| Acamptonectes | |
|---|---|
 | |
| Specimen SNHM1284-R inState Natural History Museum, Braunschweig | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Family: | †Ophthalmosauridae |
| Subfamily: | †Ophthalmosaurinae |
| Genus: | †Acamptonectes Fischeret al.,2012 |
| Species: | †A. densus |
| Binomial name | |
| †Acamptonectes densus Fischeret al., 2012 | |
Acamptonectes is agenus ofophthalmosauridichthyosaurs, a type of dolphin-likemarine reptiles, that lived during theEarly Cretaceous around 130 million years ago. The first specimen, a partial adult skeleton, was discovered inSpeeton, England, in 1958, but was not formally described until 2012 by Valentin Fischer and colleagues. They also recognised a partialsubadult skeleton belonging to the genus fromCremlingen, Germany, and specimens from other localities in England. The genus contains the singlespeciesAcamptonectes 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. Thegeneric name refers to unusual adaptations in the body ofAcamptonectes that made its trunk rigid, including tightly fitting bones in theocciput (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 witha tuna-like form of locomotion. Other distinguishing characteristics include an extremely slender snout and unique ridges on thebasioccipital bone of the braincase. As an ichthyosaur,Acamptonectes had largeeye sockets and atail fluke.Acamptonectes was similar inmorphology to the related but earlierophthalmosaurinesOphthalmosaurus andMollesaurus.
The discovery ofAcamptonectes had significant implications for the evolutionary history of ichthyosaurs. Thegeneralisedplatypterygiine ophthalmosaurids were long believed to be the only lineage of ichthyosaurs that survived into the Early Cretaceous following amass extinction of ichthyosaurs across theJurassic–Cretaceous boundary. As one of the first-known ophthalmosaurine ophthalmosaurids from the Early Cretaceous, the discovery ofAcamptonectes provided evidence against such a mass extinction. Although the larger eyes ofAcamptonectes 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.
Over a series of weekends in1958, four students and a technician fromHull University's geology departmentcollected a fossil specimen from theSpeeton Clay Formation atSpeeton in northern England. The fossil belonged to anichthyosaur or "fish lizard", aMesozoic group of marine reptiles;[1] it consists of a partial adult skeleton that includes a fragmentaryskull roof, amandible,vertebrae,ribs, and thescapular girdle (the shoulder area). In 1991, it was transferred to theHunterian Museum of theUniversity of Glasgow (GLAHM) when the geology department of Hull University was closed. It was catalogued under the specimen number GLAHM 132855,[2] and was also known as the "Speeton Clay ichthyosaur". PalaeontologistRobert M. Appleby described the specimen and assigned it to the genusPlatypterygius as the species "P. speetoni" (which he considered primitive within that genus), in amonograph that remained unpublished at the time of his death in 2003. A second specimen of the species was found in1985, also in the Speeton Clay, and is catalogued as NHMUK R11185 at theNatural History Museum, London (NHMUK). This specimen consists of a partialrostrum (snout) and mandible, fragmentary ribs, and a complete righthumerus (upper-arm bone).[3][4]
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 fromCremlingen in northern Germany about which he had recently written a draft paper with several colleagues. This German specimen was discovered in2005 when private fossil collector Hans-Dieter Macht found some vertebrae in a construction area. Macht notified the director of theState Natural History Museum of Braunschweig (SNHM), whereafter excavation began; the specimen was collected within three days because construction work had to continue. It wasprepared 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.[3][4] It was assigned to the genusPlatypterygius in a 2008 article.[5]
