| Keilhauia | |
|---|---|
 | |
| Diagram of holotype specimen as preserved | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Family: | †Ophthalmosauridae |
| Genus: | †Keilhauia Delsettet al., 2017 |
| Species: | †K. nui |
| Binomial name | |
| †Keilhauia nui Delsettet al., 2017 | |
Keilhauia is a genus ofophthalmosauridichthyosaur, a type of dolphin-like, large-eyed marine reptile, from theEarly Cretaceous shallow marine Slottsmøya Member of theAgardhfjellet Formation inSvalbard,Norway. The genus contains a single species,K. nui, known from a single specimen discovered in 2010 and described by Delsettet al. in 2017. In life,Keilhauia probably measured approximately 4 metres (13 ft) in length; it can be distinguished by other ophthalmosaurids by the wide top end of itsilium and the relatively short ischiopubis (the fusion of theischium and thepubis) compared to thefemur. Although it was placed in a basal position within the Ophthalmosauridae byphylogenetic analysis, this placement is probably incorrect.
Judging by the partially preserved holotype specimen,Keilhauia has been estimated at 3.8–4.3 metres (12–14 ft) based on comparisons to the relatedophthalmosauridCryopterygius.[1][2] This specimen was probably mature or close to maturity at time of death, judging by the convex head of thehumerus and the smooth texture of the humeral shaft.[1][3]
Although only a small portion of the snout is known, it seems to be taller than that ofAegirosaurus,[4] instead being more similar to those ofCryopterygius[2] andCaypullisaurus.[5] A shallow groove that housed blood vessels, known as thenutrient foramen, is visible on the side of the jaw. About ten teeth are preserved; they are conical, narrow at the tip, and slightly curved.[1]
It is difficult to estimate the exact number of vertebral segments that were present in front of thesacrum; the preservedcentra suggest that there are at least 43. This is more than the 42 inAthabascasaurus,[6] the 41 inNannopterygius, and the 37 inPlatypterygius americanus,[7] but less than the 46 inPlatypterygius australis[8] and the 50 or more inCryopterygius,[2]Aegirosaurus,[4] andPlatypterygius platydactylus. The posteriordorsal vertebrae are wider and taller than those near the front, and their front and rear faces are rounded. The first few caudal vertebrae are the proportionally tallest and widest of all the vertebrae,[9] but the vertebrae quickly become shorter vertically prior to the bend in the tail that supported the tail fluke.[1]
Eachneural spine is about the same length as the underlying centrum, which is similar toCryopterygius[2] but notOphthalmosaurus (in which they are proportionally longer in the posterior cervical vertebrae). Similar toGengasaurus,[10] the first few neural spines are taller than the corresponding centra, but then decrease in height gradually. The tops of the neural spines, which are the thinnest parts of the bones, are straight, instead of being notched likeCryopterygius[2] orPlatypterygius australis.[8] The ribs are shaped like afigure eight in cross section near their top ends, but this is less obvious closer to the bottom; such a morphology is typical among ophthalmosaurids, except forAcamptonectes[11] andMollesaurus.[12] The dorsal ribs are around 75–81 cm (30–32 in) long, while the first few caudal ribs are only 2–5 cm (0.79–1.97 in) long.[1]
InKeilhauia, thescapula has a relatively straight blade that was broadened at its front end into a fan-likeacromion process. This process is less prominent than that ofAcamptonectes[11] andSveltonectes,[13] instead being more similar to that ofCryopterygius,[2]Platypterygius hercynicus, andSisteronia.[14] The scapula formed more of theglenoid than thecoracoid; likeSveltonectes[13] but unlikeCryopterygius,[2] the glenoid does not extend onto the bottom face of the scapula. Thede facto blade of the scapula was widest near the middle, and angled slightly downwards; likeAcamptonectes[11] but unlike all other ophthalmosaurids, the blade has a relatively uniform thickness along its entire length.[1]
Theclavicle, which is not fused to other elements in thepectoral girdle, bears a thickened process on its frontal bottom edge which points towards the midline of the torso. This process is relatively short, square-shaped from the front, and bordered by a rim on its back edge. In most other ophthalmosaurids, this same process is more finger-like, although some specimens ofOphthalmosaurus have similar processes. The back part of the clavicle gradually narrows into a curved point, as inOphthalmosaurus,Baptanodon, andJanusaurus.[15][1]
