Acamptonectes

Acamptonectes Temporal range: Hauterivian ~
Specimen SNHM1284-R in State Natural History Museum, Braunschweig
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	†Ichthyosauria
Family:	†Ophthalmosauridae
Subfamily:	†Ophthalmosaurinae
Genus:	†Acamptonectes Fischer et al., 2012
Species:	†A. densus
Binomial name
†Acamptonectes densus Fischer et al., 2012

Acamptonectes is a genus of ophthalmosaurid ichthyosaurs, a type of dolphin-like marine reptiles, that lived during the Early Cretaceous around 130 million years ago. The first specimen, a partial adult skeleton, was discovered in Speeton, England, in 1958, but was not formally described until 2012 by Valentin Fischer and colleagues. They also recognised a partial subadult skeleton belonging to the genus from Cremlingen, Germany, and specimens from other localities in England. The genus contains the single species Acamptonectes densus; the generic name means "rigid swimmer" and the specific name means "compact" or "tightly packed".

A small ichthyosaur, Acamptonectes is estimated to have been 3 metres (9.8 ft) long. The generic name refers to unusual adaptations in the body of Acamptonectes that made its trunk rigid, including tightly fitting bones in the occiput (back and lower part of the skull) and interlocking vertebral centra ("bodies" of the vertebrae), which were likely adaptations that enabled it to swim at high speeds with a tuna-like form of locomotion. Other distinguishing characteristics include an extremely slender snout and unique ridges on the basioccipital bone of the braincase. As an ichthyosaur, Acamptonectes had large eye sockets and a tail fluke. Acamptonectes was similar in morphology to the related but earlier ophthalmosaurines Ophthalmosaurus and Mollesaurus.

The discovery of Acamptonectes had significant implications for the evolutionary history of ichthyosaurs. The generalised platypterygiine ophthalmosaurids were long believed to be the only lineage of ichthyosaurs that survived into the Early Cretaceous following a mass extinction of ichthyosaurs across the Jurassic–Cretaceous boundary. As one of the first-known ophthalmosaurine ophthalmosaurids from the Early Cretaceous, the discovery of Acamptonectes provided evidence against such a mass extinction. Although the larger eyes of Acamptonectes would have made it better adapted than platypterygiines to depth diving, it was probably a generalist predator. Its teeth, which were slender and textured with longitudinal ridges, were adapted for impaling prey, which suggests it likely fed on soft, fleshy prey such as fish and squid.

History of discovery

Basicranium (A–E), stapes (F–G), and teeth (J–K) of the holotype: The scientific name partially refers to the tightly fitting bones of the occiput; the arrow by the exoccipital (E) shows a notch that would closely fit a bump on the basioccipital.

Over a series of weekends in 1958, four students and a technician from Hull University's geology department collected a fossil specimen from the Speeton Clay Formation at Speeton in northern England. The fossil belonged to an ichthyosaur or "fish lizard", a Mesozoic group of marine reptiles; it consists of a partial adult skeleton that includes a fragmentary skull roof, a mandible, vertebrae, ribs, and the scapular girdle (the shoulder area). In 1991, it was transferred to the Hunterian Museum of the University of Glasgow (GLAHM) when the geology department of Hull University was closed. It was catalogued under the specimen number GLAHM 132855, and was also known as the "Speeton Clay ichthyosaur". Palaeontologist Robert M. Appleby described the specimen and assigned it to the genus Platypterygius as the species "P. speetoni" (which he considered primitive within that genus), in a monograph that remained unpublished at the time of his death in 2003. A second specimen of the species was found in 1985, also in the Speeton Clay, and is catalogued as NHMUK R11185 at the Natural History Museum, London (NHMUK). This specimen consists of a partial rostrum (snout) and mandible, fragmentary ribs, and a complete right humerus (upper-arm bone).

Palaeontologist Jeff Liston recognised the significance of the Speeton Clay ichthyosaur while working at the Hunterian Museum, and Appleby's widow Valerie asked him to help finish Appleby's unpublished monograph. Liston approached ichthyosaur specialist Valentin Fischer about writing a description of the animal. Fischer examined the specimen in 2011 and realised it represented the same ichthyosaur as a specimen from Cremlingen in northern Germany about which he had recently written a draft paper with several colleagues. This German specimen was discovered in 2005 when private fossil collector Hans-Dieter Macht found some vertebrae in a construction area. Macht notified the director of the State Natural History Museum of Braunschweig (SNHM), whereafter excavation began; the specimen was collected within three days because construction work had to continue. It was prepared and mounted in 2005 at the museum, where it is catalogued as SNHM1284-R. It consists of a partial skeleton of a subadult and includes a fragmentary skull roof, a complete mandible, a partial axial skeleton, and a partial scapular girdle. It was assigned to the genus Platypterygius in a 2008 article.

Historically, the genus Platypterygius has been treated as a catch-all wastebasket taxon for Cretaceous ichthyosaurs which contained multiple distantly-related species. Liston and Fischer recognised the specimens were distinct from other species referred to Platypterygius and belonged to a new species and genus. After determining the Speeton Clay specimen is much larger than the Cremlingen specimen and thus likely more mature, Liston and Fischer decided to make it the holotype of the new species because juvenile specimens often have characteristics absent in adults. The Cremlingen specimen and the other Speeton Clay specimen became paratypes (additional specimens in the series of type specimens). In 2012, a team of palaeontologists led by Fischer formally named the new genus and species Acamptonectes densus. The generic name is derived from the Greek words akamptos and nektes, which together mean "rigid swimmer"; the specific name means "compact" or "tightly packed". In full, the scientific name refers to the robust, tightly fitting bones of the occiput (back and lower part of the skull) and the tightly interlocking centra ("bodies") of the cervical (neck) and dorsal (back) vertebrae. The holotype, GLAHM 132855, was listed under the incorrect specimen number GLAHM 132588 in the original version of the description.

