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Fig. 16 | BMC Evolutionary Biology

Fig. 16

From: Internal cranial anatomy of Early Triassic species of †Saurichthys (Actinopterygii: †Saurichthyiformes): implications for the phylogenetic placement of †saurichthyiforms

Fig. 16

Cranial fossae of the occipital and otic regions and hyoopercular muscle attachment fields of selected actinopterygians. aLawrenciellla schaefferi (redrawn from [67]); b Polypterus (redrawn from [79]); cSaurichthys (based on NHMD_157546_A); d Acipenser brevirostrum (based on FMNH 113538); e Amia calva (redrawn from [70]). Craniospinal fossa: fossa on the posterior surface of the craniospinal processes, for the accommodation of epaxial muscle segments. Absent when craniospinal processes are absent; Tectosynotic fossa: paired fossae bounded laterally by the arch of the posterior semicircular canal. Non-homologous across taxa. Fossa bridgei: depression roughly constrained by the planes of the three semicircular canals. Absent when the dermatocranium is fused to the dorsal part of the neurocranium (e.g., in Polypterus); Posttemporal fossa: on the posterior part of the otic region, but lateral to the posterior semicircular canal. In most neopterygians it is confluent with the fossa bridgei, which opens posteriorly to receive epaxial segments; Spiracular fossa: depression formed around the dorsal exit of the spiracle; Prespiracular fossa: small fossa lateral to the spiracle and the anterior semicircular canal, dorsal to the horizontal semicircular canal and near to the dorsolateral margin of the braincase. Present in some late Paleozoic–early Mesozoic generalized actinopterygians; Hyoopercular constrictor fields: Origins of the hyoid and opercular constrictor muscles. Hypothesized in fossil taxa. These attachment fields migrate according to the changes in the orientation of the suspensorium. In †Saurichthys the hyoopercular musculature likely originated in the deeper, posterior part of the fossa bridgei and the tectosynotic fossa. In Amia there is no fossa developed, but the origin of the muscle corresponds topologically to the subtemporal fossa in †Australosomus and many fossil neopterygians. Drawings not to scale

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