Acamptonectes densus was a 4 meter (13 foot) long ichthyosaur which swam in the seas around western Europe during the early Cretaceous Period 131-129 million years ago.
The first specimen was found in the seaside village of Speeton, Yorkshire, England in 1958 (collection ID code: GLAHM 132855). The skeleton was found within rocks dated to the early part of the Hauterivian Stage of the Early Cretaceous, approximately 131 MYA. The skeleton belongs to an adult. It was originally identified by the British paleontologist Robert Appleby as a specimen of the ichthyosaur Platypterygius, meaning “flat fin”, which existed in the seas around Europe (and possibly elsewhere) during the middle Cretaceous Period circa 115-95 MYA. However, the genus Platypterygius was treated by paleontologists as a “waste basket taxon” for many years, with any ichthyosaur remains which couldn’t be positively identified as belonging to a particular species being classified as this animal. Understandably, this can get confusing (Fischer et al 2012: e29234).
A second specimen was found in 1985 six miles to the north at the seaside town of Filey (collection ID code: NHMUK R11185), and is also dated to the early Hauterivian Stage (Fischer et al 2012: e29234).
In 2005, another specimen was found in the small town of Cremlingen, located a few miles east of the city of Brunswick, Germany (collection ID code: SNHM1284-R). This skeleton belongs to a sub-adult. Unlike the two English specimens which date to the early Hauterivian Stage, this skeleton is dated to the late Hauterivian, approximately 129 MYA (Seibertz and Krüger 2008, page 273; Fischer et al 2012: e29234).
Partial skeleton of Acamptonectes densus, on display in the State Natural History Museum in Brunswick, Germany (collection ID code: SNHM1284-R). Public domain image, Wikimedia Commons.
In 2012, these specimens were officially re-named as Acamptonectes densus. The name Acamptonectes literally means “non-bent swimmer”, but it more appropriately means “rigid swimmer”. Anatomical features included interlocking vertebrae, making the spinal column stiff and rigid, which has been proposed as an adaptation for cutting through the water at high speed. This is a lifestyle that is seen in many of the “ophthalmosaurid” ichthyosaurs, which are best seen as prehistoric analogs of swordfish or tuna. Its ribs were also rounded in cross-section rather than the hourglass-shaped cross-section of other ophthalmosaurids, which has also been suggested to be an adaptation for strengthening the body while moving very fast through water (Fischer et al 2012: e29234; Tetrapod Zoology. “Rigid Swimmer’ and the Cretaceous Ichthyosaur Revolution (part I)”). Acamptonectes‘ slender teeth seem to be adapted to feed on soft prey like fish and squid rather than hard-shelled ammonites (Zammit 2012, page 17).
Fischer et al (2012) did not state how large Acamptonectes would have grown. Although a complete skeleton hasn’t been found, enough partial remains have been discovered from England and Germany (including skull material) to give us a good idea of its anatomy and its overall size. By comparing the size of the bones of Acamptonectes with those of other ophthalmosaurid ichthyosaurs, Acamptonectes appears to have measured 4 meters (13 feet) long.
There might have been a fourth specimen which was discovered near Hannover, Germany in the early 1900s. In 1909, it was named by Ferdinand Broili as a new species of Ichthyosaurus, called Ichthyosaurus brunsvicensis, meaning “fish-lizard from Brunswick”. However, even Broili himself was skeptical as to whether or not this identification was proper. It was described in Broili’s 1909 paper as dating to the “upper Neocomian”, which is an outdated term which was used to refer to the lowermost stages of the Cretaceous Period (Broili 1909, pages 295-302). This specimen possibly belonged to a juvenile. Unfortunately, it was destroyed during World War II. In 1972, the Canadian paleontologist Christopher McGowan assigned this German specimen as belonging to the genus Platypterygius (McGowan 1972, pages 9-29). However, in 2012, Fischer and his colleagues observed that many of the remains which were described bore a noticeable similarity to those belonging to Acamptonectes. Therefore, they re-assigned this specimen as “cf. Acamptonectes”, meaning “similar to Acamptonectes” (Fischer et al 2012: e29234).
