Endennasaurus acutirostris

Reconstruction of Endennasaurus acutirostris（圖片來源：Ghedo，採用 CC BY-SA 4.0 授權）

Age

Triassic（Norian）

227–208.5 Ma

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Taxonomy

Kingdom: Animalia

Phylum: Chordata

Class: Sauropsida

Order: Thalattosauria

Family: Endennasauridae

Genus: Endennasaurus

Species: Endennasaurus acutirostris

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Morphological description

Endennasaurus acutirostris was a Triassic marine reptile of modest size, reaching an estimated total length of about 1.1 m when complete, with the tail making up at least three fifths of the total length, so that the body proportions were characterized by a relatively short precaudal region and a much longer posterior region. The skull was narrow and elongate, and the premaxillae extended forward to form a long pointed rostrum. The external nares were retracted and did not lie close to the tip of the snout. The orbits were large and placed dorsally, with a narrow interorbital region between them. The upper temporal fenestra was very narrow and almost closed. No teeth were present at all, and the margins of the jaws formed a sharp edge that may in life have been covered by a horny beak.

The neck was elongate and included about 12 cervical vertebrae; the trunk had 16 dorsal vertebrae and 2 sacral vertebrae, while the tail contained at least 67 caudal vertebrae. The limbs were not fully transformed into flippers as in some highly specialized aquatic reptiles, but instead retained a clearly recognizable limb structure. The shoulder girdle was massive, and the clavicles were expanded and curved. The humerus was stout, with an expanded and flattened distal end. The femur was longer than the humerus.

Holotype specimen MBSN 5170 of Endennasaurus acutirostris（圖片來源：Ghedoghedo，採用 CC BY-SA 3.0 授權）

Skull of Endennasaurus acutirostris（圖片來源：Müller J et al. (2005)，採用 CC BY 4.0 授權）

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Etymology

Generic name Endennasaurus is derived from Endenna, the fossil locality near Zogno in Lombardy, northern Italy, combined with the suffix -saurus, meaning "lizard" or "reptile".

Specific name acutirostris comes from the Latin acutus, meaning "sharp", and rostrum, meaning "snout" or "beak-like front end", so the name means "sharp-snouted".

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Biological description

Endennasaurus acutirostris lived in northern Italy during the Norian stage of the Late Triassic, when the region consisted of a complex marine setting surrounded by a huge carbonate platform and containing several deep basins. The bottoms of these basins were often anoxic, whereas the margins were shallower and better oxygenated, and a few small islands were scattered through the area.

Endennasaurus acutirostris was completely toothless, which was rather unusual among marine reptiles. It had a long, slender, pointed rostrum, and the sharp margins of the jaws may in life have been covered by a horny beak. This suggests that the mouth was adapted for seizing, probing, pecking, or rapidly taking in small prey at the front of the snout, indicating a highly specialized feeding niche. Soft-shelled invertebrates, fish fry, and small crustaceans have all been suggested as plausible prey.

Given its long tail and aquatic setting, it might easily be imagined as an eel-like swimmer moving by strong lateral undulations of the whole body in the water. However, the study suggested that aquatic reptiles can broadly be divided into two types: one relying mainly on lateral undulations of the trunk and tail, usually with a more flexible body and shorter limbs, and another with a relatively stiff, thickened, or flattened body that relied more on both the tail and the limbs in propulsion. Endennasaurus acutirostris appears to have belonged more closely to the latter type. Its gastralia were strongly developed, the trunk skeleton was robust, parts of the skeleton showed a tendency toward pachyostosis, and the shoulder girdle and clavicles were also solidly built. This means that the middle part of the body was probably not highly flexible like that of a snake, but rather functioned as a stable trunk while the tail and limbs generated propulsion. Accordingly, Endennasaurus acutirostris was probably a steady nearshore forager that could move slowly or at moderate speed in shallow seas, lagoon margins, or coastal waters, using its elongate rostrum to search for small targets in the water and then rapidly seize or suck them in.

The eyes were placed in a relatively dorsal position on the skull, suggesting that in the water it probably held the body low and the head rather flat, looking upward or anterodorsally at the surrounding environment. This kind of visual arrangement would have been advantageous for detecting prey in shallow-water habitats.

At the same time, the limbs were not weakly ossified, and the limb bones themselves were quite strong, indicating that Endennasaurus acutirostris probably still retained some ability to move on land. One study proposed the reasonable inference that, although it was mainly adapted to aquatic life, it may still have needed to come ashore, at least in connection with reproduction.

Reconstruction of Endennasaurus acutirostris（圖片來源：Nobu Tamura，採用 CC BY 3.0 授權）

（Author: Shui-Ye You）

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References

1. Müller J et al. (2005). The marine diapsid reptile Endennasaurus from the Upper Triassic of Italy. Palaeontology.

2. Renesto S. (1992). The anatomy and relationships of Endennasaurus acutirostris (Reptilia, Neodiapsida), from the Norian (late Triassic) of Lombardy. Rivista Italiana Di Paleontologia E Stratigrafia.

45.82, 9.65

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