Multiple basioccipitals (a bone at the lower part of the occiput), stapes (one of the ear bones), and a basisphenoid (a bone within the lower part of the braincase) from the Cambridge Greensand Formation of Cambridge, England, were also assigned to Acamptonectes sp. (i.e., an uncertain species within the genus Acamptonectes) by Fischer and colleagues in 2012. This assumed that Acamptonectes was the only Cretaceous ichthyosaur of the subfamily Ophthalmosaurinae from Eurasia. Some of the specimens are housed at the Sedgwick Museum of Earth Sciences, University of Cambridge (CAMSM), and several others are housed at the Hunterian Museum and the Natural History Museum. Several of the bones are essentially identical to those of A. densus, while others differ in some details. The bones are generally small so their differences from A. densus were considered to be related to either the ages of the animals or evolutionary changes. In 2014, Fischer and colleagues identified a basioccipital and humerus belonging to Ophthalmosaurus (or a closely related ichthyosaur) from Berriasian-aged rocks (dating to between 145 and 139.8 million years old) near Nettleton, Lincolnshire. Therefore, since their prior assumption no longer held, Fischer and colleagues re-listed the Cambridge Greensand specimens as belonging to indeterminate ophthalmosaurines that are not identifiable below the subfamily level.

Additional Acamptonectes remains known may also include an ichthyosaur specimen from Hannover, Germany. In 1909, German palaeontologist Ferdinand Broili named it as a new species of the genus Ichthyosaurus, Ichthyosaurus brunsvicensis, but considered its assignment to the genus tentative. Broili's specimen consisted of an incomplete basicranium (skull base) and an incomplete interclavicle (a bone between the clavicles, or collar bones); however, the specimen was destroyed during World War II. Palaeontologist Christopher McGowan regarded it as a member of Platypterygius in 1972 and 2003 but Fischer and colleagues assigned it to cf. Acamptonectes (i.e., possibly Acamptonectes or a related animal). They found it similar in several features to Acamptonectes but also different in others; they suggested the specimen was a juvenile because of the size and shape of its basicranium. Due to its fragmentary and inaccessible nature, they considered I. brunsvicensis a nomen dubium (dubious name).

Description

Life restoration

Acamptonectes was a small ichthyosaur, with an estimated body length of 3 metres (9.8 ft). Like other ichthyosaurs, it had a long, thin snout, large eye sockets, and a tail fluke that was supported by vertebrae in the lower half. Ichthyosaurs were superficially similar to dolphins and had flippers rather than legs, and most (except for early species) had dorsal fins. Although the colour of Acamptonectes is unknown, at least some ichthyosaurs may have been uniformly dark-coloured in life, which is evidenced by the discovery of high concentrations of eumelanin pigments in the preserved skin of an early ichthyosaur fossil. Acamptonectes was similar in morphology to the related but earlier ophthalmosaurines Ophthalmosaurus and Mollesaurus. Features of the humerus in specimen SNHM1284-R are indicative of its immaturity; however, the humerus lacks the sandpaper-like texture of the humeral shafts (shafts of the upper arm bones) of juvenile ichthyosaurs and is thus thought to represent a subadult. The holotype and specimen NHMUK R11185 are large compared to other members of the wider family Ophthalmosauridae; the holotype is thought to have been an adult because of the extensive fusion of its bones, including within the occiput, and the smooth texture of the humerus.

Skull

Nasal bones (A–B), supratemporal (C), and supraoccipital (D–F) of the holotype

The snout of Acamptonectes was elongated and extremely slender; in the holotype, it is only 45 mm (1.8 in) wide in front of the bony nostrils. The snout was also only 0.044 times as deep as it was long, one of the lowest ratios among ophthalmosaurids. Much of the snout was formed by the premaxillae, which formed the front portion of the upper jaw. The fossa praemaxillaris, a groove that ran parallel to the tooth row of the upper jaw, was deep and continuous, and ended in a series of aligned foramina (depressions). Behind and above the premaxillae were the nasals, which the holotype preserves in three dimensions, documenting the shape of the upper side of the snout. The back part of the nasal had a downward-extending bulge that was similar to that of related genera such as Ophthalmosaurus. This bulge gave rise to a short, robust, wing-like extension that formed an overhang over the rear of the bony nostril; this feature was also present in Ophthalmosaurus and Platypterygius australis. The edge of this overhang was roughened, indicating this was probably the attachment site for a soft tissue structure. The back part of the skull roof is incompletely known from the hind part of the lacrimal bone (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 that formed the rear corners of the skull roof. A forward-directed extension of the supratemporal formed the internal rear edge of the supratemporal fenestra, an opening in the skull roof situated behind the eyes. The parietal, which would have formed the inner margin of the supratemporal fenestra, had a convex front margin that would have interdigitated (interlocked) with either the frontal or postfrontal bones, which are not preserved in the known specimens.

Skull and partial neck of SNHM1284-R seen from below, with interpretative diagram

When viewed from the side, the quadrate bone, which connected to the lower jaw to form the jaw joint, was C-shaped. Two probable