Dinosaurs and Other Reptiles from the Mesozoic of Mexico

6. Mexican Ichthyosaurs

Frey and Stinnesbeck 100 Plate 3; Fig. 6.2). They averaged 3 to 4 m long, but some species were up to 21 m in length, such as Shastasaurus sikanniensis. This gigantic ichthyosaur was discovered in the Late Triassic Pardonet Formation in British Colombia, Canada (Nichols and Manabe, 2004). The vertebral column consists of numerous discoid and deeply amphicoelous vertebrae, which occasionally show a central perforation (Fig. 6.3). The intervertebral articulation most likely was synovial. In many Triassic forms a notochordal tunnel, or foramen, is present that passed through most of the trunk vertebrae (Callaway, 1989; Massare and Callaway, 1990). The rib cage consisted of long and slender doubleheaded ribs with a furrow along the lateral face that stabilized the ribs against compression. Toward the pelvis the ribs taper and become singleheaded . While the caudal vertebral column of early ichthyosaurs remained straight throughout its length, that of late forms shows a characteristic tail bend where the vertebral column turns ventrally (Fig. 6.2). The ventrally curving part of the vertebral column supported the ventral lobe of a vertical tail fin that was hypocercal in Triassic and some Early Jurassic ichthyosaurs. In most Late Jurassic and Cretaceous ichthyosaurs 6.1. Schematic map of the central western hemisphere during Late Jurassic. The broken-line arrows show the possible migration routes of Ophthalmosaurus icenicus from the European Archipelago into the Paleo-Gulf of Mexico. Mexican Ichthyosaurs 101 6.2. Skeleton with body outline of a typical Jurassic ichthyosaur, Ophthalmosaurus icenicus. The dark areas represent the remains of specimen CPC 238 (see Fig. 6.6). 6.3. Ophthalmosauridae, longitudinal section of a sequence of four thoracic vertebrae of an ophthalmosaurid from the Tyndall area (Torres del Paine National Park, southern Chile); note the deeply concave articular faces of the centra. the tail fin was semi-lunate, suggesting fast locomotion (Massare, 1988; Motani, 2002a, 2002b; McGowan and Motani, 2003; Fig. 6.2). A few Triassic, most Jurassic, and all Cretaceous ichthyosaurs had a triangular dorsal fin, which, together with the limbs, controlled yaw and roll movements of the animals and thus acted as a three-dimensional stabilizer (Massare, 1988; McGowan and Motani, 2003; Lingham-Soliar and Plodowski, 2007). A dorsal fin was likely missing in the most primitive ichthyosaurs, which probably had an eel-like fin seam around the tail that continued onto the dorsum. These early ichthyosaurs supposedly were slow undulatory swimmers (Massare, 1988; Motani et al., 1996, 1998; Motani, 2002a, 2002b, 2005; McGowan and Motani, 2003). The limbs of all ichthyosaurs are flipper-shaped (Fig. 6.2). In earliest forms, such as Chaohousaurus, Utatsusaurus, or Grippia and to a lesser extent the Late Triassic ichthyosaurs too, the zeugopodials and metapodials are elongate, with an interosseal space between both zeugopodials. From the Late Triassic onward there is a trend to reduce digit 1, shorten Frey and Stinnesbeck 102 the zeugopodials, and increase the number of phalanges (polyphalangy; McGowan and Motani, 2003). While the outline of the phalanges in Utatsusaurus and Mixosaurus is hourglass-shaped, those of later forms become rectangular, hexagonal, or polygonal in outline (Young and Dong, 1972; Shikama et al., 1978; Mazin, 1981; Motani et al., 1998). In a few Late Triassic, many Late Jurassic, and most Cretaceous ichthyosaurs, the fins are characterized by a densely packed mosaic of zeugopodials, basipodials , metapodials, and autopodials (Fig. 6.2). Polydactyly frequently occurs, as does hypodactyly, the latter especially in the hind fins. Ichthyosaurs fed on squid, belemnites, and fishes, as evidenced by preserved gastric contents such as hooklets, fishbone, and scales. The elongate rostrum provided a large functional mouth opening during a lateral prey strike similar to other longirostrine aquatic vertebrates (e.g., Riess, 1986), or, in the case of edentulous or brevirostrine forms, suction feeding (Sander et al., 2011). Because of the restricted mobility and the flipper shape of the extremities, ichthyosaurs were unable to go on land to lay eggs. Instead, they gave birth to living young. Pregnant females might contain up to 11 embryos (Böttcher, 1990). The babies were born tail first, as documented by the fossil record. Immediately after birth, ichthyosaur babies had to swim to the water surface to take their first breath. After this they hunted together with their parents along the continental shelves or archipelagos, where their preferred prey lived in abundance (e.g., PardoP érez et al., 2012). Ichthyosaurs had huge eyes with respect to their skulls and thus likely relied on...