Browsing by Person "Miedema, Feiko"

Now showing 1 - 1 of 1

Ontogeny and reproductive paleobiology in the Mesozoic marine reptile clade Ichthyosauria
(2023) Miedema, Feiko; Schoch, Rainer
Ichthyosaurs were viviparous marine reptiles that occupied many consumer niches in the marine ecosystems of the Mesozoic. Their viviparous reproductive strategy lends us the opportunity to study their prenatal development in great detail and lets us assess their birthing strategies. Moreover, some species are common in Lagerstätten deposits, which means we are likewise able to assess postnatal ontogeny. The assessment of ontogeny is important in our understanding of an extinct animal’s paleobiology and necessary for its’ correct phylogenetic placement. After studying the prenatal osteological development in the ichthyosaur Stenopterygius we now recognize 4 different stages on the basis of relative cranial ossification. The overall ossification sequence was very similar to other diapsids. The establishment of multiple prenatal stages was a first for an extinct sauropsid. We likewise erected 3 postnatal stages in Stenopterygius on the basis of cranial ossification, fusion and element morphology. Recognizing multiple postnatal ontogenetic stages in model (fossil) organism such as Stenopterygius will help recognize the ontogenetic stages of less well-known species and single specimens as well as refine phylogenetic characters which may be prone to major ontogenetic differences. Moreover, the cranial ossification and ontogeny of Mixosaurus cornalianus was studied. Due to sample size and the difficult crushed material, we were unable to establish the same stages as in Stenopterygius. However, we did clarify certain parts in Mixosaurus osteology, established some ontogenetically variable traits, most notably regarding the braincase and lower jaw, and were able to assess in detail a prenatal stage. Early cranial development in Mixosaurus differs from that of Stenopterygius regarding the precise ossification of the basioccipital and exoccipital, whereby Mixosaurus possibly shows a more ancestral developmental pathway. The notochord is a structure established early in development of vertebrates and recedes during the ossification of the vertebral centra. After studying much fetal material of several taxa of ichthyosaurs across phylogeny we established that the relative size of the notochord pit can function as a proxy for determining prenatal stage (as compared to Stenopterygius). This shows that the developmental pathway is relatively conservative across ichthyosaurs, as expected, and will help in determining prenatal stage in the absence of cranial material in subsequent finds. Lastly the birth preference of ichthyosaurs was re-evaluated. A longstanding idea within the community was that ichthyosaurs preferentially (and almost exclusively) gave birth tail-first as head-first birth would increase the risk of drowning of the fetus during birth. An early diverging ichthyosaur, Chaohusaurus, was found to have head-first birth. The authors argued that it must be the ancestral and terrestrial condition, citing the drowning hypothesis. After studying three pregnant specimens of the ichthyosaur Mixosaurus, we assessed birthing orientation in all ichthyosaurs in which pregnant specimens are known. Moreover, we looked at birthing in aquatic and terrestrial extant and all extinct aquatic amniotes. We see no link between birthing in an aquatic medium and tail-first birth and thereby deem the drowning hypothesis improbable. We propose two new hypotheses for birth orientation preference. Furthermore, based on all known ichthyosaur fetuses we propose that a preference for tail-first birth originated much later in ichthyosaur evolution than previously anticipated.