Historically, the genusPlatypterygius has been treated as a catch-allwastebasket taxon for Cretaceous ichthyosaurs which contained multiple distantly-related species.[6][7] Liston and Fischer recognised the specimens were distinct from other species referred toPlatypterygius 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 theholotype of the new species because juvenile specimens often have characteristics absent in adults. The Cremlingen specimen and the other Speeton Clay specimen becameparatypes (additional specimens in the series of type specimens). In 2012, a team of palaeontologists led by Fischer formally named the new genus and speciesAcamptonectes densus. The generic name is derived from the Greek wordsakamptos andnektes, which together mean "rigid swimmer"; the specific name means "compact" or "tightly packed". In full, thescientific name refers to the robust, tightly fitting bones of theocciput (back and lower part of the skull) and the tightly interlocking centra ("bodies") of thecervical (neck) anddorsal (back) vertebrae.[3][4][8] The holotype, GLAHM 132855, was listed under the incorrect specimen number GLAHM 132588 in the original version of the description.[2]
Multiplebasioccipitals (a bone at the lower part of the occiput),stapes (one of the ear bones), and abasisphenoid (a bone within the lower part of thebraincase) from theCambridge Greensand Formation ofCambridge, England, were also assigned toAcamptonectes sp. (i.e., an uncertain species within the genusAcamptonectes) by Fischer and colleagues in 2012. This assumed thatAcamptonectes was the only Cretaceous ichthyosaur of thesubfamilyOphthalmosaurinae fromEurasia. Some of the specimens are housed at theSedgwick 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 ofA. densus, while others differ in some details. The bones are generally small so their differences fromA. densus were considered to be related to either the ages of the animals or evolutionary changes.[3] In 2014, Fischer and colleagues identified a basioccipital and humerus belonging toOphthalmosaurus (or a closely related ichthyosaur) fromBerriasian-aged rocks (dating to between 145 and 139.8 million years old) nearNettleton, 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.[9]
AdditionalAcamptonectes remains known may also include an ichthyosaur specimen fromHannover, Germany. In 1909, German palaeontologistFerdinand Broili named it as a new species of the genusIchthyosaurus,Ichthyosaurus brunsvicensis, but considered its assignment to the genus tentative. Broili's specimen consisted of an incompletebasicranium (skull base) and an incompleteinterclavicle (a bone between theclavicles, or collar bones); however, the specimen was destroyed duringWorld War II. Palaeontologist Christopher McGowan regarded it as a member ofPlatypterygius in 1972 and 2003 but Fischer and colleagues assigned it tocf.Acamptonectes (i.e., possiblyAcamptonectes or a related animal). They found it similar in several features toAcamptonectes 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 consideredI. brunsvicensis anomen dubium (dubious name).[3][10][11]
Acamptonectes is a small ichthyosaur, with an estimated body length of 3 metres (9.8 ft).[12] Like other ichthyosaurs, it had a long, thin snout, large eye sockets, and atail fluke that was supported by vertebrae in the lower half. Ichthyosaurs were superficially similar todolphins and had flippers rather than legs, and most (except for early species) haddorsal fins.[1][13] Although the colour ofAcamptonectes is unknown, at least some ichthyosaurs may have been uniformly dark-coloured in life, which is evidenced by the discovery of high concentrations ofeumelanin pigments in the preserved skin of an early ichthyosaur fossil.[14]Acamptonectes was similar inmorphology to the related but earlier ophthalmosaurinesOphthalmosaurus andMollesaurus. 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 widerfamilyOphthalmosauridae; 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.[3]
The snout ofAcamptonectes 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 thepremaxillae, 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 alignedforamina (depressions). Behind and above the premaxillae were thenasals, 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 asOphthalmosaurus. 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 inOphthalmosaurus andPlatypterygius australis. The edge of this overhang was roughened, indicating this was probably the attachment site for asoft tissue structure. The back part of the skull roof is incompletely known from the hind part of thelacrimal bone (in front of the eye opening), thepostfrontal (above and behind the eye opening), theparietal (at the rear of the skull roof), and parts of asupratemporal that formed the rear corners of the skull roof. A forward-directed extension of the supratemporal formed the internal rear edge of thesupratemporal 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 thefrontal or postfrontal bones, which are not preserved in the known specimens.[3][15]: 20–22
