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 fossil belonged to an ichthyosaur or "fish lizard", a Mesozoic group of marine reptiles;[1] it consists of a partial adult skeleton that includes a fragmentary skull roof, a mandible, vertebrae, ribs, and the scapular girdle (the shoulder 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.

[3] 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.

[3][10][11] Acamptonectes, like other ichthyosaurs, had a long, thin snout, large eye sockets, and a tail fluke that was supported by vertebrae in the lower half.

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).

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 stapes had a shaft that was more slender than in any other ichthyosaur, and its head was large and square; these features are regarded as an autapomorphy—a characteristic that distinguishes the genus from related genera.

[3][14] The supraoccipital at the upper rear of the braincase (part of the skull which encloses the brain) was only weakly arched; it thus differed from those of Platypterygius and Baptanodon, which were U-shaped.

The only-known complete tooth crown was small compared to those of other ophthalmosaurids; it was also slender and sharply pointed, and similar to the teeth from the rear of the jaw in Baptanodon.

In the rear dorsal vertebral column, the centra became shorter and higher; this trend peaked at the first caudal (tail) vertebra, which was 3.12 times as high as it was long.

The front dorsal vertebrae have diapophyses (sideways-protruding processes to which ribs attach) fused to the centra; this feature was shared with several other ophthalmosaurids.

[3] The coracoid (a paired bone in the scapular girdle) was roughly hexagonal, contrasting with the rounded shape in Platypterygius, and had outer and midline edges that were straight and parallel.

At the front, the mid-line margin was strongly deflected outward, forming the rugose (roughened and wrinkled) edge of a wide, sheet-like process similar to that in Ophthalmosaurus.

The lower part of the scapula was expanded from front to back, forming a wide, rugose, articular, tear-drop-shaped surface that articulated with the coracoid and glenoid facets.

[3] The deltopectoral crest (to where the deltoid muscle attached) on the upper-front part of the humerus was more prominent in Acamptonectes than in Ophthalmosaurus and Arthropterygius, but less so than in Sveltonectes and Platypterygius.

On the opposite side of the upper humerus, the trochanter dorsalis (a tubercle or protrusion where muscles attached) was tall and narrow, as in Sveltonectes and many species of Platypterygius.

[3] In 2012, a phylogenetic analysis conducted by Fischer and colleagues found Acamptonectes to be a member of the family Ophthalmosauridae based on several characteristics.

[8] Fischer and colleagues placed Acamptonectes was placed in the former clade, although its placement there represented a secondary reversal of the group's only uniting characteristic; a notch on the bottom of the basioccipital.

natans was formed on account of the reduced presence of striations on the teeth, although Fischer and colleagues indicated this characteristic was homoplastic so they did not consider it sufficient to resurrect the previously used genus name Baptanodon for "O."

natans and O. icenicus to form a clade with the exclusion of Mollesaurus and then Acamptonectes,[27] which was also recovered by Megan Jacobs and David Martill in their 2020 description of Thalassodraco.

[28] A 2014 analysis of the description of Janusaurus conducted by Aubrey Roberts and colleagues found Acamptonectes to be the sister group to a clade consisting of O. icenicus and Leninia, which collectively constituted one branch of the Ophthalmosaurinae.

[33] In 2019, Maxwell, Dirley Cortés, Pedro Patarroyo, and Parra Ruge recovered a poorly-resolved Ophthalmosauridae containing Acamptonectes in a large polytomy.

[8][35] This decline was thought to have been associated with a transition in the dominant ichthyosaur lineage; the large-eyed, thunniform (tuna-like) ophthalmosaurines, which were successful and widespread notwithstanding their hyper-specialisation, would have been replaced by the more generalised platypterygiines, which had smaller eyes and longer bodies.

[8] The tightly packed occipital bones and cervical vertebrae would have allowed limited movement in the neck, suggesting Acamptonectes must have "shot through the water like a dart", according to fellow describer Ulrich Joger.

[47] In the related genus Ophthalmosaurus, the maximum diameter of the eyeball would have been 23 centimetres (9.1 in), allowing movement to be detected at depths of 300 metres (980 ft) in the mesopelagic zone.

[49] Acamptonectes is known from rocks dating to the Hauterivian stage of the Lower Cretaceous (approximately 133 to 129 million years old[50]) in the Speeton Clay Formation of England, which is composed of claystone and mudrock, and is generally about 100–130 metres (330–430 ft) thick.

[51] Evidence of photosynthetic organisms indicate the Speeton Clay environment was at least partially located in the photic zone (the layer in the ocean that light reaches).

[54] Numerous other organisms have been recovered from the Speeton Clay Formation; many of these were borers, including foraminiferans, fungi, chlorophyte algae, and various animals such as sponges, polychaetes, brachiopods, barnacles, bivalves, and echinoids.