Thecoracoid is a kidney-shaped bone that is about as long as it is wide, compared toCryopterygius,[2]Nannopterygius, andSveltonectes,[13] where it is not as wide proportionally. LikeOphthalmosaurus andArthropterygius,[16] the coracoid bears a prominent notch on its front edge; it also bears a ridge on the frontal part of its midline, likeOphthalmosaurus andAcamptonectes,[11] but unlikeCaypullisaurus[5] andPlatypterygius australis.[8] Itsarticular facets with the scapula and glenoid are clearly separate, as inSveltonectes[13] but notAcamptonectes.[11] The former is only about 45% of the latter in length; this figure is 50% inCryopterygius,[2] and the two facets are effectively the same length inJanusaurus.[15][1]
There are two prominent processes on thehumerus. The longer, less prominent, and more ridge-like of these is the dorsal process. This process is remarkably short compared to other ophthalmosaurids, being less than half of the length of the humeral shaft, a condition also seen inUndorosaurus andAegirosaurus;[4] inOphthalmosaurus,Brachypterygius,Paraophthalmosaurus, andCryopterygius,[2] the process reaches the midpoint of the humerus, while it is even longer inArthropterygius,[16]Platypterygius americanus,[7]Platypterygius australis,[8] andPlatypterygius hercynicus.[17] The other process is the deltopectoral crest, which is similarly small among ophthalmosaurids (i.e. less than half of the shaft length) in the same manner asArthropterygius,[16]Platypterygius hercynicus,[17] andJanusaurus;[15] it is about half of the shaft length inOphthalmosaurus, and is almost as long as the entire shaft inSisteronia,[14]Acamptonectes,[11] andPlatypterygius americanus.[7][1]
At the midpoint of the humeral shaft, the bone is mildly constricted, being approximately 20% narrower than the maximum width. The bottom end of the humerus is larger than the top end; it bears three articular facets, with one for the preaxial accessory element (the smallest of the three), one for theradius (the tallest of the three), and one for theulna (which forms an angle of 120° with the radial facet).Sveltonectes,[13]Nannopterygius,Platypterygius hauthali,[18] andPlatypterygius platydactylus all only have two;Maiaspondylus,[19]Aegirosaurus,[4]Brachypterygius, andPlatypterygius americanus[7] also have three, but the second facet articulates with a different bone;Cryopterygius has two on the left humerus and three on the right;[2] andPlatypterygius australis[8] andPlatypterygius hercynicus[17] have up to four facets.[1]
Theilium ofKeilhauia is rather short compared to that of other ophthalmosaurids;Aegirosaurus is the only other ophthalmosaurid that possibly approachesKeilhauia in this respect.[4] The side of the ilium directed towards the back of the body is concave, likeOphthalmosaurus,Athabascasaurus,[6] andJanusaurus,[15] with the curve being more pronounced in the latter two. Theacetabulum, on the lower end of the ilium, is thickened relative to the rest of the bone, and does not bear any distinct articular facets.Autapomorphically, the top end of the ilium is 1.5 times the width of the acetabular end.[1]
Theischium and thepubis are fused together into a single, continuous, solid trapezoid-shaped element known as the ischiopubis, with the wider edge (1.4 times the width of the other end, which is shorter proportionally than that ofAegirosaurus,[4]Janusaurus,[15]Ophthalmosaurus, andAthabascasaurus[6]) being at the midline of the body. This complete fusion is also seen inJanusaurus,[15]Sveltonectes,[13]Athabascasaurus,[6]Aegirosaurus,[4]Caypullisaurus,[5] and possiblyPlatypterygius australis;[8] meanwhile,Ophthalmosaurus,Cryopterygius,[2]Undorosaurus, andParaophthalmosaurus retain a small hole in the ischiopubis. The probable portion of the ischiopubis that represents the pubis is thicker than the rest of the bone. Also uniquely among ophthalmosaurids, this fused element is shorter than thefemur.[1]
Both ends of the femur ofKeilhauia are approximately the same width, with the middle of the femoral shaft being slightly narrower. The top end was slightly thicker than the rest of the bone along its front rim. LikeArthropterygius[16] but unlikeOphthalmosaurus, the femoral dorsal and ventral processes were rather reduced. At the bottom end, there are two facets, one for thetibia and one for thefibula; this is typical of ophthalmosaurids, butPlatypterygius hercynicus,[17]Platypterygius americanus,[7]Platypterygius australis,[8] andParaophthalmosaurus all have three. Both facets inKeilhauia were roughly the same length, and the fibular facet was directed slightly to the back, forming an angle of 120° with the tibial facet.[1]
