''Adriosaurus'' is an extinct genus of

squamate Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest Order (biology), order of reptiles; most members of which are commonly known as Lizard, lizards, with the group also including Snake, snakes. With over 11,991 species, it i ...

which lived in what is now

Slovenia Slovenia, officially the Republic of Slovenia, is a country in Central Europe. It borders Italy to the west, Austria to the north, Hungary to the northeast, Croatia to the south and southeast, and a short (46.6 km) coastline within the Adriati ...

and other parts of

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

during the Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

. It was a small, snake-like reptile, with the type species ''Adriosaurus suessi'' measuring up to in length. ''Adriosaurus'' represents the first occurrence of vestigial limbs in fossil lizards, having lost its manus and forearm completely in order to elongate its axial skeleton. These unique anatomical features led to discussions of the evolutionary patterns of limb reduction in Squamata.Alessandro Palci and Michael W. Caldwell. 2007. Vestigial Forelimbs and Axial Elongation in a 95 Million-Year-Old Non-Snake Squamate. Journal of Vertebrate Paleontology Vol. 27, No. 1. pp. 1-7 ''Adriosaurus'' includes three species: ''A. microbrachis'' (“micro”, meaning small, and “brachis”, meaning arm, referring to the vestigial forelimb composed of only the humerus), ''A. skrbinensis'' (named after the location where they found the fossil, Skrbina, northwest of Komen,

) and ''A. suessi''. ''A. microbrachis'' lacks many crucial characters to be qualified for cladistic analysis.MICHAEL W. CALDWELL and ALESSANDRO PALCI. 2010, A NEW SPECIES OF MARINE OPHIDIOMORPH LIZARD, ADRIOSAURUS SKRBINENSIS, FROM THE UPPER CRETACEOUS OF SLOVENIA. Journal of Vertebrate Paleontology Vol. 30, No. 3. pp. 747-755

History and Discovery

''Adriosaurus'' was first described by Seeley (1881) based on a single specimen found near Comen,

. This fossil consists of the posterior half of the vertebral column, the pelvis and hindlimbs. Later Nopsca (1908, 1923) described a nearly complete skeleton from Hvar,

Croatia Croatia, officially the Republic of Croatia, is a country in Central Europe, Central and Southeast Europe, on the coast of the Adriatic Sea. It borders Slovenia to the northwest, Hungary to the northeast, Serbia to the east, Bosnia and Herze ...

. Nopsca's conclusions about the animals systematics were not accurate. In 2000, Michael S. Y. Lee and Michael W. Caldwell redescribed specimen NHMR2867, which is attributed to ''A. suessi''.Michael S. Y. Lee and Michael W. Caldwell. 2000. Adriosaurus and the Affinities of Mosasaurs, Dolichosaurs, and Snakes. Journal of Paleontology Vol. 74, No. 5 pp. 915-937 Four years later, in 2004, Lee and Caldwell went on to reevaluate '' Acteosaurus crassicostatus,'' a species that was then based on a small squamate fossil found in Comen,

. They concluded that this species is synonymous with ''Adriosaurus suessi''. In 2007, Caldwell and Alessandro Palci described a new species of ''Adriosaurus, A. microbranchis''. The description was based on specimen MCSNT 7792, which consists of an articulated skeleton missing the skull and part of the cervical and caudal skeleton. They described another species of ''Adriosaurus'' in 2010. This new species, ''A. skrbinensis'', was about 60% larger than the other two known species within ''Adriosaurus''. The description was based on specimen SMNH 2158, which was also found near a small village in Comen,

. The specimen represents a relatively complete skeleton, with most of the skull, parts of the vertebrae, parts of limbs and pelvic girdle being preserved.

Description

''A. suessi'' was a small marine

with an elongated neck and body, with the type species (''A. suessi'') measuring long. It had 10 cervical, 29 dorsal, and at least 65 caudal vertebrae. Its bones exhibit

pachyostosis Pachyostosis is a non-pathological condition in vertebrate animals in which the bones experience a thickening, generally caused by extra layers of lamellar bone. It often occurs together with bone densification ( osteosclerosis), reducing inner c ...

