Hyposaurus is a genus of extinct marine dyrosaurid crocodyliform. Fossils have been found in Paleocene aged rocks of the Iullemmeden Basin in West Africa, Campanian–Maastrichtian (Late Cretaceous) Shendi Formation of Sudan and Maastrichtian (Late Cretaceous) through Danian (Early Paleocene) strata in New Jersey, Alabama and South Carolina. Isolated teeth comparable to Hyposaurus have also been found in Thanetian (Late Paleocene) strata of Virginia. It was related to Dyrosaurus. The priority of the species H. rogersii has been debated, however there is no sound basis for the recognition of more than one species from North America. The other North American species (i.e. H. fraterculus, H. ferox and H. natator) are therefore considered nomina vanum (i.e. empty names).
Introduction[]
Hyposaurus is an extinct reptile whose fossils are found in marine sediments from the Maastrichtian (latest Cretaceous age) to the Danian (earliest Paleocene age). It is a mesosuchian crocodyliform in the family Dyrosauridae. It is closely related to dyrosaurs and congosaurs (Schwarz-Wings). The earliest fossils were found in North America, and they were later discovered in Africa and South America. The genus is believed to have originated in Africa. Hyposaurus lived in a shallow, near shore marine environment and has many aquatic adaptations In 2009, the disorganized phylogeny of crocodyliforms was treated and reliable diagnostic traits established, but remaining questions are unanswered.
Early discoveries[]
Owen first recognized the genus in 1849. This first fossil was two amphicoelous vertebrae, vertebrae with two concave sides of the centrum, discovered by Professor Henry Roger. It was found in the greensand beds in New Jersey. The different greensand beds of New Jersey represent a complete record from the Cretaceous to the Paleocene. They are estimated to cover 10,000 square miles (26,000 km2) of sea floor but are limited on land to coastal environments. In honor of professor Roger, Owen named this new fossil Hyposaurus rogersii. The genus name is meant to describe the unique "hypapophyseal keel extended on the ventral surface of the centrum". This is an extension of the vertebrae centrum which point down towards the belly, similar to a boat keel. The second fossil find was by Cope in 1886. This fossil was found in Brazil and comprises a left molar, quadratojugal bone, a lower jaw, many vertebrae from the middle to posterior parts of the column, a humerus, a coracoid bone, teeth, and several other bones. It had been hypothesized Hyposaurus was related to Teleosaurus and this fossil evidence allowed Cope to propose Hyposaurus was part of the family Teleosauridae. The differences between Hyposaurus and Teleosaurus are described as "the robust size and vertical direction" of the teeth of Hyposaurus, as well as Hyposaurus had hypapophyses[clarification needed] on more dorsal vertebrae than Teleosaurus, which only has these on the first and second dorsal vertebrae. Cope remarks the characteristics of H. rogersii and his new specimen are very similar, but the articular faces of the centrum are less concave than H. rogersii. The species was named Hyposaurus derbianus after professor Orville Derby, the director of the department of Geology at the National Museum of Brazil.
Description[]
In 2006, Schwarz and colleagues, described the postcranial skeletons of new specimens of Hyposaurus, focusing mostly on the vertebrae. From partial skeletons a proatlas, atlas, axis, a third to ninth cervical vertebrae, and at least 16 dorsal, two sacral, and 45 caudal vertebrae have been reconstructed. The vertebrae are weakly amphicoelous, meaning both sides of the centrum are concave. The dorsal shield is made of two columns of paravertebral osteoderms and two lateral columns of accessory osteoderms. At least 12 horizontal rows of these make up the shield.
The three main differences between the axial skeletons of Hyposaurus and modern crocodylians are the tall neural spines, vertically oriented thoracic ribs and osteoderm which lack external keels. This indicates that they also have a different epiaxial musculature (muscles above the axial skeleton). Along with the specialized osteoderm morphology, Hyposaurus probably had a specialized trunk bracing system which suggests that individuals with low body mass could have only high walked or galloped.