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Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia.

ABSTRACT: The phylogenetic relationships within crown Crocodylia remain contentious due to conflicts between molecular and morphological hypotheses. However, morphology-based datasets are mostly constructed on external characters, overlooking internal structures. Here, we use 3D geometric morphometrics to study the shape of the intertympanic sinus system in crown crocodylians during ontogeny, in order to assess its significance in a taxonomic context. Intertympanic sinus shape was found to be highly correlated with size and modulated by cranial shape during development. Still, adult sinus morphology distinguishes specimens at the family, genus and species level. We observe a clear distinction between Alligatoridae and Longirostres, a separation of different Crocodylus species and the subfossil Malagasy genus Voay, and a distinction between the Tomistoma and Gavialis lineages. Our approach is independent of molecular methods but concurs with the molecular topologies. Therefore, sinus characters could add significantly to morphological datasets, offering an alternative viewpoint to resolve problems in crocodylian relationships.

SUBMITTER: Perrichon G

PROVIDER: S-EPMC10184552 | biostudies-literature | 2023 Jun

REPOSITORIES: biostudies-literature

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Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia.

Perrichon Gwendal G Hautier Lionel L Pochat-Cottilloux Yohan Y Raselli Irena I Salaviale Céline C Dailh Benjamin B Rinder Nicolas N Fernandez Vincent V Adrien Jérôme J Lachambre Joël J Martin Jeremy E JE

Journal of anatomy 20230129 6

The phylogenetic relationships within crown Crocodylia remain contentious due to conflicts between molecular and morphological hypotheses. However, morphology-based datasets are mostly constructed on external characters, overlooking internal structures. Here, we use 3D geometric morphometrics to study the shape of the intertympanic sinus system in crown crocodylians during ontogeny, in order to assess its significance in a taxonomic context. Intertympanic sinus shape was found to be highly corre ...[more]

PMID: 36709416

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Project description:Modern crocodylians, including Alligator mississippiensis, have a greatly elaborated system of pneumatic sinuses invading the cranium. These sinuses invade nearly all the bones of the chondrocranium and several bony elements of the splanchnocranium, but patterns of postnatal paratympanic sinus development are poorly understood and documented. Much of crocodylomorph--indeed archosaurian--evolution is characterized by the evolution of various paratympanic air sinuses, the homologies of which are poorly understood due in large part to the fact that individual sinuses tend to become confluent in adults, obscuring underlying patterns. This study seeks to explore the ontogeny of these sinuses primarily to clarify the anatomical relations of the individual sinuses before they become confluent and thus to provide the foundation for later studies testing hypotheses of homology across extant and extinct Archosauria. Ontogeny was assessed using computed tomography in a sample of 13 specimens covering an almost 19-fold increase in head size. The paratympanic sinus system comprises two major inflations of evaginated pharyngeal epithelium: the pharyngotympanic sinus, which communicates with the pharynx via the lateral (true) Eustachian tubes and forms the cavum tympanicum proprium, and the median pharyngeal sinus, which communicates with the pharynx via the median pharyngeal tube. Each of these primary inflations gives rise to a number of secondary inflations that further invade the bones of the skull. The primary sinuses and secondary diverticula are well developed in perinatal individuals of Alligator, but during ontogeny the number and relative volumes of the secondary diverticula are reduced. In addition to describing the morphological ontogeny of this sinus system, we provide some preliminary exploratory analyses of sinus function and allometry, rejecting the hypothesis that changes in the volume of the paratympanic sinuses are simply an allometric function of braincase volume, but instead support the hypothesis that these changes may be a function of the acoustic properties of the middle ear.

Project description:Testudines are a group of reptiles characterized by the presence of a shell covered by keratinous shields. Stigmochelys pardalis is the most widely distributed terrestrial testudine in southern Africa. Although relatively common with some life history traits being well known, the growth of this species has yet to be studied in any detail. The bone microanatomy of this clade differs from that found in other amniotes, where terrestrial species tend to display characteristics normally seen in aquatic species and vice versa. A detailed histological analysis of the limb bones of S. pardalis reveals extensive variation through ontogeny. Cortical bone becomes increasingly thicker through ontogeny and is finally resorbed in the late sub-adult stage, resulting in a thin cortex and a large infilled medullary cavity. The predominant bone tissues are parallel-fibred and lamellar-zonal for the forelimbs and hind limbs respectively. The oldest individual displayed an External Fundamental System indicating that the growth rate had decreased substantially by this stage. Variability is prevalent between the forelimb and hind limb as well as between early and late sub-adults Forelimb elements exhibit characteristics such as faster growing parallel-fibered bone tissue, slightly higher vascularization and a predominance of annuli over Lines of Arrested Growth (LAG) compared to the hind limb which exhibits poorly vascularized, slower growing lamellar-zonal bone interrupted by LAGs. These differences indicate that the forelimb grew more rapidly than the hind limb, possibly due to the method of locomotion seen in terrestrial species. The extensive bone resorption that occurs from the early sub-adult stage destroys much of the primary cortex and results in a significantly different ratio of inner and outer bone diameter (p = 3.59 × 10--5; df = 28.04) as well as compactness (p = 2.91 × 10--5; df = 31.27) between early and late sub-adults. The extensive bone resorption seen also destroys the ecological signal and infers an aquatic lifestyle for this species despite it being clearly terrestrial. This supports the results of other studies that have found that using bone microanatomy to determine lifestyle in testudines does not produce accurate results.

Dataset Information

Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia.

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Ontogenetic variability of the intertympanic sinus distinguishes lineages within Crocodylia.

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