, an anatomical feature that thickens the periosteal bone by increasing the osteogenic activity of the periosteum. Both its forelimbs and hindlimbs were reduced in size, which suggests that it lived in a marine environment. ''A. skrbinensis'' was a small marine squamate with 30 dorsal vertebrae. Unlike ''A. suessi'', ''A. skrbinesis'' had forelimbs which were much more reduced than the hindlimbs. It had a humerus/femur ratio of 0.40, compared to 0.61 in ''A. suessi'' and 0.62 in ''A. microbrachis''. A phosphatic matter was recovered from its gastric contents, which probably represents the remains of small fishes. This suggests that ''A. skrbinensis'' was likely a

piscivore A piscivore () is a carnivorous animal that primarily eats fish. Fish were the diet of early tetrapod evolution (via water-bound amphibians during the Devonian period); insectivory came next; then in time, the more terrestrially adapted repti ...

Vertebrae

''A. suessi'' has 10 cervical vertebrae, none of which exhibit pachyostosis. The neural arches of the cervical vertebrae are wide with both anterior and posterior lateral expansions. The neural spines extend vertically along the length of the neural arch. The shoulder girdle is placed after the tenth presacral and the following vertebrae connect to large, pachyostotic ribs. This has been used to infer that the distinction between the cervical and dorsal vertebrae lies between vertebrae 10 and 11. There are 29 dorsal vertebrae, with each being attached to large ribs. Compared to the cervical vertebrae, the neural arches of the dorsal vertebrae are much wider and more swollen. The dorsal vertebrae have less prominent transverse processes, and the ribs articulate with the body of the centrum. Most of the ribs are heavily ossified and pachyostotic. They are most ossified and thickened near the middle of the dorsal region, while the anterior and posterior part of the dorsal region appear to be less affected. The pachyostosis is most prominent in the proximal half of the ribs, with their distal half being less ossified. After the dorsal vertebrae, ''A. suessi'' preserves two sacral vertebrae. They are similar to the dorsal vertebrae, but lack ribs and are instead fused to the pelvis. 65 caudal vertebrae are preserved, with all of them possessing narrow neural arches. Transverse processes are present on the anterior caudals and gradually diminish in size posteriorly, disappearing around the 27th caudal vertebra. For ''A. skrbinensis'', 10 cervical vertebrae, 30 dorsal vertebrae, and 2 sacral vertebrae are preserved. The centrum of the dorsal vertebrae in this species presents a procoelous pattern.

Classification and species

As a pythonomorph with reduced limbs, ''Adriosaurus'' is important in regards to discussions on the origin of snakes, a topic on which paleontologists and

zoologists This is a list of notable zoologists who have published names of new taxa under the International Code of Zoological Nomenclature. A * Abe – Tokiharu Abe (1911–1996) * Abeille de Perrin, Ab. – Elzéar Abeille de Perrin (1843–1910) * ...

have long held different opinions. After the discovery of mid-Cretaceous snake-like lizards, paleontological research linked snakes to mosasauroid lizards which once lived in marine environments, while zoologists linked snakes to squamates with reduced limbs, which implied a terrestrial origin.Ad'A. Bellairs and G. Underwood . 1951. The origin of snakes. Biological Reviews 26:193–237. The precise origin of snakes is still a subject of scientific debate. ''Adriosaurus'' has three species: ''A. microbrachis'', ''A. skrbinensis'' and ''A. suessi''. ''A. skrbinensis'' and ''A. suessi'' are present on the cladogram below. The

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

is based on Palci and Caldwell (2010a & 2010b):

Paleobiology

Locomotion

''Adriosaurus'' mainly would have swum by using lateral undulation because of its laterally compressed body, flattened tail, and small limbs. The thicked parts of the middle dorsal region of the skeleton would have reduced swimming speed and maneuverability. Thus, ''Adriosaurus'' was a relatively slow swimmer. Its living environment most likely consisted of calm, near-shore environments. In this environment, the flat, distal regions of the limbs could have been used as paddles for slower swimming and maneuvering of the animal in aquatic environments. It could have also walked on land, but with its small limbs would only have been able to walk slowly. Most of the fossils of ''Adriosaurus'' have been found in

and were preserved in limestone.

Diet

''Adriosaurus'' had sharp, recurved teeth and a large skull relative to its body size. These features indicate that it was a predator, though its swimming abilities were not excellent. This means that it was most likely an ambush hunter. Considering its small body size, it most likely would have hunted various small fish and invertebrates.

References

{{Taxonbar, from=Q3605829 Late Cretaceous lepidosaurs of Europe Cretaceous lizards Fossil taxa described in 1881 Taxa named by Harry Seeley