On the “cartilaginous rider” in the endocasts of turtle brain cavities

Ingmar Werneburg,Gabriel Ferreira,Serjoscha W Evers

doi:10.3897/vz.71.e66756

Abstract

Abstract In recent years, paleoneurology became a very popular research field and hundreds of brain-endocasts were described. The interpretation of a dorsal protuberance of the brain-endocast puzzled researchers for a long time, the so-called (cartilaginous) rider. This is mainly because of technical limitations in the past and due to non-accessibility of comparative material. Using turtles as a case-study, we conducted a literature review and studied embryological data in addition to fossil and extant species’ endocasts. We assessed three hypotheses on the origin of the rider as relating to 1) the pineal gland, to 2) the blood vessel system, and to 3) skull roof elements. Based on our integrated anatomical observations, we refute the pineal gland hypothesis (1) and an exclusive blood vessel explanation (2). However, we show that, in most cases, the cartilaginous origin applies (3). The related cartilages, mainly the anterior process of the chondrocranial tectum synoticum, can persist until adulthood. Its diversity is interpreted in regard to the mechanical support for the temporal skull region, the shape of which has been shown to be in turn related to neck retraction and jaw mechanics. Finally, we highlight the value of embryological data to provide profound hypotheses for evolutionary research despite its low quantitative evaluability. We argue that it should be studied in conjunction with modern computer-aided data acquisition whenever possible.

Highlights

Thanks to the development of computed tomography (CT) in the last 20 years and its increasing application to earth and life sciences, non-destructive analyses of anatomical structures, otherwise not assessable for macroscopical research, became possible
We compared a series of μCT-scans and brain-endocast reconstructions that we prepared for other studies (i.e., Evers et al 2019; Ferreira et al 2020; Ferreira et al in press; Lautenschlager et al 2018) as well as literature data on turtle brain endocasts to analyze the anatomy of the rider (Table 1; Werneburg et al 2021)
The endocasts reconstructed based on enhanced contrast-stained specimens do not show the rider, in contrast to those based on μCT images of unstained macerated or fossil skull specimens (Fig. 1I–J)

Summary

Introduction

Thanks to the development of computed tomography (CT) in the last 20 years and its increasing application to earth and life sciences, non-destructive analyses of anatomical structures, otherwise not assessable for macroscopical research, became possible The brain, does not attach closely to the braincase in most living reptiles (Edinger 1929; Hopson 1979), whereas they are closer in macrocerebral birds and mammals (Early et al 2020; Orliac et al 2014; Knoll and Kawabe 2020). Instead, it is embedded in cerebrospinal fluid, which is produced via ultrafiltration by a highly vascular choroid plexus (tela choroidea) above the myelencephalon (Wyneken 2001). It is difficult to correlate the internal shape of the braincase directly to the external anatomy of the brain

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