Geosciences

A nearly complete skull and associated osteoderms from the Middle/Upper Triassic Madygen Formation of Kyrgyzstan are referred to a new chroniosuchid genus and species. The new taxon is characterized by a parabolic skull outline, pustular ornamentation, tabular-squamosal contact, marked postparietal embayments, and the lack of an antorbital fontanelle. The palate is only preserved in part, showing broad palatines and ectopterygoids. Presence of a preorbital fenestra and characteristic osteoderm morphology are synapomorphies shared with all other chroniosuchids. According to the phylogenetic analysis performed, the new chroniosuchid nests with Chroniosaurus, with which it shares the wide, transversely extended osteoderms and pustular ornamentation. The chroniosuchians are robustly supported as a natural group, but their position within the reptiliomorph (stem-amniote) clade is not adequately understood. Whereas the parasphenoid is similar to that of anthracosaurs, most other characters support a higher nesting of chroniosuchians within the stem-amniotes.

Dermal ossifications are widespread in Permian and Triassic tetrapods, but only members of the Chroniosuchia possess a series of dorsal osteoderms with a complex plate-to-plate articulation mechanism in addition to a contact between each osteoderm and its associated vertebral spine. The stratigraphically youngest chroniosuchid, Madygenerpeton pustulatus, from the Triassic of Kyrgyzstan provides new insight on the function of the chroniosuchian osteoderm system. Osteoderms of M. pustulatus are broad, peaked-roof-shaped to arched, with enlarged posterodorsal and anteroventral articulation facets bearing unique sets of concentric rail-like ridges and furrows. Supplementing the multiple-overlap chroniosuchiantype articulation, the interlocking ridges and furrows confined the relative motion of two neighboring osteoderms to a rotation in slightly oblique and curved contact planes. Given the significant lateral narrowing of the dorsal ornamented non-overlap area, the horizontal component of the plate-to-plate rotation angles could reach up to 7.5°, enabling more extensive lateral flexion of the trunk than in other chroniosuchids. Considering functional analogs, the chroniosuchian osteoderm system probably stabilized the vertebral column against shearing, torsion, tension, and compression loads and thus facilitated terrestrial locomotion at the expense of trunk flexibility. With its particular morphology, the carapace of M. pustulatus, however, was more suitable for locomotion styles featuring lateral body undulation than the carapaces of Permian chroniosuchids. We interpret this speciality as a secondary adaptation to an aquatic habitat.

The microstructure of dorsal osteoderms referred to the chroniosuchid taxa Chroniosuchus, Chroniosaurus, Madygenerpeton and cf. Uralerpeton is compared to existing data on the bystrowianid chroniosuchian Bystrowiella and further tetrapods. Chroniosuchid osteoderms are marked by thin internal and relatively thick external cortices that consist of lowly vascularised parallel-fibred bone. They are structured by growth marks and, in case of Madygenerpeton, by lines of arrested growth. The cancellous middle region is marked by a high degree of remodelling and a primary bone matrix of parallel-fibred bone that may include domains of interwoven structural fibres. Whereas the convergence of Bystrowiella and chroniosuchid osteoderms is not confirmed by our observations, the internal cortex of the latter displays a significant peculiarity: It contains distinct bundles of shallowly dipping Sharpey’s fibres with a cranio- or caudoventral orientation. We interpret this feature as indicative for the attachment of epaxial muscles which spanned several vertebral segments between the medioventral surface of the osteoderms and the transversal processes of the thoracic vertebrae. This finding endorses the hypothesis that the chroniosuchid osteoderm series was part of a crocodylian-like trunk bracing system that supported terrestrial locomotion. According to the measured range of osteoderm bone compactness, some chroniosuchian species may have had a more aquatic lifestyle than others.

Of the recent sauropsid skin appendage types, only feathers develop from a cylindrical epidermal invagination, the follicle, and show hierarchical branching. Fossilized integuments of Mesozoic diapsids have been interpreted as follicular and potential feather homologues, an idea particularly controversially discussed for the elongate dorsal skin projections of the small diapsid Longisquama insignis from the Triassic of Kyrgyzstan. Based on new finds and their comparison with the type material, we show that Longisquama’s appendages consist of a single-branched internal frame enclosed by a flexible outer membrane. Not supporting a categorization either as feathers or as scales, our analysis demonstrates that the Longisquama appendages formed in a two-stage, feather-like developmental process, representing an unusual early example for the evolutionary plasticity of sauropsid integument.

