Welcome to Acamptonectes
Name Definition
Rigid swimmer
Name Given By
Fischer et al., 2012
Location
Cambridge Greensand Formation & Speeton Clay Formation of England, Cremlingen of Germany
Classification
Reptilia, Ichthyosauria, Ophthalmosauridae, Ophthalmosaurinae
Size
Around 3 meters long (9.8 ft)
Temporal Range
Hauterivian age of the Early Cretaceous, about 133 - 129 million years ago
Ecological niche
Aquatic pursuit predator
Species/Sub Species
A. densus
Diet
Acamptonectes had teeth that were designed to impale prey which means that they would have fed on fleshier prey like fish and squids instead of harder shelled animals
Introduction
Acamptonectes is a genus of ophthalmosaurid ichthyosaur that lived in Europe during the Early Cretaceous. The generic name means "rigid swimmer" in reference to the odd adaptations that made its trunk (basically the torso) rigid, which would most likely allow it to swim at high speeds in a tuna-like manner, where there is minimal head to body movement relative to the movement of the tail, which keeps the body in a streamlined position and reduces drag. The specific name, densus, means "compact" which refers to the tightly-packed bones of the occipital, or the bone that rests at the lower half in the back of the skull, as well as the tightly-interlocking centra (the body of a vertebra) of the cervical and dorsal vertebrae.
Acamptonectes was originally discovered in 1958 from the Speeton Clay Formation in Yorkshire, northern England, but it was never originally described at the time of its discovery. The original specimen consists of a partial adult skeleton with a partial skull roof, a mandible (lower jaw), some vertebrae, ribs, and a scapular girdle preserved. Another specimen was unearthed in 1985 and was also from the Speeton Clay Formation and was catalogued as NHMUK R11185 and currently resides in the Natural History Museum in London. In 1991, the original specimen of Acamptonectes, catalogued GLAHM 132855, was transferred to the Hunterian Museum of the University of Glasgow in Scotland. It was dubbed the "Speeton clay ichthyosaur" and was originally named as a species of Platypterygius, P. speetoni, and was considered a rather basal species within that genus. However, this specimen was examined again in 2011 by Fischer and was realized to have represented the same species of ichthyosaur as a specimen from Cremlingen in northern Germany that he had recently written a paper about with some of his colleagues, and this German specimen was discovered in 2005 by private fossil collector Hans-Dieter Macht who found some vertebrae in a construction area. Fischer recognized this specimen as a distinct species from other species of Platypterygius and that these specimens belonged to a new genus and species. Then, in 2012, Acamptonectes was officially named by Fischer and colleagues, and the original specimen from the Speeton Clay had become the holotype specimen, while the second one from the Speeton Clay and the Cremlingen specimen from Germany were paratypes, in other words, some other specimens that the authors of a genus's description wanted to add along with the holotype. Acamptonectes was also discovered to be closer to ichthyosaurs like Ophthalmosaurus and Thalassodraco instead of Platypterygius.
Like any ichthyosaur, Acamptonectes was a homeotherm with a high metabolic rate, indicating a rather active lifestyle. Acamptonectes was probably also viviparous, or that it gave live birth, a feature rather unusual in reptiles. The later more derived Jurassic and Cretaceous ichthyosaurs such as Acamptonectes adopted a thunniform (tuna-like) method of swimming, compared to the relatively earlier ichthyosaurs that used anguiliform locomotion (anguiliform means eel-like and anguiliform locomotion is usually undulatory). The more derived thunniform ichthyosaurs were capable of reaching higher swimming speeds more efficiently than not only the earlier ichthyosaurs, but also other marine reptiles of similar sizes, and they were better adapted to a pelagic (open ocean) lifestyle.
Unlike most other derived ichthyosaurs however, Acamptonectes had an unusually rigid trunk, which made undulatory or side to side motion virtually impossible at the front of the skeleton. Its snout was also shallow compared to related species, and its ribs were more rounded in cross section. Paleontologist Darren Naish, one of the describers of Acamptonectes, says that the rigid trunk may have been an adaptation to further increase the stiffness of the animal's body and also make these body parts more resistant to bending. The tightly packed occipital bones which are the only bones that connect to the cervical vertebrae, as well as the compact cervical vertebrae, would have severely limited movement in the neck, and this suggests that Acamptonectes would have "shot through the water like a dart", according to describer Ulrich Joger.
Like its related ophthalmosaurines, Acamptonectes was most likely a opportunistic generalist that fed on soft bodied prey such as squid or fish. The rigid trunk and streamlined body also suggest that Acamptonectes was built for speed, making it a pursuit predator. In 1987, paleontologist Judy A. Massare (who specializes in the work of Mesozoic marine reptiles) proposed "feeding guilds" as a way to classify marine reptiles. Some ichthyosaurs possessed small needle-like teeth that were adapted for piercing small prey, while others had more robust dentition for crushing which would make them durophages (an animal that feeds on hard-shelled prey). In 2012, Maria Zammit suggested that the longitudinal ridges and the slender tooth crown observed in Acamptonectes would have been used for piercing/impaling prey rather than grasping onto it, which suggests that Acamptonectes hunted soft fleshy prey rather than animals with hard exoskeletons. Regardless, she also noted that the shallow snout and the unique dental morphology may indicate a different lifestyle from other Cretaceous ichthyosaurs.
Ichthyosaurs are known to have the largest eyes of any known vertebrate group, and not only does this apply to the proportions, but also to the literal size. This suggests that most ichthyosaurs would have been able to live in low-light conditions which would be important for catching prey in the ocean's depths. The related genus Ophthalmosaurus has an eye diameter of 23 cm (9.1 in) at maximum, suggesting that it could detect movement at depths of 300 meters (980 ft) in the mesopelagic zone (also known as the twilight zone). Ophthalmosaurus could also likely dive at a depth of 600 meters (2,000 ft) and stay in the water for around 20 minutes before having to go to the surface again for air. Additionally, the enlarged olfactory region of the brain suggests that ichthyosaurs had a good sense of smell, making them efficient pursuit predators along with the low-light vision and speed. This would probably all have applied to Acamptonectes and other ophthalmosaurines.
Sources:
- https://en.wikipedia.org/wiki/Acamptonectes
- http://www.prehistoric-wildlife.com/species/a/acamptonectes.html