Acamptonectes

Acamptonectes is an extinct genus of ophthalmosaurid ichthyosaur known from England and Germany. Acamptonectes is one of only eight genera of ichthyosaurs known to have existed into the Cretaceous. Fossils have been collected from the Hauterivian stage of England and Germany and from the Cambridge Greensand Formation, eastern United Kingdom, dating to late Albian or early Cenomanian stage, of the Early Cretaceous-Late Cretaceous boundary. Acamptonectes was first described in 2012 and the type species is Acamptonectes densus. The first and most complete specimen of Acamptonectes was discovered in 1958 in the Speeton Clay formation of North Yorkshire in England, but its description was never formally published. At the time, its describer regarded the fossil as a new species of Platypterygius, another genus of ophthalmosaurid ichthyosaur. In 2003, a new ophthalmosaurid ichthyosaur was discovered in Cremlingen, Germany. This was determined to be a new taxon, and the characteristics unique to it were also determined to exist in the 1958 Speeton Clay 'Platypterygius'. A second Speeton Clay specimen was discovered in 1985. Ultimately, a collaboration was created amongst the various scientists who had studied these three specimens, and the taxon was formally named in 2012 by Valentin Fischer and colleagues, using the original 1958 specimen (which was determined with certainty not to represent Platyptergius after all) as the holotype and the two later finds as paratypes. Ichthyosaurus brunsvicensis described in 1909 was regarded as cf. Acamptonectes in the 2012 study. The holotype (GLAHM 132588), an adult individual, consists of a fragmentary skull roof, a complete mandible, a fragmentary spine and a partial scapular girdle. Paratype SNHM1284-R is an incomplete sub-adult skeleton including parts of the skull roof, a complete mandible, as well as parts of the spine and shoulder girdle. Paratype NHMUK R11185 comprises parts of the snout and mandibles as well as a right humerus (upper arm bone) and fragmentary ribs. Acamptonectes is similar in morphology to the related but earlier ophthalmosaurids Ophthalmosaurus and Mollesaurus. It is morphologically distinct from other ophthalmosaurines in a variety of ways. Its binomial name, meaning 'tightly-packed rigid swimmer', refers to the tightly-packed occipital bones and cervical vertebrae that would have allowed little movement in the neck, indicating it must have 'shot through the water like a dart.' Most of its skeleton appears to be unusually rigid, which would have in effect severely limited the amount of side-to-side motion that was possible in the anterior part of the skeleton. Its snout is also shallower and its teeth more slender-crowned and sharply pointed than related species, and its ribs are also more robust rounded in cross-section, which may be a further adaptation to increase the stiffness of the animal's body, as they were likely more resistant to bending. Like other ichthyosaurs, Acamptonectes was a predator and probably fed on fish and squid, and its overall body plan would have been similar in appearance to that of a dolphin. The snout was elongated and extremely slender – in front of the bony nostrils, it was only 45 mm wide in the holotype specimen. The snout also was only 0.044 times as deep as long, which is one of the lowest ratios found in ophthalmosaurids. Much of the snout was formed by the premaxillae, which formed the front portion of the upper jaw. Behind and above were the nasals, which are three-dimensionally preserved in the holotype, documenting the shape of the upper side of the snout. On its back part, the nasal featured a downward extending bulge, similar to that seen in some related genera including Ophthalmosaurus. This bulge also gave rise to a short but robust winglike extension that formed an overhang over the hind part of the bony nostril; this feature was also present in Ophthalmosaurus icenicus and Platypterygius australis. The edge of this overhang was roughened, indicating that this was probably the attachment site for a soft tissue structure. The hind part of the skull roof is only incompletely known, including the hind part of the lacrimal (in front of the eye opening), the postfrontal (above and behind the eye opening), the parietal (at the rear of the skull roof) and parts of a supratemporal (which formed the rear corners of the skull roof). An forward-directed extension of the supratemporal formed the inner-rear edge of the supratemporal fenestra, an opening through the skull roof behind the eyes. The parietal, which would have formed the inner margin of the supratemporal fenestra, had a complex front margin that was interdigitating with either the frontal or postfrontal bones, which are not preserved in the known specimens.:20–22 The quadrate bone, which connected to the lower jaw to form the jaw joint, was C-shaped in side view. Two probable hyoid bones (tongue-bones) are preserved with specimen SNHM1284-R; these bones were rod-like, with one end spatula-shaped. The stapes (bone one the ear region that is used for hearing) had a shaft that was more slender than in any other ichthyosaur, and its head was large and square—these features are regarded an autapomorphy (unique feature) and serve to distinguish the genus from related genera. The basisphenoid, a bone of the lower part of the braincase, had a well-developed crest on its upper surface, which also is considered an autapomorphy. In other ichthyosaurs the basisphenoid instead formed a wide plateau. At its front end, the basisphenoid was fused to the parasphenoid, and no suture (border between the two bones) can be seen. The supraoccipital at the upper rear of the braincase was only weakly arched and thus different from Platypterygius and Ophthalmosaurus natans, where it was U-shaped. Below the supraoccipital were the two exoccipitals, which formed the sides of the foramen magnum, the canal for the spinal cord. Further below, and forming the floor of the foramen magnum, was the basioccipital. The midline canal that formed this floor was bordered by ridges, giving a bilobed appearance when seen from above, which is regarded an autapomorphy of the genus. Below the foramen magnum, the basioccipital formed the occipital condyle to connect with the first vertebra of the neck, forming the head joint. The occipital condyle was well demarcated from the remainder of the bone by a constricted band, unlike in most other ophthalmosaurids. The condyle was rounded and had visible growth rings, as in related genera. The opisthotics, which sat at both sides of the basioccipital, had extensions pointing backwards and upwards, the paroccipital processes. These processes were elongate and slender in Acamptonectes and Ophthalmosaurus icenicus but short and stout in other ophthalmosaurids. Acamptonectes is a significant find in that it demonstrates that the more specialized ichthyosaurs were not entirely wiped out at the extinction marking the end of the Jurassic period, which up until now had been the common interpretation of the fossil record for this clade. The discovery of Acamptonectes indicates that ophthalmosaurids remained diverse until their total extinction at the end of the Cretaceous. The phylogenetic analysis performed by Fischer et al. indicates that the family Ophthalmosauridae underwent an early divergence into two very different subfamilies, Ophthalmosaurinae and Platypterygiinae. Both clades crossed the Jurassic-Cretaceous boundary and lasted at least until the Albian of the early Cretaceous.However, because of the lack of complete skeletal remains for most of the specimens, Acamptonectes forms a polytomic clade with Cryopterygius kristiansenae, Ophthalmosaurus icenicus, O. natans, Paraophthalmosaurus kabanovi, P. saveljeviensis, and Undorosaurus gorodischensis. With their dolphin-like bodies, ichthyosaurs were better adapted to an aquatic environment than any other group of marine reptiles. They were viviparous (gave birth to live young) and likely incapable of leaving the water. As homeotherms with high metabolic rates, ichthyosaurs would have been active swimmers. Jurassic and Cretaceous ichthyosaurs, including Acamptonectes, had evolved thunniform (tuna-like) swimming, as opposed to the anguilliform (undulating) mode of locomotion employed by earlier species. Thunniform ichthyosaurs were able to swim faster and more efficiently than other marine reptiles of similar sizes, and better adapted to a pelagic lifestyle. This method of swimming was aided by their more compact bodies and crescent-shaped caudal fins. This suggests that these ichthyosaurs were pursuit predators.