When viewed from the side, thequadrate bone, which connected to the lower jaw to form the jaw joint, was C-shaped. Two probablehyoid bones (tongue bones) are preserved in specimen SNHM1284-R; these bones were rod-like with one spatula-shaped end. 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 anautapomorphy—a characteristic that distinguishes the genus from related genera. The basisphenoid had a well-developed crest on its upper surface; this is considered another autapomorphy because this surface was a wide, flat plateau in other ichthyosaur species. At its front end, the basisphenoid was fused to theparasphenoid (another bone within the lower part of the braincase) and nosuture (border between the two bones) can be seen.[3][15]
Thesupraoccipital at the upper rear of the braincase (part of the skull which encloses the brain) was only weakly arched; it thus differed from those ofPlatypterygius andBaptanodon, which were U-shaped. Below the supraoccipital were the twoexoccipitals, which formed the sides of theforamen magnum (the canal for thespinal cord). Located further below was the basioccipital, which formed the floor of the foramen magnum. The midline canal that formed this floor was bordered by ridges, giving a bilobed appearance when seen from above; this is also regarded as an autapomorphy of the genus. Below the foramen magnum, the basioccipital formed theoccipital condyle, which connected with the first vertebra of the neck to form the head joint. The occipital condyle was well-demarcated from the remainder of the bone by a constricted band, unlike most other ophthalmosaurids. The condyle was rounded and had visible growth rings, as in related genera. Theopisthotics, which are on either side of the basioccipital, possessed extensions called the paroccipital processes which pointed backwards and upwards. These processes were elongated and slender inAcamptonectes andOphthalmosaurus, but short and stout in other ophthalmosaurids.[3][15]
The dentary (the tooth-bearing bone at the front of the lower jaw) was elongated, straight, and had a blunt front tip; this contrasts with the down-turned and beak-like tips of some ichthyosaurs in the subfamilyPlatypterygiinae. Thesplenial bones expanded in depth at their rear, forming the lower margin of the mandible and much of its midline surface. A groove similar in morphology to the fossa premaxilliaris of the upper jaw, termed the fossa dentalis, ran parallel to the dentary. TwoAcamptonectes specimens lack the "3"-shaped upper surface of theangular bones that are otherwise typical of ophthalmosaurids; instead, the surface in these specimens consists of a simple, flat groove bordered by two walls. Because the "3" shape is present in the holotype specimen, however, this feature may have varied between individuals or growth stages. Thearticular bone in one specimen was stouter than those in other ophthalmosaurids, which were nearly as thick as it was long. The teeth ofAcamptonectes had striated bases and quadrangular roots, as in numerous ophthalmosaurids, but they were not square like those ofPlatypterygius. Some of the roots of SNHM1284-R had resorption pits, indicating its teeth were still growing. The only-known completetooth 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 inBaptanodon. The bottom two-thirds of the crown had subtle, longitudinal ridges and was covered in a coarse texture that was nevertheless finer than those inAegirosaurus and somePlatypterygius specimens. The base of the crown was slightly bulbous and almost smooth, unlike those in other ophthalmosaurids.[3]
As was typical for ichthyosaurs, the vertebral centra ofAcamptonectes were disc-shaped and deeply concave on both ends. Theprocesses (bony projections that serve as muscle and rib attachments) projecting from the centra were greatly reduced as an adaptation for its fully aquatic lifestyle.[16] InAcamptonectes, the front-most cervical (neck) centra were high and short, and the following cervical and dorsal (trunk) centra become progressively longer. 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 remaining caudals became longer and lower again; the caudals, which comprised the fin, were as long as they were high, a feature that was previously identified only inP. platydactylus. The first two cervicals—theatlas andaxis—were fused into a single complex that was wide when viewed from the rear. The front dorsal vertebrae havediapophyses (sideways-protruding processes to which ribs attach) fused to the centra; this feature was shared with several other ophthalmosaurids. The centra of the dorsal vertebrae were autapomorphic, being tightly interlocking and having extensive posterolateral lamellae (ridges lining the rear surfaces of the centra). This interlocking stiffened the front section of the vertebral column in conjunction with the strong occiput of the skull. Such stiffening can be observed in other ichthyosaurs belonging to the wider groupThunnosauria, though not to the degree as seen inAcamptonectes.[3]
