Dyrosaurids

Dyrosauridae is a family of extinct neosuchian crocodyliforms that lived from the Late Cretaceous (Maastrichtian) to the Eocene. Dyrosaurid fossils are globally distributed, having been found in Africa, Asia, Europe, North America and South America. Over a dozen species are currently known, varying greatly in overall size and cranial shape. A majority were aquatic, some terrestrial and others fully marine (see locomotion below), with species inhabiting both freshwater and marine environments. Ocean-dwelling dyrosaurids were among the few marine reptiles to survive the Cretaceous–Paleogene extinction event.

The dyrosaurids were a group of mostly marine, long jawed, crocodile-like quadrupeds up to long. The largest dyrosaurid was probably ''Phosphatosaurus'' estimated at in length. Based on bone tissue evidence, it has been hypothesized that they were slow-growing near-shore marine animals with interlocking closed jaws, able to swim as well as walk on land. External nostrils at the posterior end of its snout and an internal naris in its pterygoid indicated a habit of hunting while swimming with the top of the head above the water, enabling it to breathe while stalking prey.

Overview

Dyrosaurids are known to have a very characteristic skull shape with a long and thin snout that is approximately 68% of the total skull length. The most anterior part of the skull and snout is the external naris followed posteriorly by two premaxillae bones until they reach two maxillae bones separated by a single nasal bone.

Typical in dyrosaurids is a single nasal element with a characteristic collection of small pits and a constant width until it widens to contact the
lacrimal bones, then tapering off for a short distance until it meets the boundary of the frontals and prefrontals.

Dyrosaurs have a premaxilla with shallow pits that extend posterior to the third maxillary alveoli. There are two premaxillae that are narrow as compared to the maxillary bones and extend in the two long maxilla bones which separated by the single nasal bone. The last premaxilla and first maxilla are widely separated by a fourth dentary tooth. Alveoli are widely spaced anteriorly and the space between them decreases posteriorly from the fifteenth alveolus with the diameter remaining constant.

The maxilla is long (approx. two and a half times the length of jugal) and forms most of the lateral margin of the skull. According to Jouve and Barbosa and perhaps depending on the age of the animal, each maxilla bears 13-19 teeth.Antonio Barbosa, José; Wilhelm Armin Kellner, Alexander. Somália Sales Viana, Maria. New dyrosaurid crocodylomorph and evidences for faunal turnover at the K–P transition in Brazil. 2008. Proceedings of the Royal Society. Downloaded from http://rspb.royalsocietypublishing.org/

An important feature of the dyrosaurid dentition are deep occlusal pits, present particularly in the posterior region of the maxillae that get less pronounced anteriorly. The pits are indicative of an interlocking closed jaw for dyrosaurids because the pits give a way for the upper and lower jaws to alternate. Although they are still present in dyrosaurus phosphaticus, this feature of deep occlusal pits become less developed and broad. Dyrosaurus teeth are homodont, conical, long, and slender, with asymmetrically labial and lingual surfaces. Posterior teeth are shorter and more compressed than anterior tooth indicating that the tooth size decreased anterior to posterior.

Dyrosaurs have a snout to skull length of about 68% and the genus ''Rhabdognathus'' and ''Atlantosuchus'', ''Dyrosaurus'' and ''Arambourgisuchus'' have the largest snout proportions of all dyrosaurids. Snout length was previously used it to establish dyrosaurid relationships, while considering the lengthening of the snout to be a ‘more evolved’ character. This was not congruent with Jouve's conclusion which was that the longest snout is actually the primitive condition so the shorter or longer snout appears independently at least four times in dyrosaurid evolution.

Paleobiogeography

Dyrosaurids were once considered an African group, but discoveries made starting from the 2000s indicate they inhabited the majority of the continents.Jouve ''et al.'' (2008) In fact, basal forms suggest that their cradle may have been North America.

General

Phylogeny

Jouve ''et al.'' (2005) diagnose Dyrosauridae as a clade based on the following seven synapomorphies or shared characters:

*Posteromedial wing of the retroarticular process dorsally situated ventrally on the retroarticular process
*Occipital tuberosities small
* Exoccipital participates largely to the occipital condyle
*Supratemporal fenestra anteroposteriorly strongly elongated
*Symphysis about as wide as high
*Quadratojugal participates largely to the cranial condyle for articulation with the jaw
*4 premaxillary teeth
Jouve ''et al.'' 2020 provide a comprehensive analysis of the relationships of dyrosaurids, shown below. Note the former dyrosaurids '' Sabinosuchus'' and ''Fortignathus'' are recovered as a pholidosaurid and a peirosaurid, respectively.

