Synapsids Developed the Diaphragm for Breathing Very Early

A caseian point for the evolution of a diaphragm homologue among the earliest synapsids

Authors:

Lambertz et al

Abstract:

The origin of the diaphragm remains a poorly understood yet crucial step in the evolution of terrestrial vertebrates, as this unique structure serves as the main respiratory motor for mammals. Here, we analyze the paleobiology and the respiratory apparatus of one of the oldest lineages of mammal-like reptiles: the Caseidae. Combining quantitative bone histology and functional morphological and physiological modeling approaches, we deduce a scenario in which an auxiliary ventilatory structure was present in these early synapsids. Crucial to this hypothesis are indications that at least the phylogenetically advanced caseids might not have been primarily terrestrial but rather were bound to a predominantly aquatic life. Such a lifestyle would have resulted in severe constraints on their ventilatory system, which consequently would have had to cope with diving-related problems. Our modeling of breathing parameters revealed that these caseids were capable of only limited costal breathing and, if aquatic, must have employed some auxiliary ventilatory mechanism to quickly meet their oxygen demand upon surfacing. Given caseids’ phylogenetic position at the base of Synapsida and under this aquatic scenario, it would be most parsimonious to assume that a homologue of the mammalian diaphragm had already evolved about 50 Ma earlier than previously assumed.

pop sci write up.

Cladistic analysis of Caseid Synapsids

Cladistic analysis of Caseidae (Caseasauria, Synapsida): using the gap-weighting method to include taxa based on incomplete specimens

Authors:

Romano et al

Abstract:

Occupying the role of primary consumer and having an early–middle Permian age range, caseids (Caseasauria, Synapsida) are fundamental to the interpretation of the early history of terrestrial vertebrate ecosystems. Despite this importance, no comprehensive, species-level phylogenetic study of Caseidae has yet been performed. Herein, we present a phylogenetic analysis of the group, using gap weighting to include poorly known taxa. Besides the description and comments on the resultant topologies, some more general issues concerning cladistic methodologies are briefly addressed. This study highlights the importance of a total-evidence approach, including as many within-group taxa and characters as possible. Continuously varying characters, in the form of indices derived from measurement of individual skeletal elements, proved to be highly important, adding significantly to the resolution of, and support for, recovered trees. The utility of the postcranial skeleton in understanding relationships among basal synapsids is highlighted.

Evidence of Parallel Evolution in Synapsids From Caseid Casea broilii

New Postcranial Material of the Early Caseid Casea broilii Williston, 1910 (Synapsida: Caseidae) with a Review of the Evolution of the Sacrum in Paleozoic Non-Mammalian Synapsids

Authors:

LeBlanc et al

Abstract:

Here we use the description of a new specimen of the small caseid synapsid Casea broilii that preserves the sacral, pelvic and hind limb regions in great detail and in three dimensions, as a unique opportunity to reevaluate the early stages in the evolution of the sacrum in the lineage that led to mammals. We place this new material in the context of sacral evolution in early caseid synapsids and conclude that the transition from two to three sacral vertebrae occurred in small-bodied species, suggesting that it was not an adaptation to heavy weight bearing. Furthermore, we compare descriptions of sacral anatomy among known early synapsids, including caseids, ophiacodontids, edaphosaurids, varanopids, and sphenacodontians and review sacral evolution in early synapsids. Based on the descriptions of new species of caseids, edaphosaurids, and varanopids over the past several decades, it is clear that a sacrum consisting of three vertebrae evolved independently at least four times in synapsids during the Late Carboniferous and Early Permian. Furthermore, similarities in the morphologies of the sacral vertebrae and ribs of these early synapsids lead us to conclude that an anterior caudal vertebra had been incorporated into the sacral series convergently in these groups. Given the repeated acquisition of a three-vertebra sacrum in early synapsids and no apparent link to body size, we argue that this sacral anatomy was related to more efficient terrestrial locomotion than to increased weight bearing.

Caseid Tracks in Lower Permian Oklahoma?

Dimetropus osageorum n. isp. from the Early Permian of Oklahoma (USA): A Trace and its Trackmaker

Authors:

Sacchi et al

Abstract:

A new ichnospecies is named as Dimetropus osageorum n. isp. within the ichnogenus Dimetropus Romer and Price, 1940. The new ichnotaxon comes from the Lower Permian Midco Member of the Wellington Formation, cropping out near Perry, Noble Co. (Oklahoma, USA), and differs from congeneric ichnospecies in the apparent heteropody and in the proportionally shorter digits. The characters of the new ichnotaxon, together with comparative analysis of footprints and of known skeletal remains, suggest referral of the trackmaker to the Caseidae, although edaphosaurid affinities cannot be excluded. Tracks referred to Dimetropus exhibit wide variation, and their respective trackmakers may be ascribed to an accordingly wide range of different zoological taxa among non-therapsid Synapsida and not only to Sphenacodontidae as has been generally believed. At the same time, the process of attributing ichnotaxa, on the basis of well preserved tracks and by comparison with known skeletal remains, is validated.

Alierasaurus ronchii: a new Giant Caseid Synapsid From Cisuralian Permian Sardinia

Alierasaurus ronchii, gen. et sp. nov., a caseid from the Permian of Sardinia, Italy

Authors:

Romano et al

Abstract:

A giant caseid, Alierasaurus ronchii, gen. et sp. nov., is herein described, based on a partial postcranial skeleton collected from the Permian Cala del Vino Formation (Alghero, Nurra, northwest Sardinia). Despite the highly conservative morphology of ribs and vertebral material, typical of caseids, the very well-preserved foot elements show highly autapomorphic features, warranting assignment of the specimen to a new genus. In particular, the fourth metatarsal is not short and massive, as in other large caseids, and shows a distinct axial region. Finally, the claw-shaped ungual phalanges are autapomorphic in being proportionately very short, with a strong double ventral flexor tubercle positioned very close to the proximal phalangeal rim, and a distal end that is not spatulate, but rather subtriangular in transverse section. Principal component and reduced major axis slope analyses, performed on 10 caseid specimens, suggest that the observed changes in overall shape of metatarsal IV were mainly linked to the enormous body size reached by the Sardinian specimen.

Eocasea martini: a new Carnivorous Caseid Synapsid From Pennsylvanian Carboniferous Kansas

The Oldest Caseid Synapsid from the Late Pennsylvanian of Kansas, and the Evolution of Herbivory in Terrestrial Vertebrates

Authors:

Reisz et al

Abstract:

The origin and early evolution of amniotes (fully terrestrial vertebrates) led to major changes in the structure and hierarchy of terrestrial ecosystems. The first appearance of herbivores played a pivotal role in this transformation. After an early bifurcation into Reptilia and Synapsida (including mammals) 315 Ma, synapsids dominated Paleozoic terrestrial vertebrate communities, with the herbivorous caseids representing the largest vertebrates on land. Eocasea martini gen. et sp. nov., a small carnivorous caseid from the Late Carboniferous, extends significantly the fossil record of Caseidae, and permits the first clade-based study of the origin and initial evolution of herbivory in terrestrial tetrapods. Our results demonstrate for the first time that large caseid herbivores evolved from small, non-herbivorous caseids. This pattern is mirrored by three other clades, documenting multiple, independent, but temporally staggered origins of herbivory and increase in body size among early terrestrial tetrapods, leading to patterns consistent with modern terrestrial ecosystem.