Aegirosaurus
Aegirosaurus is an extinct genus of platypterygiine ophthalmosaurid ichthyosaurs known from the late Jurassic and early Cretaceous of Europe. It was originally named as a species of Ichthyosaurus.
Discovery and species
Originally described by Wagner as a species of the genus Ichthyosaurus, the species Aegirosaurus leptospondylus has had an unstable taxonomic history. It has been referred to the species Ichthyosaurus trigonus posthumus in the past, and sometimes identified with Brachypterygius extremus. In 2000, Bardet and Fernández selected a complete skeleton in a private collection as the neotype for the species I. leptospondylus, as the only other described specimen was destroyed in World War II. A second specimen from the Munich collection was referred to the same taxon. Bardet and Fernández concluded that the neotype should be assigned to a new genus, Aegirosaurus. The name means 'Aegir lizard with slender vertebrae'.Within the Ophthalmosauridae, scientists once believed Aegirosaurus was most closely related to Ophthalmosaurus. However, many subsequent cladistic analyses found it is more closely related to Sveltonectes. Aegirosaurus lineage was found to include Brachypterygius and Maiaspondylus, too, and to nest within the Platypterygiinae, which is the sister taxon of Ophthalmosaurinae.
Stratigraphic range
Aegirosaurus is known from the lower Tithonian of Bavaria, Germany. Its remains were discovered in the Solnhofen limestone formations, which have yielded numerous well-known fossils, such as Archaeopteryx, Compsognathus, and Pterodactylus.In addition to its late Jurassic occurrence, Aegirosaurus has been discovered from the late Valanginian of southeastern France, the first diagnostic ichthyosaur recorded from the Valanginian. This shows that most types of late Jurassic ichthyosaurs crossed the Jurassic-Cretaceous boundary.
Description
Ichthyosaurs are marine reptiles showing extreme adaptations to life in the water, superficially resembling fish and dolphins in overall shape. Aegirosaurus is a medium-sized ichthyosaur. The neotype specimen is long, while the destroyed holotype was estimated to have been about long when complete. The skull of the neotype is long, while that of the smaller specimen BSPHGM 1954 I 608 is approximately in length.Skull
The skull of Aegirosaurus gently tapers into an elongate, thin snout that makes up 62-73% of the length of the lower jaw. Its jaws are densely packed with small teeth lacking or at most bearing very small enamel ridges. The teeth are firmly rooted in a groove. The external nares are long, thin, and two-lobed in shape, with one lobe protruding upwards. The orbits of Aegirosaurus are of medium size for an ichthyosaur, and each contains a ring of 14 bony plates that occupies most of the space within. A flange projects outwards from above the orbit. The jugals is short and extends in front of the orbit.The region of the skull behind the orbits is not very extensive, the length of this region being only about 6-8% of the mandible's length. The postorbitals lie on top of the quadratojugals, almost completely hiding them when seen from the side. Aegirosaurus also bears a pair of bones known as squamosals in its cheek region. These bones are small, triangular, and can be seen when the skull is viewed from the side. However, two pairs of bones of the skull roof, the postfrontals and supratemporals, block them from reaching the temporal fenestrae. The mandible is strongly built. The angulars, one pair rear mandibular bones, can be seen on the mandible's outer surface and extend as far forwards as the surangulars, another pair of bones in the same region.
Postcranial skeleton
Aegirosaurus is estimated to have had around 157 vertebrae, with 52 of them being presacral vertebrae and the other 105 caudal vertebrae. The tail was bent downwards at a roughly 45-degree angle. This bend was formed by four vertebrae, with 40 caudal vertebrae in front of them and 61 behind. The vertebrae of Aegirosaurus are small and both ends of their centra are concave. The centra are about twice as wide, and, in the case of the presacral vertebrae, tall as they are long. The diapophyses flow smoothly into the neural arches. Another set of projections, the parapophyses, which are located lower than the diapophyses, are positioned towards the front of the vertebrae.The slender scapulae of Aegirosaurus are expanded at both ends and constricted at their middles. The front edges of the coracoids lack notches. The pelvis of Aegirosaurus is composed of two bones on each side, each pubic bone and ischium having been fused into a single unit known as the puboischiatic complex. The lower end of the puboischiatic complex is over twice as wide as its upper, and the bone is not pierced by a foramen.
The short, massively built humeri of Aegirosaurus have a lower ends that are wider than their upper ends. Each humerus bears facets for three bones on its lower end, two larger ones for the radius and ulna and a smaller one in the middle for the wedge-shaped intermedium. The radius is significantly smaller than the ulna, and the intermedium is longer from its lower to upper end than it is wide. An additional bone is present in each front flipper, located in front of and slightly lower than the radius. This bone, known as an extrazeugopodial element, has an accessory digit attached to it. In total, Aegirosaurus has six digits in each foreflipper, five of them primary digits. The fourth digit, which contains 23 individual bones, is the longest in the foreflipper.
The hindflippers are short, with the femora only half the length of the humeri and smaller than the lower hip bones. There are two bones, the tibia and fibula that contact the lower end of each femur, and the hind flippers each bear four digits, three of them primary digits. Both the front and hind flippers have a similar bone configuration, with the bones angular and tightly packed together further up while those further towards the flipper's tip being less tightly packed and rounder. Except for the fibulae, all of the bones in the flippers lack notches.