Habitat partitioning and site fidelity of spawning grounds are well-documented phenomena in extant selachians, but little is known about the reproductive strategies of their fossil relatives. Here we describe the selachian fauna of the Middle to Late Triassic Madygen Formation in southwestern Kyrgyzstan, Central Asia, based on several dozen tooth crowns and egg capsules. The material is assigned to three new taxa: Lonchidion ferganensis, sp. nov., and Palaeoxyris alterna, sp. nov., being teeth and egg capsules of hybodontid sharks, and Fayolia sharovi, sp. nov., being egg capsules of probable xenacanthids. Teeth of L. ferganensis, sp. nov., were almost exclusively found in pelecypod-rich shallow lacustrine mudstones and belong to juvenile individuals. Oxygen and strontium isotope data of tooth enameloid indicate freshwater conditions of the ambient water at the time of tooth mineralization. The egg capsules are common findings in near-shore lake deposits as well. Considering the mass co-occurrence of juvenile teeth and egg capsules in the study area, we propose that hybodontid/ xenacanthid sharks recurrently occupied littoral zones of the Madygen lake for spawning. The small number of full-grown individuals points to habitat partitioning of juveniles and adults wherefore the study site is interpreted as a shark nursery. The oviposition strategies inferred from this fossil example are remarkably similar to those of modern sharks, suggesting that the reproductive patterns seen in extant sharks originated well before the Cenozoic.

Elongated skin projections of the reptile Longisquama insignis from the Triassic of Kyrgyzstan are preserved as imprints on the only skeletal specimen and on seven additional pairs of fossil slabs and counter-slabs from the same locality and horizon. The integumentary structures became a matter of debate when they were assessed as “non-avian feathers” homologous to avian feathers. Conflicting interpretations of their morphology and relationship to other appendage types arose from the ambiguity of the fossil skin impressions. On the basis of comparative description of the individual morphology of all yet known Longisquama specimens we address aspects of taphonomy, development, and function and define to what extent Longisquama’s appendages share characteristics of avian vaned feathers. We explain the existing feather similarity by their development from a filamentous primordium and a complex sequence of individual processes, some of which are reminiscent of processes observed in feather development. Such an interpretation is in agreement with a set of homologous mechanisms of appendage morphogenesis in an archosauromorph clade including Longisquama and feather-bearing archosaurs but does not necessarily require that the appendages of Longisquama themselves are feathers or high-level feather homologues.

ABSTRACT Chondrichthyan egg capsule fossils have been described since the 19th century, but the early finds were often misinterpreted as various plant organs like inflorescences or fructifications, stems of algae or sometimes even as coprolites. Since the initial discovery of these fossils our knowledge of their morphological and taphonomical variability, geographical and stratigraphical distribution, and potential producers has significantly advanced. Currently, ten distinct morphotypes of fossil and extant chondrichthyan egg capsules are discerned: Palaeoxyris; Vetacapsula; Crookallia; Fayolia; holocephalan callorhinchid, rhinochimaerid, and chimaerid types, fossils summarized as Chimaerotheca; batoid type, fossils summarized as Rajitheca; scyliorhinid type, fossils summarized as Scyliorhinotheca; extant orectolobid type; extant heterodontid type. These represent a range of morphological traits. Eight morphotypes are known from the fossil record with five types being exclusively represented by fossil finds. A phylogenetic analysis of all extant and currently known fossil chondrichthyan egg capsule types based on all available discrete morphological characters indicates that holocephalan morphotypes cluster together in one branch while the elasmobranchs form a sister branch and feature many character transformations since the Late Palaeozoic. Moreover, the enigmatic Carboniferous egg capsule morphotypes Vetacapsula and Crookallia form a clade together with the egg capsule morphotype of chimaerid holocephalans. Assignment of the very incompletely known Pennsylvanian type Scapellites is still unclear, but the preserved rhomboidal patterns on the fossil finds as result of an original composition of helicoidally twisted bands point to an elasmobranch producer. All chondrichthyan capsules can be traced back to one hypothetical primordial morphotype with a broad, lateral, ribbed collarette combining features from callorhinchid holocephalans and a putative placoderm capsule. Given the similarity of their egg capsules holocephalans might be more closely related to certain placoderms than to elasmobranchs.

ABSTRACT Body-size variation as a result of intra and interspecific competition is well known for recent reef-associated fishes but has not been documented for extinct reef dwellers, for example Mesozoic pycnodont fishes. This type of competition effect is mostly related to population density or overlapping territories and often occurs when related species compete for similar resources. In this study, the lengths of 217 prearticulars (=lower jaws) were measured for the pycnodont genera Gyrodus, Eomesodon, Proscinetes, and Ocleodus from the Late Jurassic of Lower Saxony (Germany). Prearticular length is directly related to body standard length and is, thus, a reasonable proxy for body size. In addition to significant overall size variation among taxa, comparisons of subsamples from different locations reveal a clear order in size ranges of co-occurring taxa, and that the presence or absence of similar-sized pycnodont taxa might affect prearticular length. Apart from interspecific competition, several other factors might have contributed to the observed size variation, for example taphonomic and ecological differences between locations and different age and species composition among the populations sampled.