The neural arches of the vertebrae had narrow pre- and postzygapophyses (articular processes projecting forward and backward from the centra) that were unpaired (fused into a single element) in all vertebrae. In contrast, inP. hercynicus andSveltonectes, these processes were paired in the front of the vertebral column. The neural spines (large upward-projecting processes) were of variable height within each specimen; they were markedly longer in some dorsals than others, reaching 1.25 times the height of the largest centrum. These long spines may be bony extensions that are analogous to the extraneural processes, a row of bones located above the tops of the neural spines that are preserved in two juvenileStenopterygius specimens.[17] The top surfaces of the neural spines were often pitted, indicating they had acartilage covering. The ribs were distinct in being robust with a round cross-section; this contrasted with the "8"-shaped cross-section that is seen in other thunnosaurian ichthyosaurs.[3]
Thecoracoid (a paired bone in the scapular girdle) was roughly hexagonal, contrasting with the rounded shape inPlatypterygius, and had outer and midline edges that were straight and parallel. The upper and lower surfaces were slightly paddle-shaped, and the mid-line surface was eye-shaped as inOphthalmosaurus, although it was not as thick as those inSveltonectes andP. australis. The mid-line surface was unfinished and had deep pits, indicating the presence of a thick layer of cartilage. 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 inOphthalmosaurus. The process was separated from the scapular facet (articulation with thescapula) by a deep, wide notch, as in manyOphthalmosaurus specimens. The scapular facet of the coracoid was small, deeply pitted, and triangular while theglenoid facet (articulation with the humerus) was large and eye-shaped. These facets were not markedly separated, unlike those inSveltonectes, where they were set at an angle of 100°. The hind margin of the coracoid was sheet-like and lacked a notch.[3]
As with its coracoid, the scapula (shoulder blade) ofAcamptonectes was similar to that ofOphthalmosaurus. It was strongly compressed from side to side, unlike that inP. hercynicus, in which the shaft was thick and rod-like. 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. It had a large, flat, fan-likeacromial process at the front (which connected with the clavicle), like those inOphthalmosaurus andP. americanus. The coracoid facet of the scapula was triangular and continuous with the larger glenoid facet, as inOphthalmosaurus but unlikeP. australis. The side and mid-line surfaces of the acromial process were slightly concave.[3]
The deltopectoral crest (to where thedeltoid muscle attached) on the upper-front part of the humerus was more prominent inAcamptonectes than inOphthalmosaurus andArthropterygius, but less so than inSveltonectes andPlatypterygius. On the opposite side of the upper humerus, thetrochanter dorsalis (atubercle or protrusion where muscles attached) was tall and narrow, as inSveltonectes and many species ofPlatypterygius. The humerus had three facets on its lower side, including a facet for a bone at the front and a backward-deflected facet for theulna, which was also similar to that ofOphthalmosaurus, and had five articular processes. The expanded upper surface that articulated with the humerus was slightly concave and pitted, unlike that ofArthropterygius, in which the ulna's humerus facet formed a pyramidal-shaped process. The facet for theradius was straight and trapezoidal, and merged with facets for two wrist bones, the intermedium and the ulnare. The facet for the pisiform, another wrist bone, was small and triangular, and was located at the back of the ulna's lower side. The ulna had a concave and edge-like hind margin. Thephalanx bones (finger bones within the flipper) were oval as inOphthalmosaurus,Arthropterygius, and some species ofBrachypterygius; they tapered away from the body, and the edges of the peripheral phalanx bones were irregular and slightly concave.[3]
