Acamptonectes
Acamptonectes | |
---|---|
Specimen SNHM1284-R in State Natural History Museum, Braunschweig
| |
Scientific classification | |
Domain: | Eukaryota |
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
The
The discovery of Acamptonectes had significant implications for the evolutionary history of ichthyosaurs. The
History of discovery
Over a series of weekends in
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Clay ichthyosaur while working at the Hunterian Museum, and Appleby's widow Valerie asked him to help finish Appleby's unpublished monograph.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
Historically, the genus Platypterygius has been treated as a catch-all
Multiple
Additional Acamptonectes remains known may also include an ichthyosaur specimen from
Description
Acamptonectes, like other ichthyosaurs, had a long, thin snout, large eye sockets, and a
Skull
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
When viewed from the side, the quadrate bone, which connected to the lower jaw to form the jaw joint, was C-shaped. Two probable hyoid 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 an autapomorphy—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 the parasphenoid (another bone within the lower part of the braincase) and no suture (border between the two bones) can be seen.[3][14]
The
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 subfamily
Postcranial skeleton
As was typical for ichthyosaurs, the vertebral centra of Acamptonectes were disc-shaped and deeply concave on both ends. The
The neural arches of the vertebrae had narrow pre- and postzygapophyses (
The
As with its coracoid, the scapula (shoulder blade) of Acamptonectes was similar to that of Ophthalmosaurus. It was strongly compressed from side to side, unlike that in P. 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-like acromial process at the front (which connected with the clavicle), like those in Ophthalmosaurus and P. americanus. The coracoid facet of the scapula was triangular and continuous with the larger glenoid facet, as in Ophthalmosaurus but unlike P. australis. The side and mid-line surfaces of the acromial process were slightly concave.[3]
The deltopectoral crest (to where the
Classification
In 2012, a
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.[17][18][19] The phylogenetic analysis conducted by Fischer and colleagues in 2012 recovered two novel clades (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[20]—but had not yet been supported robustly by the results of phylogenetic analyses.[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.[3]
Within the Ophthalmosaurinae, various positions have been recovered for Acamptonectes due to the same issues. In 2012, Fischer and colleagues found that it grouped closest with "Ophthalmosaurus" natans, with Ophthalmosaurus icenicus and Mollesaurus 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 name Baptanodon for "O." natans.
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.[29] The same arrangement was recovered by a 2017 analysis of the description of Keilhauia conducted by Lene Delsett and colleagues.[30] In 2019, another analysis by the same authors found Acamptonectes closer to Janusaurus, Keilhauia, and Palvennia than to Paraophthalmosaurus, "O." natans (as Baptanodon), O. icenicus, or Gengasaurus in successive order of closeness to the base of the Ophthalmosaurinae.[31] In each case, however, the Bremer support—a measure of the likelihood of a phylogenetic tree's arrangement over alternatives—of the groupings was low.[29][30][31]
Other analyses also found Acamptonectes within unresolved polytomies. For the 2016 description of Muiscasaurus, Erin Maxwell and colleagues found O. icenicus, "O." natans, Undorosaurus, and Acamptonectes in a polytomy at the base of the Ophthalmosauridae. Contrary to most analyses, they did not recover a distinct Ophthalmosaurinae.[32] Also in 2016, Fischer and colleagues found Ophthalmosaurinae to consist of Mollesaurus as the sister group to a polytomy including O. icenicus, "O." natans, Leninia, Acamptonectes, and a group containing Cryopterygius, Janusaurus, and Palvennia.[33] In 2019, Maxwell, Dirley Cortés, Pedro Patarroyo, and Parra Ruge recovered a poorly-resolved Ophthalmosauridae containing Acamptonectes in a large polytomy.[34] In their 2020 description of Arthropterygius thalassonotus, Lisandro Campos and colleagues placed Acamptonectes in a polytomy with O. icenicus, Leninia, and Athabascasaurus, which formed the sister group to a clade of Keilhauia and Undorosaurus; the base of the Ophthalmosaurinae was formed by a polytomy of those species, and Baptanodon and Gengasaurus.[19]
The phylogenetic tree from the analysis of Páramo-Fonseca and colleagues in 2020 is reproduced below.[26]
Ophthalmosauridae |
| ||||||||||||||||||||||||||||||||||||||||||||||||
Palaeobiogeography
Ichthyosaurs were traditionally thought to have been affected by three extinction events; one at the
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 Nettleton Ophthalmosaurus specimens. They also cited reports of the Late Jurassic-aged platypterygiines Brachypterygius, Aegirosaurus, Caypullisaurus, and Yasykovia—which has been synonymised with Nannopterygius[37]—from the Early Cretaceous.[3][8][38][39][40]
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 the cladogenesis (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]
Palaeobiology
With their dolphin-like bodies, ichthyosaurs were better adapted to their aquatic environment than any other group of marine reptiles.
Most of the skeleton of Acamptonectes 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 palaeontologist Darren 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, suggesting Acamptonectes must have "shot through the water like a dart", according to fellow describer Ulrich Joger.[12]
Diet and feeding
As an ophthalmosaurine, Acamptonectes would likely have been an opportunistic
Ichthyosaurs had the largest eyes of any known vertebrate group, which can be inferred from bones in the eye sockets known as
Palaeoecology
Speeton Clay
Acamptonectes is known from rocks dating to the
Numerous other organisms have been recovered from the Speeton Clay Formation; many of these were
Lower Saxony Basin
SNHM1284-R, the German specimen of Acamptonectes, comes from late Hauterivian rocks of the
See also
- List of ichthyosaur genera
- List of ichthyosaur type specimens
- Timeline of ichthyosaur research
References
- ^ a b c Marek, R. (2015). "Fossil Focus: Ichthyosaurs". Palaeontology Online. 5: 8. Archived from the original on 18 January 2021. Retrieved 13 June 2020.