Excavations led by the Spitsbergen Mesozoic Research Group from 2004 to 2012 recovered 29ichthyosaur specimens from outcrops of the Slottsmøya Memberlagerstätte, which belongs to the greaterAgardhfjellet Formation, on the island ofSpitsbergen,Svalbard,Norway. These outcrops likely date to theTithonian-Berriasian based onammonitebiostratigraphy. From these, the new genera and speciesCryopterygius kristiansenae,Palvennia hoybergeti,[2] andJanusaurus lundi[15] have previously been described.[1]
Four ichthyosaur specimens were prepared at theUniversity of Oslo and subsequently described in 2016; they include PMO 222.655, the holotype ofKeilhauia, discovered from the Berriasian portions of the Slottsmøya Member in 2010. This articulated partial skeleton, which was preserved lying on its left side, consists of part of the snout, the dorsal and anterior caudal vertebrae, the right forelimb andpectoral girdle, the majority of thepubic girdle, and both of thefemora. Since all of the cervical vertebrae and parts of the dorsal vertebrae are missing, it is difficult to assign precise numberings to the existing dorsal vertebrae. The other three described specimens were PMO 222.670 and PMO 227.932, both discovered in 2011; and PMO 222.662, discovered in 2007.[1]
The genus nameKeilhauia honours the Norwegian geologistBaltazar Mathias Keilhau, who conducted an expedition to Spitsbergen in 1827. Meanwhile, the species namenui is derived from the acronym of the environmental organizationNatur og Ungdom, the fiftieth anniversary of which occurred in 2017.[1]
In 2017,Keilhauia was referred to the cladeOphthalmosauridae. This was on account of it bearing anarticular facet on thehumerus for the attachment of an anterior accessory element (a specialized limb bone that ichthyosaurs gained), as well as its forelimb elements lacking notches on their front edges.[11] Aphylogenetic analysis likewise recoveredKeilhauia as an ophthalmosaurid. Its specific relationships within the ophthalmosaurids, however, were difficult to elucidate due to the lack of cranial material. Thus, the authors considered thebasal placement ofKeilhauia in the below phylogenetic tree somewhat suspect (a similar problem occurs forUndorosaurus, which was also placed basally despite having a forelimb similar toCryopterygius).[1]
| Ophthalmosauridae |
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Keilhauia can be distinguished from its contemporaries by various characters. UnlikeCryopterygius, theulnar facet of thehumerus is angled backwards, and there is no opening in the fused ischiopubis.[2]Janusaurus has comparatively more robust humeri and more strongly-developed dorsal and ventral processes on thefemur.[15] Additionally, theautapomorphies, or traits unique to this genus, provide further evidence for its distinctness.[1]
A statistical analysis ofichthyosaurian proportions conducted in the description ofKeilhauia showed that, throughout their evolutionary history, ichthyosaurs did not significantly reduce the relative size of theirilium, but theirfemora did become proportionally longer relative to theirhumeri.Keilhauia has a smallpelvic girdle as well as a small humerus and femur, but it falls within the normal range of variation betweenparvipelvian ichthyosaurs.[1]
The Slottsmøya Member of theAgardhfjellet Formation, from whichKeilhauia is known, consists of a mix ofshales andsiltstones. It was deposited in a shallow marine environment, near a patch of deepermarine sediment.[20] The seafloor, which was located about 150 metres (490 ft) below the surface, seems to have been relatively dysoxic, or oxygen-poor, although it was periodically oxygenated byclastic sediments.[21] Despite this, near the top of the member, various diverse assemblages of invertebrates associated withcold seeps have been discovered; these includeammonites,lingulate brachiopods,bivalves,rhynchonellate brachiopods,tubeworms,belemnoids,tusk shells,sponges,crinoids,sea urchins,brittle stars,starfish,crustaceans, andgastropods, numbering 54 taxa in total.[22] Outside of the cold seeps, many of these invertebrates were also present in abundance.[21]
BesidesKeilhauia, 16 otherichthyosaur specimens are also known from the Slottsmøya Member. These include the three that were described alongside it, as well as the type and only specimens ofCryopterygius kristiansenae,Palvennia hoybergeti,[2] andJanusaurus lundi.[15] Additionally, 21plesiosaurian specimens are also known from the site, including two belonging to the largePliosaurus funkei, three toColymbosaurus svalbardensis, and one each toSpitrasaurus wensaasi andS. larseni. Many of these specimens are preserved in three dimensions and partially in articulation; this is correlated with high abundance of organic elements in the sediments they were buried in, as well as a lack of invertebrates locally.[21]