In 2012, aphylogenetic analysis conducted by Fischer and colleagues foundAcamptonectes to be a member of the family Ophthalmosauridae based on several characteristics. These include: the reduced extracondylar area (a band of bone surrounding the occipital condyle), the plate-like dorsal trochanter of the humerus, the presence of a facet at the front of the humerus' bottom end for a paddle bone, and the lack of notching in the paddle bones that was considered to behomoplastic (independently acquired). It was also found to be more closely related to other ophthalmosaurids thanArthropterygius based on the large processes of the basipterygoids (bones at the base of the braincase), the lack of a peg on the basioccipital, and the large trochanters of the femur.[3]
Relationships within Ophthalmosauridae have historically been unstable in analyses due to the fragmentary nature of many ophthalmosaurid specimens; furthermore, many ophthalmosaurid genera are known from a single specimen. Removal of these fragmentary genera, however, has degraded the resolution of analyses even further.[18][19][20] The phylogenetic analysis conducted by Fischer and colleagues in 2012 recovered two novelclades (groups) within Ophthalmosauridae; the Ophthalmosaurinae and Platypterygiinae, the existence of which had long been suspected by ichthyosaur researchers—Maxim Arkhangelsky had named the clades as subfamilies as early as 2001[21]—but had not yet been supported robustly by the results of phylogenetic analyses.[8] Fischer and colleagues placedAcamptonectes 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.[3]
Within the Ophthalmosaurinae, various positions have been recovered forAcamptonectes due to the same issues. In 2012, Fischer and colleagues found that it grouped closest with"Ophthalmosaurus" natans, withOphthalmosaurus icenicus andMollesaurus being successively less-closely related. The relationship with"O." 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 nameBaptanodon for"O." natans.[3] In 2013, they recovered the same arrangement in a derivative analysis for the description ofMalawania,[22] as did Nikolay Zverkov and colleagues in a 2015 analysis focusing onGrendelius—albeit with a clade consisting ofCryopterygius,Undorosaurus, andParaophthalmosaurus being closer toAcamptonectes thanMollesaurus.[23] Arkhangelsky and Zverkov previously recovered all of these species with the exception ofMollesaurus in apolytomy (unresolved clade) in 2014.[24] A 2019 analysis by Zverkov and Vladimir Efimov found an otherwise identical arrangement, in which the positions ofMollesaurus andAcamptonectes were exchanged,[25] which was also found in another 2019 analysis by Zverkov and Natalya Prilepskaya,[26] and in the 2020 description of a new specimen ofMuiscasaurus by María Páramo-Fonseca and colleagues, in whichMuiscasaurus was the next-closest relative of these species.[27] In their description ofAcuetzpalin, a 2020 analysis by Jair Barrientos-Laraa and Jesús Alvarado-Ortega found"O." natans andO. icenicus to form a clade with the exclusion ofMollesaurus and thenAcamptonectes,[28] which was also recovered by Megan Jacobs andDavid Martill in their 2020 description ofThalassodraco.[29]
A 2014 analysis of the description ofJanusaurus conducted by Aubrey Roberts and colleagues foundAcamptonectes to be thesister group to a clade consisting ofO. icenicus andLeninia, which collectively constituted one branch of the Ophthalmosaurinae.[30] The same arrangement was recovered by a 2017 analysis of the description ofKeilhauia conducted by Lene Delsett and colleagues.[31] In 2019, another analysis by the same authors foundAcamptonectes closer toJanusaurus,Keilhauia, andPalvennia than toParaophthalmosaurus,"O." natans (asBaptanodon),O. icenicus, orGengasaurus in successive order of closeness to the base of the Ophthalmosaurinae.[32] In each case, however, theBremer support—a measure of the likelihood of aphylogenetic tree's arrangement over alternatives—of the groupings was low.[30][31][32]