Another specimen which was discovered near Cambridge, England has also been tentatively ascribed to the genus Acamptonectes, but this specimen dates to a much later time than the species Acamptonectes densus. This Cambridgeshire specimen comes from the Cambridge Greensands, and probably dates to the lower Cenomanian Stage of the middle Cretaceous, approximately 100 MYA (Fischer et al 2012: e29234).
Phylogenic analysis made by Fischer and his colleagues in 2012 placed Acamptonectes as the sister species of Baptanodon natans, an ichthyosaur which lived in the Sundance Sea of North America during the middle and late Jurassic Period (Fischer et al 2012: e29234; Zammit 2012, page 19). This analysis was revised in 2020, where Acamptonectes was placed as the most basal member of the sub-family Ophthalmosaurinae, closely related to Ophthalmosaurus, Mollesaurus, and Baptanodon (Zverkov and Jacobs 2020, page 266).
Isotopic analysis of fossilized foraminifera shells shows that there was an increase in oxygen in the atmosphere, which in turn indicates that Earth’s global temperatures had undergone a period of cooling (Hart et al 2009, pages 434-435).
Careful examination of the geological strata from both northern England and northern Germany shows that, during the early Cretaceous Period 130 million years ago, this area was a shallow sea near the coast. Islands of varying sizes were located offshore (Möller and Mutterlose 2014, pages 502-504). A corridor of saltwater which ran through what’s now Poland directly connected the northern Boreal Sea with the warmer Tethys Sea which lay to the south (Mutterlose and Ruffell 1999, page 134; Weinkauf et al 2013, page 246). Hart et al (2009) went further and made comparisons between the early Cretaceous rock layers of the Speeton Clay of northern England to modern-day Plymouth Sound, in which the bottom consisted of soft sediment such as clay, mud, and sand (Hart et al 2009, pages 248-250). Being a native New Yorker, I myself am tempted to make comparisons to the Long Island Sound.
Map of northern Europe, including Britain, northern France, northern Germany, and southern Scandinavia, as it would have appeared during the Hauterivian Stage of the early Cretaceous Period, 131-129 MYA. Image from Mutterlose, Jörg; Ruffell, Alastair (1999). “Milankovitch-scale paleoclimate changes in pale-dark bedding rhythms from the Early Cretaceous (Hauterivian and Barremian) of eastern England and northern Germany”. Palaeogeography, Palaeoclimatology, Palaeoecology, volume 154, issue 3 (November 1999). Page 134.
Map of northern Germany during the Hauterivian Stage of the early Cretaceous Period, 131-129 MYA. Image from Möller, Carla; Mutterlose, Jörg (2014). “Middle Hauterivian biostratigraphy and palaeoceanography of the Lower Saxony Basin (Northwest Germany)”. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, volume 165, issue 4 (December 2014). Page 503.
By comparing the levels of the isotope oxygen-18 in fossilized belemnites and comparing them with similar periods in Earth’s history which had similar levels, we can conclude that the temperature of the water where the ichthyosaur fossils were found in northern England ranged from 11-15 degrees Celsius (52-59 degrees Fahrenheit). Because this waterway was connected the cold Boreal Sea to the north and the warmer Tethys Sea to the south, the water was subjected to fluctuating periods of warming and cooling, with the warmest temperatures occurring during the middle of the Hauterivian Stage about 130 MYA (Mutterlose and Ruffell 1999, pages 133-160; McArthur et al 2004, pages 253, 269-270; Seibertz and Krüger 2008, pages 273-287; Weinkauf et al 2013, pages 241-271; Möller and Mutterlose 2014, page 502). Examination of the rock layers in northern Germany suggest that the deep parts of the sea in that area were oxygen-poor (Möller and Mutterlose 2014, page 503).
