Ichthyosaura

Macaluso, Loredana, Wencker, Lukardis C M, Castrovilli, Maria, Carnevale, Giorgio & Delfino, Massimo, 2023, A comparative atlas of selected skeletal elements of European urodeles (Amphibia: Urodela) for palaeontological investigations, Zoological Journal of the Linnean Society 197 (3), pp. 569-619 : 598-600

ICHTHYOSAURA SONNINI & LATREILLE, 1801

Species: Ichthyosaura alpestris * (Laurenti, 1768) .

Otic–occipitum complex ( Fig. 5C )

The prominentiae semicircularis anterioris and posterioris are clearly visible, whereas the prominentia semicircularis lateralis is variably visible (see below). Among them, the deep middle depression is smooth. The prominentia semicircularis posterioris extends posteriorly and ventrally between the cotyle and the parotic process. The circular or elliptical fenestra ovalis is ventral to the prominentiae posterioris and lateralis and between cotyle and parotic process. The fenestra is poorly visible in posterior view. The low parietal and parotic crests are variably present. When present, the parietal crest starts from the midlength of the distal edge of the tectum synoticum, being inclined on the anteriormost half of tectum synoticum until it reaches the posteriormost edge of the prominentia semicircularis anterioris, then becoming straight until the otic process. The parotic crest, when present, is laterally convex at the level of the parietal crest and laterally concave or straight close to the parotic process. In some specimens, the parotic and parietal crests are clearly visible, covering the prominentia semicircularis lateralis. In these specimens, those crests meet posterior to the anterior edge of the prominentia semicircularis anterioris, and the parietal crest, lower than the parotic crest, runs straight until the anterior edge of the prominentia and onto the otic process, reaching the anteriormost edge of the complex. The parotic process is subtriangular or rectangular in dorsal view; in posterior and anterior views, its dorsal portion develops in a more lateral direction than the ventral part. In lateral view, the parotic process is inclined, with the ventral tip being more anterior than the dorsal one. The anterior edge of the tectum synoticum in medial view is posterior to the mid-length of the complex. This structure is almost as medially extended as the hypochordal commissure, extending medially beyond the prefacial commissure, although a concavity is visible between the two commissures. A lamina connecting the fenestra ovalis with the ventral edge of the cotyle is present. The articular surface of the short, cylindrical, anteriorly pointing otic process is formed by at least two small facets with different orientations, or by a single, circular articular surface. In MDHC 352, the otic process is long. The kidney-shaped articular surface of the processus basalis forms a short, laterally developed spine. The sulcus petrosus is deep between the processus basalis and the otic process. The elliptical postoticum foramen is surrounded posteriorly by the cotyle, whereas anteriorly, the foramen prooticum is not entirely surrounded by bone. The foramen faciale is posterodorsal to the processus basalis. The auditory cavity is well defined and deep (except for MDHC 352, in which it is not deep). The basicapsular commissure extends medially beyond the prefacial one, being almost as developed medially as the hypochordal commissure, with the fenestra basicranialis clearly visible between the basicapsular and hypochordal commissures. In MDHC 391, two concavities are present in ventral view between the basicapsular and hypochordal commissures. The ventral surface is generally smooth, with a couple of foramina (other than the foramen faciale), and bears a low or no medial crest.

Remarks: In MDHC 352, the parotic crest is absent and, as a consequence, the prominentia semicircularis lateralis is clearly visible. In this specimen, the parotic process is only slightly visible, and the parietal crest does not reach the anteriormost edge of the otic–occipitum complex. In MDHC 416, the parietal and parotic crests are present, although anteriorly interrupted before meeting each other; the parietal crest is higher than the parotic one.

Atlas ( Fig. 7C, D )

The neural canal is circular in anterior view and twice as high as each occipital joint. In posterior view, the neural canal is approximately twice as wide as the circular cotyle. The occipital joints are circular or elliptical, with a horizontal (or sub-horizontal) major (or rarely minor; e.g. Fig. 7D ) axis. The articular facets of the odontoid process are separated by a wide (rarely narrow) groove on the ventral surface. In ventral view, the base of the odontoid process is wider than each occipital joint. The posteriorly enlarged neural crest is low or absent, whereas the secondary crests are variably developed, but always present. The secondary crests run parallel to the neural crest and do not contact the neural crest and the posterior edge of the neural arch. The neural spine can be present or absent. The lateral surface of the atlas is variably perforated. The incisura vertebralis cranialis is small or absent. In lateral view, the dorsal edge of the neural arch is sub-horizontal (inclination of 10–30° relative to the horizontal). The neural arch between the incisura caudalis and the cotyle is convex or sub-vertical. The maximum concavity of the incisura vertebralis caudalis is dorsal to the horizontal plane containing the maximum concavity of the incisura cranialis. The well-developed lateral crests reach the elliptical postzygapophyses posteriorly, extend beyond the latter in some cases, and generally reach the posterior edge of the neural arch. The inferior crests are low or absent. In posterior view, the neural arch is dorsally convex (U- or V-shaped), but the incisura dorsalis is sometimes visible. Less than half of the postzygapophyses extend posteriorly beyond the cotyle in lateral view. In dorsal view, the neural arch is anteriorly straight or concave (U- or V-shaped concavity), and the incisura dorsalis is clearly recognizable posteriorly, formed by the forked neural crest or by the neural arch. In dorsal view, the cotyle is not visible or slightly visible. The ventral surface is smooth or bears more than two foramina.