Other analyses also foundAcamptonectes within unresolved polytomies. For the 2016 description ofMuiscasaurus, Erin Maxwell and colleagues foundO. icenicus,"O." natans,Undorosaurus, andAcamptonectes in a polytomy at the base of the Ophthalmosauridae. Contrary to most analyses, they did not recover a distinct Ophthalmosaurinae.[33] Also in 2016, Fischer and colleagues found Ophthalmosaurinae to consist ofMollesaurus as the sister group to a polytomy includingO. icenicus,"O." natans,Leninia,Acamptonectes, and a group containingCryopterygius,Janusaurus, andPalvennia.[34] In 2019, Maxwell, Dirley Cortés, Pedro Patarroyo, and Parra Ruge recovered a poorly-resolved Ophthalmosauridae containingAcamptonectes in a large polytomy.[35] In their 2020 description ofArthropterygius thalassonotus, Lisandro Campos and colleagues placedAcamptonectes in a polytomy withO. icenicus,Leninia, andAthabascasaurus, which formed the sister group to a clade ofKeilhauia andUndorosaurus; the base of the Ophthalmosaurinae was formed by a polytomy of those species, andBaptanodon andGengasaurus.[20]
The phylogenetic tree from the analysis of Páramo-Fonseca and colleagues in 2020 is reproduced below.[27]
Ichthyosaurs were traditionally thought to have been affected by three extinction events; one at theTriassic–Jurassic boundary, one at the Jurassic–Cretaceous boundary, and a final extinction in the Cretaceous at the boundary of theCenomanian andTuronianages that left no survivors. Some researchers suggested theirspecies diversity declined after the mid-Jurassic, with the ichthyosaurs continuing until they disappeared at the end of the Cenomanian.[8][36] 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][37]
Acamptonectes is a significant find because it is an ophthalmosaurine from the Early Cretaceous, demonstrating the ophthalmosaurines were not entirely wiped out at the Jurassic–Cretaceous boundary. Fischer and colleagues also found evidence of other ophthalmosaurines in the Early Cretaceous by reanalyzing known material, including the NettletonOphthalmosaurus specimens. They also cited reports of the Late Jurassic-aged platypterygiinesBrachypterygius,Aegirosaurus,Caypullisaurus, andYasykovia—which has been synonymised withNannopterygius[38]—from the Early Cretaceous.[3][8][39][40][41]
By tabulating the number of genera that disappeared in each age, Fischer and colleagues found no clear boundary between individual ages from the Late Jurassic (Oxfordian) to Early Cretaceous (Aptian) that could be considered an extinction event for ophthalmosaurids. The Jurassic–Cretaceous boundary had a net extinction rate of 0 and even the highest survival rates. By counting the number of new clades that emerged, however, they computed thecladogenesis (clade formation) rate to have been lower in the Cretaceous. They concluded—contrary to traditional thinking—the Jurassic-Cretaceous extinction event had a negligible impact on ichthyosaurs compared to its impact on other marine reptiles, and that ophthalmosaurids remained diverse until their final extinction.[3]
With their dolphin-like bodies, ichthyosaurs were better adapted to their aquatic environment than any other group of marine reptiles.[16] They wereviviparous that gave birth to live young and were likely incapable of leaving the water. Ashomeotherms ("warm-blooded") with high metabolic rates, ichthyosaurs would have been active swimmers.[42] Jurassic and Cretaceous ichthyosaurs, includingAcamptonectes, had evolved a thunniform method of swimming rather than theanguilliform (undulating or eel-like) methods of earlier species.[16] Thunniform ichthyosaurs were able to swim faster and more efficiently than other marine reptiles of similar sizes,[43] and were better adapted to apelagic (open-ocean) lifestyle.[22] Their swimming was aided by their compact bodies and crescent-shapedtail fins.[16]
Most of the skeleton ofAcamptonectes appears to have been unusually rigid, which would have severely limited the extent of side-to-side motion in the front part of the skeleton. Its snout was also shallower than those in related species, and its ribs were more rounded in cross-section. According to palaeontologistDarren Naish, one of the describers of the genus, these may have been further adaptations to increase the stiffness of the animal's body by making these body parts more resistant to bending.[8] The tightly packed occipital bones and cervical vertebrae would have allowed limited movement in the neck, suggestingAcamptonectes must have "shot through the water like a dart", according to fellow describerUlrich Joger.[13]