Palaeobiology

Growth

Evidence for the semi-aquatic life of dyrosaurids comes from careful analysis of bone structure. There are two types of structural bone organization that can occur in aquatic tetrapods: osteoporotic or pachyostotic. Osteoporotic bone is spongy and porous whereas pachyostotic involves an increase in skeletal mass. Spongy/porous bone such as osteoporotic is associated with faster swimming and better maneuverability in water because of the reduction in bone tissue, many extant cetaceans and marine turtles have osteoporotic bone which enables them to be good swimmers. Pachyostotic bone is a general/local increase in skeletal mass which can be caused by osteosclerosis (inner compaction of bone), pachyostosis (hyperplasy of compact cortices) or pachyeosclerosis (combination of the two). Research on dyrosaur bone performed by Rafael César Lima Pedroso de Andrade and Juliana Manso Sayao revealed that this family had osteoporotic bone tissue indicative of a fast-swimming ecology as well as some osteosclerosis which is a component of pachyostotic bone tissue. Osteoporosis is associated with a fully aquatic lifestyle whereas pachyostotic is not fully aquatic but is associated with fast swimming ecology. Therefore, dyrosaurs are semi-aquatic fast swimmers as indicated by their bone structure. Other evidence for near shore, semi-aquatic lifestyle is where the fossils are found, often in transitional marine sediments. -using axial frequency swimming (that used primarily by extant crocodylians) with a greater undulatory motion and frequency of the tail due to highly developed musculature allowing a more powerful forward thrust.

Dyrosaurids have a tissue pattern that is indicative of a slow-growth animal that was determined by careful analysis of a right femur and left tibia. In the left tibia, the cortex had a lamellar zonal bone with five lines of arrested growth (LAGs) which were spaced 300 mm apart, well as a clear vascular networks of primary osteons that decreased in density towards the membrane (periostially). The right femur had double LAGs and an EFS later as well as secondary osteons occurring in the deep cortex and the spongiosa. This tissue growth pattern is a common characteristic of slow growing animals.

Habitat

Dyrosaurids are found in transitional marine sediments from the Late Cretaceous to lower Eocene. This family is known mainly from Maastrichthian deposits in New Jersey and the late Cretaceous to early Paleogene rock from the Tethys Sea in northern and western Africa. Fossils have also been found from the Paleocene and Eocene strata of Pakistan, as well as South America, Brazil, India, Southern Asia as well as coastal. Generally dyrosaurids are recovered from coastal and estuarine deposits through North Africa and the Middle East confirming their existence as semi-aquatic animals.

Dyrosauridae had its greatest taxonomic diversity during the Early Paleogene but it appears as though the clade was able to obtain a greater and more widespread geographic distribution during the Late Cretaceous. The earliest records of dyrosaurids are either in or close to Africa with fragmentary occurrences from the Cenomanian of Sudan and Portugal and several other pre-Maastrichtian, Late Cretaceous discoveries in Egypt. Later, by the Maastrichtian of North America, the record of dyrosaurids became more complete by establishing a widespread distribution that appears to be maintained through the Paleocene and Eocene.

Dyrosaurids have also been found from non-marine sediments. In northern Sudan, dyrosaurids are known from fluvial deposits, indicating that they lived in a river setting. Bones from indeterminate dyrosaurids have been found in inland deposits in Pakistan as well. Some dyrosaurids, such as those from the Umm Himar Formation in Saudi Arabia, inhabited estuarine environments near the coast. The recently named dyrosaurids ''Cerrejonisuchus'' and ''Acherontisuchus'' have been recovered from the Cerrejón Formation in northwestern Colombia, which is thought to represent a transitional marine-freshwater environment surrounded by rainforest more inland than the estuarine environment of the Umm Himar Formation. ''Cerrejonisuchus'' and ''Acherontisuchus'' lived in a neotropical setting during a time when global temperatures were much warmer than they are today.

Reproduction

In 1978, it was proposed that dyrosaurids lived as adults in the ocean but reproduced in inland freshwater environments. Remains belonging to small-bodied dyrosaurids from Pakistan were interpreted as juveniles. Their presence in inland deposits was viewed as evidence that dyrosaurids hatched far from the ocean. From the lower Eocene Oulad Abdoun Basin, there are very few juvenile dyrosaurids, but numerous similarly-sized adult specimens. This has furthered the assumption that juveniles lived in freshwater environments and adults lived in marine environments. Recently however, the large-bodied and fully mature dyrosaurids of the Cerrejón Formation have shown that some dyrosaurids lived their entire lives in inland environments, never returning to the coast.

Locomotion

A study on ''Cerrejonisuchus'' suggest this genus was more terrestrial than other dyrosaurids, and also shows that modern crocodylians are not good functional analogues for Dyrosauridae.

References

*Barbosa, J.A., Kellner, A.W.A. and Viana, M.S.S. (2008). New dyrosaurid crocodylomorph and evidences for faunal turnover at the K–P transition in Brazil. ''Proceedings of the Royal Society B: Biological Sciences'': Firstcite
*Buffetaut, E. (1985). L'evolution des crocodiliens. Les animaux disparus-Pour la science, Paris 109.
*
*

External links

Jouve et al. (2005)

{{Taxonbar, from=Q548118

Late Cretaceous crocodylomorphs
Paleogene crocodylomorphs
Prehistoric reptile families
Prehistoric marine crocodylomorphs
Maastrichtian first appearances
Eocene extinctions