Metoposaurus

*Metoposaurus* is an extinct genus of Temnospondyl that lived in Europe during the Late Triassic.

## History

Von Meyer (1842) mentions a Metoposaurid skull roof from near Feuerbacher Heide, attempting reconstruction of the cranium and naming it *Metopias diagnosticus*. Lydekker (1890) moved the species to *Metoposaurus* because "*Metopias*" was preoccupied.

## Description

The lacrimal touches the nasal at the medial, at the maxilla laterally, at the prefrontal posteromedially and at the jugal posteriorly. Most Metoposaurs are classified by their lacrimal position, but others disagree with this methodology. Hunt (1993) published an image showing the lacrimal entering the orbit, contrary to what Fraas (1889) describes. However, Lucas notes that this does not seem to be the case, and that poor preservation may have led to this misinterpreting. Sulej (2007) notes a narrow variability in the lacrimal postition that may support it's use in phylogenetic analyses, but states it to be used with caution. Sulej also shows the parietal contacts the frontal anteriorly, the postfrontal anterolaterally, the supratemporal laterally and the postparietal posteriorly. The pineal foramen sits at the posterior of the parietal. *M. d. krasiejowensis* is also noted by Sulej to have a shorter prepineal region rather than *M. d. diagnosticus*, with the expansion angle of the suture separating the parietal and supratemporal at a lower value. The maxilla forms a large, dentigerous shelf that has 83-107 teeth. The first few teeth are large, posteriorly dectreasing in size. The maxilla constacts the ectopterygoid, palatine and vomer at the ventral. The maxilla slightly broadens mediallty on the palatal side bordering the choana of the choanal region. The choanal margin is variable. Most skulls have this weakly distinguished and rounded, but some are more solid and have a sharp outline.

Sulej notes intercentra of the cervicals and thoracis vertebrae being fully ossified. The pleurocentra are not preserved, but not evidence of cartilages are present. The atlas, axis and 3rd and 4th cervicals are similar to other Stereospondyls. They also suggest it had a relatively flexible neck. The intercentra where the spine comtacts the shoulder are flat anteriorly and posteriorly. The prezygapophyses are almost upright, which suggests limited lateral bending in the vertebrae. It likely connected through the spine/shoulder articulation. Where the spine contacts the limbs, the spine is stiffened, which is essential for swimming. *M. d. krasiejowensis* has paropophyses that become poteriorly shorter, similar to plagiosaurids. The intercentra of the dorsals and sacrals are fully ossified with short disks that are disconnected from the neural arches. The dorsal and sacral region has concave anterior and posterior surfaces. This is similar to plesiosaur trunk vertebrae and somewhat to ichthyosaurs. These all suggest *Metoposaurus* was aquatic. *M. d. krasiejowensis* is one of the most common metoposaurids from Krasiejów site, Poland.

## Paleobiology

Biomechanics studies show that *Metoposaurus* swam with their flippers with simultaneous and symmetrical strokes similar to plesiosaurs. The broad and flat head and arms with wide hands and a long tail in *M. diagnosticus* suggest they swam in ephemeral lakes during the wet season and used their heads and forearms to burrow during the dry season. The medullary region is filled with well-developed trabecular bone. Thick layers of high vascularization and thick, compact annuli with many rest lines are seen in growth marks of the bone. This may correlate favourably with wet seasons and unfavourably with dry seasons. Associated phytosaurs in bonebeds with *Metoposaurus* may represent potential predators.

## Paleoecology

*Metoposaurus* was likely ambush predators that lived on the shores and riverbanks of Pangea. They likely preyed on small animals such as *Eoraptor*, as prey of this size would have only required several snaps to quickly eat whole.

## Species

* *M. diagnosticus*
	+ *M. d. diagnosticus*
	+ *M. d. krasiejowensis*
* *M. algarvensis*
* *M. ultimus*

### *Reassigned Species*

* *M. maleriensis*
* *M. azerouali?*
* *M. stuttgartensis (=M. diagnosticus)*
* *M. santaecrucis (*dubious*?)*
* *M. heimi (=M. diagnosticus)*
* *M. krasiejowensi*
* *M. bakeri*

## Notable Specimens

* FCT-UNL 600: A skull from the Penina Bonebed, the holotype of *M. algarvensis*.
* MNHN.F.ARG 5: A relatively well-preserved and somewhat deformed skull. It is the holotype of *M. azerouali*, sometimes known as *Arganasaurus*.
* UMMP 13055: Holotype skull of possible species *M. bakeri*.
* SMNS 10825: A partial skull from Feuerbacher Heide, assigned the type of *M. d. diagnosticus*.
* SMNS 11423, 12712, 12714: A clavicle, interclavicle, a vertebra and ribs assigned syntypes of *M. stuttgartensis*, now referred to *M. diagnosticus*. Fraas (1913) named it, but it is traditionally considered synonymous. It is now deposited in the Stuttgart Museum.
* BSPG 1931 X 3: A skull from Ebrach bed number 7, assigned to *M. heimi* by Kuhn (1932). It is now deposited in the Museum of Paleontology and Historical Geology of Munich.
* ZPAL Ab III 358: A partial skull roof and holotype of *M. d. krasiejowensis*.
* Algarve bonebed: A bonebed of *Metoposaurus* found in Algarve, Portugal by Octávio Mateus. Hundreds of individuals were scrambled into 0.5 meters of mud. It was excavated multiple times over several years, since the bonebed was so concentrated and massive. Their death was likely due to climate change 230 million years ago.

## References

* https://en.wikipedia.org/wiki/Metoposaurus
* https://paleobiodb.org/classic/basicTaxonInfo?taxon\_no=37164
* https://www.chapters.indigo.ca/en-ca/books/the-rise-and-fall-of/9780062490438-item.html