Abbreviations used for stratigraphical ages are the same as in Figs. 4 and 6 except for Induan (IND) and Changhsingian (CHA). Abbreviations used for model used and clade assignment are the same as in Fig. 7

Microanatomy and growth of the mesosaurs Stereosternum tumidum and Brazilosaurus sanpauloensis (Reptilia, Parareptilia)

Histology and microanatomy of vertebrae, ribs, haemalarch, and humeri and femora of 10 individuals of Stereosternum and two dorsal ribs of1 individual of Brazilosaurus were studied. All individuals had achieved a body lengthof 50 cm (equal to 65 % of the maximum known body length) or larger. All sampledbones are highly osteosclerotic due to the reduction of medullary cavitiesand the filling of medullary regions by endosteal bone. Calcified cartilageoccurs – if at all – only locally in small clusters in the medullaryregions of midshaft and in higher amounts only in non-midshaft sections oflong bones and towards the medio-distal rib shaft, respectively. The primarybone tissue consists of highly organized parallel-fibred tissue and/orlamellar tissue, which is in most samples relatively lightly vascularized oreven avascular. If present, vascular canals are mainly longitudinallyoriented; some show a radial orientation. Simple vascular canals as well asprimary osteons occur. Some of the latter are secondarily altered, i.e. widened. Remodelling of the periosteal cortex is only documented by fewscattered erosion cavities and secondary osteons. The tissue is regularlystratified by lines of arrested growth (LAGs), which usually appear as double or multiple rest lines,indicating strong dependence on exogenous and endogenous factors. Because ofthe inhibition of periosteal remodelling the growth record is complete andno inner cycles are lost. Individuals of Stereosternum show a poor correlation of bodysize and number of growth marks, which might be the result of developmentalplasticity. Brazilosaurus shows a highly organized, avascular lamellar tissue and a highnumber of regularly deposited rest lines throughout the cortex of the ribs.The medullary region in the ribs of Brazilosaurus is distinctly larger when compared to ribsof Stereosternum. However, strong osteosclerosis is obvious in both taxa, pointing to ahigh degree of aquatic adaption. Ribs of Stereosternum, Brazilosaurus, and Mesosaurus are clearly distinguishablefrom each other by the distribution of the periosteal and endostealterritory. Furthermore, Brazilosaurus differs in its growth pattern (i.e. spacing of restlines) when compared to Stereosternum and Mesosaurus