- ^ S2CID 215213303.
- ^ PMID 22235274.
- ^ a b c Liston, J. (7 January 2012). "Missing Skull-Bones: Hidden Sea Dragon – A guest post by Jeff Liston". Mr Wood's Fossils. Archived from the original on 13 February 2020. Retrieved 13 February 2020.
- ^ a b c Seibertz, E.; Krüger, F. J. (2008). "Biostratigraphie und Paläobiogeographie des Hauterivium von Cremlingen bei Braunschweig bestimmt mit Cephalopoden (Unterkreide, Ostniedersachsen)". Braunschweiger Naturkundliche Schriften (in German). 8: 273–287. Archived from the original on 18 September 2021. Retrieved 24 July 2020.
- .
- PMID 27781178.
- ^ a b c d e f Naish, D. (3 January 2012). "'Rigid Swimmer' and the Cretaceous Ichthyosaur Revolution (part I)". Tetrapod Zoology. Scientific American Blogs. Archived from the original on 4 April 2012. Retrieved 9 November 2020.
- ^ PMID 24465427.
- ^ Broili, F. (1909). "Neue Ichthyosaurierreste aus der Kreide Norddeustschlands und das Hypophysenloch bei Ichthyosauriern". Palaeontographica (in German). 55: 295–302. Archived from the original on 24 July 2020. Retrieved 10 February 2020.
- ^ McGowan, C. (1972). "The systematics of Cretaceous ichthyosaurs with particular reference to the material from North America". Rocky Mountain Geology. 11 (1): 9–29. Archived from the original on 18 September 2021. Retrieved 20 February 2020.
- ^ a b c "German marine reptile find rewrites fossil record". BBC News. London: BBC. 5 January 2012. Archived from the original on 12 November 2020. Retrieved 9 November 2020.
- S2CID 4468035.
- ^ ISBN 3-89937-007-4.
- ^ S2CID 85352593.
- doi:10.1139/e92-034.
- S2CID 128464793.
- ^ Fernandez, M. S.; Campos, L. (2015). "Ophthalmosaurids (Ichthyosauria: Thunnosauria): alpha taxonomy, clades and names". Publicación Electrónica de la Asociación Paleontológica Argentina. 15 (1). Archived from the original on 13 June 2020. Retrieved 13 June 2020.
- ^ .
- ^ Arkhangelsky, M. S. (2001). "The historical sequence of Jurassic and Cretaceous ichthyosaurs". Paleontological Journal. 35 (5): 521–524.
- ^ S2CID 9981885.
- (PDF) from the original on 24 June 2016. Retrieved 18 June 2020.
- (PDF) from the original on 9 March 2021. Retrieved 2 July 2019.
- from the original on 18 September 2021. Retrieved 28 September 2020.
- PMID 31106052.
- ^ S2CID 228965354.
- S2CID 213102661.
- PMID 33296370.
- ^ PMID 25084533.
- ^ S2CID 28124852.
- ^ hdl:10852/76089.
- S2CID 128464793.
- PMID 26953824.
- S2CID 146059015.
- PMID 21536898.
- ^ Bakker, R. T. (1993). "Plesiosaur extinction cycles — events that mark the beginning, middle and end of the Cretaceous". Geological Association of Canada, Special Papers. 3: 641–664.
- .
- S2CID 130457040.
- S2CID 130393321.
- from the original on 16 June 2020. Retrieved 28 September 2020.
- from the original on 7 July 2019. Retrieved 8 July 2019.
- ^ a b Massare, J. A.; Buchholtz, E. A.; Kenney, J. M.; Chomat, A.-M. (2006). "Vertebral morphology of Ophthalmosaurus natans (Reptilia: Ichthyosauria) from the Jurassic Sundance Formation of Wyoming". Paludicola. 5 (4): 242–254. Archived from the original on 18 September 2021. Retrieved 8 July 2019.
- ^ S2CID 5432366.
- JSTOR 4523132.
- S2CID 15064470.
- (PDF) from the original on 27 April 2019. Retrieved 8 July 2019.
- PMID 11893757.
- S2CID 54742104.
- from the original on 18 September 2021. Retrieved 8 July 2019.
- ^ Cohen, K.M.; Finney, S.; Gibbard, P.L. (2015). "International Chronostratigraphic Chart" (PDF). International Commission on Stratigraphy. Archived (PDF) from the original on 2 April 2015. Retrieved 28 November 2020.
- ^ .
- ^ (PDF) from the original on 19 June 2020. Retrieved 18 June 2020.
- JSTOR 41701824.
- ^ .
- .
- ^ from the original on 18 September 2021. Retrieved 24 July 2020.
- ^ a b c Weinkauf, M. F. G.; Keupp, H.; Mutterlose, J. (2013). "Calcareous dinoflagellates from the Late Hauterivian (Early Cretaceous) of Frielingen, Germany". Documenta Naturae. 192 (3): 241–271. Archived from the original on 18 September 2021. Retrieved 28 September 2020.