As an ophthalmosaurine,Acamptonectes would likely have been an opportunisticgeneralist predator[9] that fed on fish and squid.[3][13] Adaptations for speed suggest thatAcamptonectes and other ophthalmosaurines were likely alsopursuit predators.[43] In 1987, paleontologistJudy A. Massare proposed "feeding guilds" as a way to group marine reptiles; some ichthyosaur species were thought to have "pierced" small prey using needle-like teeth, and others to have "crunched" hard-shelled prey using robust teeth.[1][45] In 2012, palaeontologist Maria Zammit suggested that the slender tooth crowns with longitudinal ridges seen inAcamptonectes (which placed it in the "pierce II/generalist" guild) were likely used to impale rather than grasp prey, and its diet may thus have consisted of fleshy prey that did not have a hard exterior. Nevertheless, she noted that its shallow snout and unique tooth morphology may indicate a different diet and lifestyle from other known Cretaceous ichthyosaurs.[46]
Ichthyosaurs had the largest eyes of any known vertebrate group, which can be inferred from bones in the eye sockets known assclerotic rings,[44] and would therefore have possessed sensitive low-light vision[47] that would have aided prey capture at great depths.[48] In the related genusOphthalmosaurus, 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 themesopelagic zone.Ophthalmosaurus could likely dive for around 20 minutes and reach depths of at least 600 metres (2,000 ft).[49] In addition to good eyesight, the enlargedolfactory region of the brain indicates ichthyosaurs had a sensitive sense of smell.[50]
Acamptonectes is known from rocks dating to theHauterivianstage of theLower Cretaceous (approximately 133 to 129 million years old[51]) in the Speeton Clay Formation of England, which is composed ofclaystone andmudrock, and is generally about 100–130 metres (330–430 ft) thick.[52][53] TheAcamptonectes holotype came from the D2D beds and the specimen NHUMK R11185 came from the slightly older D2C beds. Material preserved in these sediments is sometimesreworked from the underlying olderValanginian rocks rather than originating from the Hauterivian. The holotype ofAcamptonectes is partially articulated, as were some nearbycrinoid fossils, indicating the specimen was not reworked and genuinely came from the Hauterivian.[3]Carbon-13 concentration (δ13C levels in the Speeton Clay Formation increased during the Valanginian and the early Hauterivian. This may have occurred when land submerged by rising sea levels released carbon-13 into the oceans. Concentrations ofoxygen 18 (δ18O), however, increased during this time, indicating an episode of cooling;[54] specifically, δ18O levels inbelemnite fossils indicate the temperature of the Speeton Clay was about 11 °C (52 °F) at the beginning of the Hauterivian, rose to 15 °C (59 °F) during the middle part of this stage, and reverted to 11 °C (52 °F) by its end.[52] Evidence ofphotosynthetic organisms indicate the Speeton Clay environment was at least partially located in thephotic zone (the layer in the ocean that light reaches).[55]
Numerous other organisms have been recovered from the Speeton Clay Formation; many of these wereborers, includingforaminiferans,fungi,chlorophytealgae, and various animals such assponges,polychaetes,brachiopods,barnacles,bivalves, andechinoids.[55] In addition to the crinoids,[3] otherinvertebrates in the Speeton Clay Formation are represented by a wide variety ofammonites and belemnites.[56][52] While bothbony fish andcartilaginous fish—the latter group represented bysharks andrays of various types[53]—are known from the Speeton Clay, they are poorly preserved and not very abundant. Marine reptiles are uncommon in this formation; other thanAcamptonectes, they are represented by some fragmentaryplesiosaur remains.[3]
SNHM1284-R, the German specimen ofAcamptonectes, comes from late Hauterivian rocks of theLower Saxony Basin near Cremlingen in easternLower Saxony.[3] The Lower Cretaceous sediments of this basin are rich insiliclastic rocks that were deposited in the southern region of the proto-North Sea, anepicontinental sea covering much ofNorthwest Germany during the Lower Cretaceous. Since this region linked the warmerTethys Sea and the colderBoreal Sea, its environment was very susceptible to change.[57] The late Hauterivian rocks of the region were deposited in theneritic zone (shallow seas) during a time of alternatingmarine transgression andregression (rising and falling sea levels).[58] The surface waters were generally cool, although they sometimes warmed when warmer water from the Tethys Sea entered the region.[58][5]Sedimentation rates were high and the bottom waters were somewhatanoxic (oxygen deprived).[57] Organisms that inhabited this sea includedinoflagellates, ammonites, and belemnites.[58][5]
