Razanandrongobe

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Razanandrongobe
Temporal range: Bathonian, 167–164 Ma
Razanandrongobe.jpg
Holotype of Razanandrongobe, showing teeth and associated maxillary fragment
Scientific classification e
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Sebecosuchia
Genus: Razanandrongobe
Maganuco et al., 2006
Type species
Razanandrongobe sakalavae
Maganuco et al., 2006

Razanandrongobe (meaning "ancestor [of the] large lizard" in Malagasy) is a genus of carnivorous sebecosuchian from the Middle Jurassic of Madagascar. It is based on MSNM V5770, a fragment of maxilla, the main tooth-bearing bone of the upper jaw, and several isolated teeth have also been assigned to it. The remains come from the Bathonian-age Isalo Illb rock unit of Mahajanga. The jaw fragment and teeth are similar to the representative bones of theropod dinosaurs and some types of crocodylomorphs, and the authors refrained from assigning the genus to either group. They interpreted the stout teeth as having been used for crushing bones, and their denticles are remarkably large, even larger than those of Tyrannosaurus.[1] Remains described in 2017 have given further insight to the phylogeny of R. sakalavae, revealing it to be the oldest known notosuchian.

Description[edit]

Life reconstruction; the body is speculatively restored

Based on available remains, Razanandrongobe is the largest known Jurassic member of the Mesoeucrocodylia. In life, the length of its skull likely surpassed that of Barinasuchus,[2] which has been estimated at 88 cm (35 in) long.[3]

Snout bones[edit]

Reconstruction of the snout, viewed from the front; bony nostrils marked as ano

Razanandrongobe has a highly specialized skull, with a robust and rounded (U-shaped) snout that is taller than it is wide (oreinirostral), like Dakosaurus. At the front of the snout, the openings of the bony nostrils - the apertura nasi ossea - face forward, and are fused at the midline. Smooth fossae known as the perinarial fossae extend down from the nostrils to the level of the teeth. The remainder of the premaxilla has a roughened surface, covered in crests, ridges, and pits. On the palate, two sub-circular depressions are visible at the front of the snout, where the first pair of teeth from the lower jaw would have been located when the mouth was closed. The palatal portion of the maxilla does not close off the bottom edge of the premaxillae, leaving a large opening - an incisive foramen - which is about half as long as the premaxilla is wide. On the inside of the tooth row, there is a paradental shelf covered in ridges and furrows.[2]

The maxilla is similarly tall and robust to the premaxilla, and the paradental shelf continues onto it. The surface of the palate is placed unusually high above the tooth row, at about halfway up the depth of the tooth sockets. Below the eye sockets, the maxillary palate is thickest. At this position, it meets the portion of the palate formed by the palatine bones, and borders the openings known as the suborbital fenestrae. In this way, the palate of Razanandrongobe is similar to the Ziphosuchia, including Araripesuchus. On the interior of the maxilla, there is a smooth groove, which may correspond to a pneumatic opening in the skull that is also seen in Alligator.[2]

Lower jaw[edit]

Like the snout, the lower jaw is also tall and robust. Uniquely, the tip of the lower jaw that is devoid of teeth, for a section of the dentary bone corresponding to the diameter of more than one tooth. The front of the jaw would have been fused; on the inside of the bone, there is a scar running along the rear 20% of the fused portion, representing the attachment of the splenial bone. Like the premaxilla, the outer surface of the dentary is textured, bearing a dense network of zigzagging canals for blood vessels (i.e., vascular canals). On the interior surface, immediately adjacent to the tooth row, there is a row of pits, which become enclosed in a groove towards the back of the jaw. The top margin of the bone is convex at the front, then transitions into a concave region.[2]

Teeth[edit]

Razanandrongobe had five teeth in each premaxilla, at least ten in each maxilla, and eight in each half of the dentary. Most of the tooth sockets are sub-circular, although the inner half of the sockets in the maxilla and the front of the dentary are rectangular. All of them are wider than they are long, and are nearly vertical. Larger sockets are separated by narrower distances than smaller teeth, and the separating surfaces are ornamented like the paradental shelves. The teeth themselves are unusual; they bear large serrations on both the front and rear edges, which are proportionally even larger than those of dinosaurs such as Tyrannosaurus. They are also thick, non-constricted, and slightly recurved (pachydont), and are sub-oval to U-shaped (or salinon-shaped) in cross section. The smallest teeth are globe-shaped. None of the teeth are particularly hypertrophied like the canine teeth of mammals (i.e., caniniform), but the first three dentary teeth are larger than the rest.[2]

References[edit]

  1. ^ Maganuco, S.; Dal Sasso, C.; Pasini, G. (2006). "A new large predatory archosaur from the Middle Jurassic (Bathonian) of Madagascar, with remarks on its affinities and paleobiology". Atti della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale in Milano. 147 (1): 19–51.
  2. ^ a b c d e Dal Sasso, C.; Pasini, G.; Fleury, G.; Maganuco, S. (2017). "Razanandrongobe sakalavae, a gigantic mesoeucrocodylian from the Middle Jurassic of Madagascar, is the oldest known notosuchian". PeerJ. 5: e3481. doi:10.7717/peerj.3481.
  3. ^ Molnar, R.E. (2013). "Cenozoic dinosaurs in South America revisited". Abstracts with Programs. 47th Annual Meeting of the Geological Society of America. 45 (3). San Antonio: Geological Society of America. p. 83.

External links[edit]

  • Razanandrongobe sakalavae - Dinosaur Mailing List posting that announces the genus and includes the abstract of Maganuco et al.'s article.