Rider diversity and skull categories. A) Braincase endocast in Emydura subglobosa (IW92) with no rider on the endocast; B) Brain tissue of the same specimen as in A in lateral (B1) and dorsal view (B2); C) an elongated rider in Chelydra serpentina (UFR VP1); D) a bulbus-like rider in Caretta caretta (NHMUK1940.3.15.1); E–F) † Plesiochelys etalloni (modified after Paulina-Carabajal et al. 2013), in posterodorsal view (E) and in oblique lateral (F) view – gray labels based on Paulina-Carabajal et al. (2013), black labels with “?” based on the presented hypothesis; G) four rider categories discussed herein, outlines based on the endocast of † P. etalloni; H) morphotypes of posterior temporal emargination discussed herein, images modified after Werneburg (2012) and Ferreira and Werneburg (2019): from left to right: Dermochelys coriacea, Chelonia mydas, Cuora trifasciata, Emydura macquarii, Chelodina expansa. I) Image of a µCT-scan of a macerated skull of Malacochersus tornieri (SMF-58702) in the cerebellum region. J) Image of a µCT-scan of a macerated skull of Podocnemis unifilis (SMF-55470) in the cerebrum region; * = partly calcified cartilage of the anterior tectal process is visible. K–Q) Contrast-enhanced stainings using PTA-solution in K) Platysternon megacephalum (R12559); L) a juvenile Dermochelys coriacea (IW1476), M) a juvenile Chelus fimbriatus (IW1148), N–O) a hatchling Caretta caretta (IW1681), P–Q) Apalone spinifera aspera (R12970). Abbreviations (following Werneburg 2011): No. 19 = musculus (m.) adductor mandibulae externus Pars profundus, No. 21 = m. adductor mandibulae externus Pars superficialis, No. 23 = m. adductor mandibulae internus Pars pseudotemporalis. For institutional abbreviations see caption to Table 1. Arrows in A–H indicate anterior.

  Part of: Werneburg I, Evers SW, Ferreira G (2021) On the “cartilaginous rider” in the endocasts of turtle brain cavities. Vertebrate Zoology 71: 403-418. https://doi.org/10.3897/vz.71.e66756