page_title: Acamptonectes
section_title: Palaeobiogeography
hierarchical_section_title: Acamptonectes / Classification / Palaeobiogeography
caption_attribution_description: Ophthalmosaurus icenius, from the time of the Callovian period of the Middle Jurassic. This specimen was found in Oxford Clay, in Peterborough, England. Photo lightly touched up to remove lighting flares.
context_page_description: Acamptonectes is an extinct genus of ophthalmosaurid ichthyosaur that lived between 134 and 132 million years ago during the Early Cretaceous. Known from fossil deposits in England and Germany, the genus currently contains the single species A. densus, which was first described in 2012. Its discovery was important in the research of ichthyosaurs, as being one of the first known ophthalmosaurines from the Early Cretaceous, it provided evidence that no mass extinction of ichthyosaurs occurred during the Jurasic-Cretaceous boundary as previously understood. Nevertheless, Acamptonectes remains one of only eight genera of ichthyosaurs known during the Cretaceous. In life, Acamptonectes was probably a generalist predator. Its teeth were slender and textured with longitudinal ridges, which were best adapted for impaling prey; as a result, Acamptonectes likely fed on soft, fleshier prey such as fish and squid. The ichthyosaur's body was rigid and compact, which was suited for swimming at high speeds in tuna-like locomotion.
context_section_description: Ichthyosaurs were traditionally thought to have been affected by three extinction events: one at the Triassic–Jurassic boundary, one at the Jurassic–Cretaceous boundary, and a final extinction in the Cretaceous at the boundary of the Cenomanian and Turonian ages that left no survivors. Some researchers suggested that their species diversity declined after the mid-Jurassic, with the ichthyosaurs lingering on until they disappeared at the end of the Cenomanian. This decline was thought to have been associated with a transition in the dominant ichthyosaur lineage: the large-eyed, thunniform (tuna-shaped) ophthalmosaurines, which were successful and widespread notwithstanding their hyperspecialization, would have been replaced by the more generalized platypterygiines, which had smaller eyes and longer bodies.
  Acamptonectes is a significant find in that it is an ophthalmosaurine from the Early Cretaceous, demonstrating that 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; this included a basoccipital and humerus from Berriasian-aged rocks near Nettleton, Lincolnshire that they referred to Ophthalmosaurus itself. Additionally, they pointed to reports of the Late Jurassic-aged platypterygiines Brachypterygius, Aegirosaurus, Caypullisaurus, and Yasykovia (which has been synonymized with Nannopterygius) from the Early Cretaceous. 
  Lastly, by tabulating the number of genera disappearing in each age, Fischer and colleagues found that no boundary existed between ages from the Late Jurassic (Oxfordian) to Early Cretaceous (Aptian) that could be considered an extinction event for ophthalmosaurids; in particular, the Jurassic-Cretaceous boundary had a net extinction rate of 0 and even the highest survival rates. (However, by counting the number of new clades that emerged, they found that the cladogenesis rate was lower in the Cretaceous.) Thus, they concluded—contrary to traditional thinking—that the Jurassic-Cretaceous extinction event had a negligible impact on ichthyosaurs compared to other marine reptiles, with ophthalmosaurids having remained diverse until their final extinction.
caption: Reconstructed skeleton of the closely related Ophthalmosaurus, Natural History Museum, London
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