Acamptonectes
Skull of the holotype of Acamponectes densus
- Speeton, UK
- Filey, UK
- Cremlingen, district Wolfenbüttel, Germany
- Acamptonectes densus Fischer et al., 2012
- Acamptonectes sp.
Acamptonectes is a genus of extinct ichthyosaurs from the Lower Cretaceous of Europe. The only known way Acamptonectes densus, was a relatively modern ichthyosaurs and lived around 130 million years ago in the field of contemporary Europe. Fossil specimens have been found in British Speeton and Filey and in German Cremlingen. Acamptonectes is placed in the Ophthalmosauridae within the ichthyosaurs, the closest relative is Ophthalmosaurus. That ichthyosaurs occurred with Acamptonectes to the Lower Cretaceous, contradicts an earlier thesis of a mass extinction in this group at the end of the Jurassic.
Features
The Anatomy of Acamptonectes densus probably largely followed the basic blueprint of Thunnosauria. The snout was long, only about 45 mm wide and very low. The lower jaw was set with relatively small teeth, similar to the closely related Ophthalmosaurus, and graduated with a blunt end from. The ribs were compared with closely related genera very strong, the vortex stood close together.
Geographical distribution
Acamptonectes was so far in the south of England ( Speeton Beds) and in Northern Germany ( Cremlingen ) found. Both regions were covered during the Lower Cretaceous from one part of the sea of Tethys.
Stratigraphy and temporal horizon
The layer of Speeton Beds, from which the Acamptonectes fossils, is the basal, the Cremlinger copy attributed to the upper Hauterivian. This corresponds to a time frame 133-130 million years ago. Maybe the genus handed even time on the Hauterivian addition to the Albian.
The existence of Acamptonectes during the early Cretaceous relativized the then popular theory of a mass extinction of the ichthyosaurs at the transition from the Jurassic to the Cretaceous period, which would have survived only a few representatives. In fact, in the Cretaceous, there were not only continuing the Platypterygiinae with Platypterygius and related taxa, but also the sister line Ophthalmosaurinae. A short time later, another Cretaceous ichthyosaurs were described with Leninia and Malawania, the evidence of a continued existence of different lines.
Systematics and Taxonomy
Malawania
Stenogopterygius
Chacaicosaurus
Arthropterygius
Mollesaurus
Ophthalmosaurus icenicus
" Ophthalmosaurus " natans
Acamptonectes densus
Platypterygiinae
The type kind Acamptonectes densus was in 2012 by Fischer et al. described on the basis of three fossils. As holotype was a partial skeleton of an adult animal got from the Speeton Beds ( GLAHM 132855 ); a find from Filey ( NHMUK R11185 ) and a 2005 found at highway construction work near Cremlingen fossil of a subadult animal ( SNHM1284 -R ) served as paratypes. The genus name is derived from the Acamptonectes Greek and means " rigid swimmer" ( ἄκαμπτος for " rigid, stiff" and νήκτης for " float " ), the epithet densus comes from Latin and means " tight" or "compact". Both names allude to the rigid spine of the kind which had tightly strung together vertebrae. Fischer et al. hold one or two more species possible. However, since the corresponding findings are very fragmentary, they dropped a precise assignment to a taxon and placed them only in or near the genus Acamptonectes.
An osteological analysis of the available fossils revealed a close relationship to the genus Ophthalmosaurus that existed during the late Jurassic. Both lineages diverged in a Oxfordian ( Late Jurassic). Acamptonectes was therefore a relatively modern representative of the Thunnosauria, but more primitive than about Platypterygius.