Due to the rather chilly temperature of the water, Acamptonectes was likely colored dark, as many marine reptiles are suspected to have been, in order to absorb as much heat as possible.
The ichthyosaur Acamptonectes shared its shallow sea ecosystem with various species of sponges, crinoids, polychaete worms, brachiopods, sea urchins, marine snails, bivalves, ammonites, and belemnites (Doyle 1989, pages 175-182; McArthur et al 2004, pages 253, 256-260; Seibertz and Krüger 2008, pages 273-287; Taylor et al 2013, pages 227-245; Möller and Mutterlose 2014, pages 502-504; Hull Geological Society. “Fossils of the Speeton Clay”). Fish fossils are rare, but we have found evidence of several species of sharks and rays including the synechodontiform shark Synechodus dubrisiensis, the hexanchiform sharks (six-gill sharks) Notidanodon lanceolatus and Notorhynchus aptiensis, the orectolobiform shark (nurse sharks, carpet sharks, and wobbegongs) Cretorectolobus doylei, the stingray Dasyatis speetonensis, and the guitarfish Spathobatis rugosus (Underwood et al, 1999, pages 287-302). Fragmentary remains from three plesiosaur specimens have also been found within the same Yorkshire rock layers as the specimens of Acamptonectes (collection ID codes: SCARB2007.51; NHMUK R6650; NHMUK 48623) (Fischer et al 2012: e29234). They have not yet been positively identified, but it’s likely that they belonged to the 10 foot long plesiosaur Leptocleidus.
Below is an illustration which I made of Acamptonectes. The animal’s appearance is strongly based upon that of Baptanodon, which it may be closely related to. As you can see, the animal is colored very darkly in order to keep it as warm as possible in the frigid water of the Boreal Sea. The illustration was made with No.2 pencil and Crayola colored pencils on printer paper.
Acamptonectes densus. © Jason R. Abdale (March 11, 2023).
Broili, Ferdinand (1909). “Neue Ichthyosaurierreste aus der Kreide Norddeustschlands und das Hypophysenloch bei Ichthyosauriern”. Palaeontographica, volume 55 (1909). Pages 295-302.
Doyle, J. C. (1989). “The stratigraphy of a late Lower Hauterivian horizon in the Speeton Clay formation (Lower Cretaceous) of East Yorkshire”. Proceedings of the Geologists’ Association, volume 100, issue 2 (1989). Pages 175-182.
Hart, Malcolm B.; Price, Gregory D.; Smart, Christopher W. (2009). “Foraminifera and sequence stratigraphy of the lower part of the Speeton Clay Formation (Lower Cretaceous) in NE England”. Annalen des Naturhistorischen Museums in Wien, 110A (January 2009). Pages 423-442.
Fischer, Valentin; Maisch, Michael W.; Naish, Darren; Kosma, Ralf; Liston, Jeff; Joger, Ulrich; Kruger, Fritz, J.; Perez, Judith Pardo; Tainsh, Jessica; Appleby, Robert M. (2012). “New Ophthalmosaurid Ichthyosaurs from the European Lower Cretaceous Demonstrate Extensive Ichthyosaur Survival across the Jurassic–Cretaceous Boundary”. PLoS One, volume 7, issue 1 (January 3, 2012): e29234.
Fischer, Valentin; Maisch, Michael W.; Naish, Darren; Kosma, Ralf; Liston, Jeff; Joger, Ulrich; Kruger, Fritz, J.; Perez, Judith Pardo; Tainsh, Jessica; Appleby, Robert M. (2012). “Correction: New Ophthalmosaurid Ichthyosaurs from the European Lower Cretaceous Demonstrate Extensive Ichthyosaur Survival across the Jurassic–Cretaceous Boundary”. PLoS ONE, volume 7, issue 1 (January 26, 2012): 10.1371/annotation/9731f93a-c28f-4234-8fd9-c587a103b572. https://journals.plos.org/plosone/article?id=10.1371/annotation/9731f93a-c28f-4234-8fd9-c587a103b572.