Remarks: In MDHC 416, the secondary crests reach the posterior edge of the neural arch.

Precaudal vertebrae ( Fig. 11B )

The precaudal vertebrae are opisthocoelous. The morphology of the neural canal is characterized by considerable variability, being pentagonal, triangular or circular and either slightly higher or lower than the condyle. The condyle is circular or elliptical, with a horizontal major axis. In lateral view, the anterior edge of the neural arch between the condyle and the elliptical prezygapophyses is inclined. Diapophyses and parapophyses are distinguishable, connected by a smooth lamina reaching their distal ends. Distally, the lamina is smooth. The transverse processes are subvertical or slightly posteriorly oriented, usually covering the incisura caudalis only marginally in lateral view. In the same view, the neural arch dorsal to the prezygapophyses is either visible or not. Onethird of the height of the vertebra is formed by the neural arch dorsal to the postzygapophyses. The neural crest is blade-like and high, starting posterior to the anterior edge of the neural arch, and being posteriorly broadened and forked. The neural spine is low or absent. In dorsal view, the medial edges of the prezygapophyses are generally convergent. The anterior zygapophyseal crests contact the dorsal part of the diapophyses. The posterior zygapophyseal crests are well developed and horizontal, contacting the diapophyses proximally or at their mid-length. Anterior and posterior ventral crests are well developed and form an asymmetrically rhomboidal or trapezoidal ventral lamina. The lateral surface of the vertebrae is generally perforated. In anterior view, a small foramen is visible in the ventral half of the proximal edge of the transverse processes (at the base of parapophyses). In lateral view, the dorsal edge of the neural arch is dorsally concave in the anterior part. The incisura vertebralis caudalis is deep, and the neural arch ventral to it is inclined or rarely convex. In posterior view, the dorsal edge of the neural arch is horizontal or dorsally convex (inverted U-shaped). The neural arch dorsal to the postzygapophyses is sigmoid or vertical in lateral view. Less than half of the postzygapophyses extends posteriorly beyond the cotyle in lateral view. In dorsal view, the neural arch is anteriorly concave (U-shaped) or flat. The condyle and the cotyle may be visible or not in dorsal view. The ventral surface is generally perforated.

Caudal vertebrae ( Fig. 13C, D )

The caudal vertebrae are high (height/length ratio ~1.3–1.4). The neural canal is pentagonal or circular, and the haemal canal is elliptical, U-shaped or triangular. The neural canal is wider and lower than the haemal canal. The transverse processes are rectangular (with the longest side vertical) in anterior view; in dorsal view, they are rectangular or triangular. The neural and haemal crests are high and posteriorly enlarged or forked; where these crests are enlarged, their dorsal surface is perforated. The lateral surface is not smooth, owing to the presence of several crests and small foramina, and with a large foramen on the haemal arch. The zygapophyseal crests are marked and horizontal or ventrally concave. The anterior ventral crests are low and generally do not reach the anterior edge of the haemal arch. The posterior ventral crests start from the posteroventral edge of the haemal arch in lateral view. In lateral view, the anterior and posterior ventral crests form an angle> 130°. In lateral view, the anterior margin of the haemal arch is convex (or anteriorly inclined) between the centrum and the anterior ventral crest and forms another convexity ventral to the anterior ventral crest. The haemal crest does not project anteriorly beyond the haemal arch. The haemal arch and crest are posteriorly convex and rounded in lateral view, both being without a posterior tip.

Figures

Figure 5. Otic–occipitum complexes of European Pleurodelinae.A, left complex of Calotriton asper (MNCN 16122). B, right complex of Euproctus montanus (BSPGM 4202). C, left complex of Ichthyosaura alpestris (MDHC 416). D, right complex of Lissotriton Ʋulgaris (MDHC 133). E, left complex of Ommatotriton Ʋittatus (MNCN 13193). F, right complex of Pleurodeles aealtl (MDHC 253). G, right complex of Triturus carnifex (MDHC 38). H, left complex of Triturus carnifex (MDHC 299). From left to right: anterior, dorsal, lateral, posterior and ventral views. Scale bars: 1 mm.

Figure 7. Atlantes of European urodeles. A, Calotriton asper (MNCN 16122). B, Euproctus montanus (BSPGM 4202). C, Ichthyosaura alpestris (MDHC 407). D, Ichthyosaura alpestris (MDHC 391). From left to right: anterior, dorsal, lateral (left lateral for A; right lateral for B–D), posterior and ventral views. Scale bars: 1 mm.

Figure 11. Precaudal vertebrae of European urodeles. A, Euproctus platycephalus (MDHC 405). B, Ichthyosaura alpestris (MDHC 352). C, Lissotriton Ʋulgaris (MDHC 135). D, Ommatotriton Ʋittatus (MNCN 13193). E, Pleurodeles aealtl (MDHC 253). F, G,Triturus carnifex (MDHC 38): F, one of the first trunk vertebrae; and G, trunk vertebra close to the pelvis. From left to right: anterior, dorsal, lateral (left lateral for A–C, E, F; right lateral for D, G), posterior and ventral views. Scale bars: 1 mm.

Figure 13. Caudal vertebrae of European urodeles.A, Calotriton asper (MNCN 16122). B, Euproctus platycephalus (MDHC 405). C, D, Ichthyosaura alpestris (MDHC 391). E, Lissotriton Ʋulgaris (MDHC 135). From left to right: anterior, dorsal, lateral (left lateral for A, E; right lateral for B–D), posterior and ventral views. Scale bars: 1 mm.