McArthur, J. M.; Mutterlose, J.; Price, G. D.; Rawson, P. F.; Ruffell, A.; Thirlwall, M. F. (2004). “Belemnites of Valanginian, Hauterivian and Barremian age: Sr-isotope stratigraphy, composition (87Sr/86Sr, δ13C, δ18O, Na, Sr, Mg), and palaeo-oceanography”. Palaeogeography, Palaeoclimatology, Palaeoecology. volume 202, issue 3-4 (2004). Pages 253-272.
McGowan, Christopher (1972). “The systematics of Cretaceous ichthyosaurs with particular reference to the material from North America”. Rocky Mountain Geology, volume 11, issue 1 (April 1, 1972). Pages 9-29.
Möller, Carla; Mutterlose, Jörg (2014). “Middle Hauterivian biostratigraphy and palaeoceanography of the Lower Saxony Basin (Northwest Germany)”. Zeitschrift der Deutschen Gesellschaft für Geowissenschaften, volume 165, issue 4 (December 2014). Pages 501-520.
Mutterlose, Jörg; Ruffell, Alastair (1999). “Milankovitch-scale paleoclimate changes in pale-dark bedding rhythms from the Early Cretaceous (Hauterivian and Barremian) of eastern England and northern Germany”. Palaeogeography, Palaeoclimatology, Palaeoecology, volume 154, issue 3 (November 1999). Pages 133-160.
Seibertz, Ekbert; Krüger, Fritz J. (2008). “Biostratigraphie und Paläobiogeographie des Hauterivium von Cremlingen bei Braunschweig bestimmt mit Cephalopoden (Unterkreide, Ostniedersachsen)”. Braunschweiger Naturkundliche Schriften, volume 8, issue 1 (October 2008). Pages 273-287.
Taylor, Paul D.; Barnbrook, Jane A.; Sendino, Consuelo (2013). “Endolithic biota of belemnites from the Early Cretaceous Speeton Clay Formation of North Yorkshire, UK”. Proceedings of the Yorkshire Geological Society, volume 59, issue 4 (November 2013). Pages 227-245.
Underwood, Charlie J.; Mitchell, Simon F.; Veltcamp, Kees J. (1999). “Shark and ray teeth from the Hauterivian (Lower Cretaceous) of north‐east England”. Palaeontology, volume 42, issue 2 (April 1999). Pages 287-302.
Weinkauf, M. F. G.; Keupp, H.; Mutterlose, J. (2013). “Calcareous dinoflagellates from the Late Hauterivian (Early Cretaceous) of Frielingen, Germany”. Documenta Naturae, volume 192, issue 3 (December 2013). Pages 241-271.
Zammit, Maria (2012). “Cretaceous Ichthyosaurs: Dwindling Diversity, or the Empire Strikes Back?”. Geosciences, volume 2 issue 2 (April 12, 2012). Pages 11-24.
Zverkov, Nikolay G.; Jacobs, Megan L. (2020). “Revision of Nannopterygius (Ichthyosauria: Ophthalmosauridae): reappraisal of the ‘inaccessible’ holotype resolves a taxonomic tangle and reveals an obscure ophthalmosaurid lineage with a wide distribution”. Zoological Journal of the Linnean Society, volume 191. Pages 228-275.
Hull Geological Society. “Fossils of the Speeton Clay” (2020). http://www.hullgeolsoc.co.uk/fossils.htm. Accessed on March 11, 2023.
Tetrapod Zoology. “Rigid Swimmer and the Cretaceous Ichthyosaur Revolution (part I)”, by Darren Naish (January 3, 2012). https://blogs.scientificamerican.com/tetrapod-zoology/cretaceous-ichthyosaur-revolution-part-i/. Accessed on March 11, 2023.
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