Sauropelta

Sauropelta (/ˌsɔːroʊˈpɛltə/ SOR-oh-PEL-tə; meaning 'lizard shield') is a genus of nodosaurid dinosaur that existed in the Early Cretaceous Period of North America. One species (S. edwardsorum) has been named. Anatomically, Sauropelta is one of the most well-understood nodosaurids, with fossilized remains recovered in the U.S. states of Wyoming, Montana, and possibly Utah. It is also the earliest known genus of nodosaurine; most of its remains are found in a section of the Cloverly Formation dated to 108.5 million years ago.

Sauropelta is a medium-sized nodosaurid, measuring about 5.2–6 metres (17.1–19.7 ft) long. Sauropelta had a distinctively long tail which made up about half of its body length. Although its body was smaller than a modern black rhinoceros, Sauropelta was about the same mass, weighing up to about 1.5–2 metric tons (1.7–2.2 short tons). The extra weight was largely due to its extensive covering of bony armor, including the characteristically large spines projecting from its neck.

Sauropelta was a heavily built quadrupedal herbivore with a body length of approximately 5.2–6 metres (17.1–19.7 ft) and a body mass of 1.5–2 t (1.7–2.2 short tons). The skull was triangular when viewed from above, with the rear end wider than the tapering snout. One skull measured 35 centimeters (13.75 in) in width at its widest point, behind the eyes. Unlike some other nodosaurids, the roof of the skull was characteristically flat, not domed. The roof of the skull was very thick and covered in flat, bony plates that are so tightly fused that there appear to be no sutures (boundaries) like the ones seen in Panoplosaurus, Pawpawsaurus, Silvisaurus, and many other ankylosaurs. This could also be an artifact of preservation or preparation. As in other ankylosaurs, thick triangular scutes projected from the postorbital bone, above and behind the eyes, as well as the jugal bone, below and behind the eyes. More typically for nodosaurids, leaf-shaped teeth lined both upper and lower jaws, used for cutting plant material. The front end of the skull is unknown, but there would have been a sharp bony ridge (tomium) at the end of both upper and lower jaws, as seen in other ankylosaurs. This ridge probably would have supported a keratinous beak.

The tail of Sauropelta was characteristically long and made up nearly half of the body length. One skeleton preserved forty caudal (tail) vertebrae, although some were missing, suggesting that the true number of caudal vertebrae may have exceeded fifty. Ossified tendons stiffened the tail along its length. Like other ankylosaurs, Sauropelta had a wide body, with a very broad pelvis and ribcage. The forelimbs were shorter than the hindlimbs, which resulted in an arched back, with the highest point over the hips. Its feet, limbs, shoulders, and pelvis were all very stoutly constructed and reinforced to support a great deal of weight.

Like other nodosaurids, Sauropelta was covered in armor formed from bony masses embedded in the skin (osteoderms). The discovery of a skeleton with the body armor preserved in situ allowed Carpenter and other scientists to accurately describe this protection. Two parallel rows of domed scutes ran down the top of the neck, along the anteroposterior axis (front to back). On the upper surface of the back and tail, the skin was covered in small, bony nodules (ossicles), which separated larger conical scutes arranged in parallel rows along the mediolateral axis (side to side). Over the hips, the ossicles and larger domed plates were interlocked very tightly to form a structure called a sacral shield. This shield is also found in ankylosaurs like Polacanthus and Antarctopelta. Large, pointed spines lined the sides of the neck, increasing in size towards the shoulders, and then decreasing in size again along the side of the body until they stopped just before the hips. Behind the hips, flat triangular plates lined the tail on both sides, pointing laterally (outwards) and decreasing in size towards the end of the tail. Carpenter originally described the cervical (neck) spines and caudal plates as belonging to a single row on each side, although more recently he and Jim Kirkland reconstructed them in two parallel rows on each side, one above the other. The upper row of cervical spines pointed backwards and upwards (posterodorsally), while the lower row pointed backwards and outwards (posterolaterally). The bases of each pair of cervical spines and each pair of caudal plates were fused together, greatly restricting mobility in both the neck and upper tail.

Since John Ostrom first described Sauropelta in 1970, it has been recognized as a member of the family Nodosauridae. The nodosaurids, along with the family Ankylosauridae, belong within the infraorder Ankylosauria. Nodosaurids are characterized by certain features of the skull, including the mandible (lower jaw), which curves downwards at the end. Overall, nodosaurids had narrower snouts than the ankylosaurids, and also lacked the heavy ankylosaurid tail clubs. Nodosaurids, like ankylosaurids, are found in North America, Asia, and Europe.

While the systematics (evolutionary relationships) of nodosaurids have not been firmly established, the genera Sauropelta, Silvisaurus and Pawpawsaurus are sometimes considered to be basal to geologically younger nodosaurids like Panoplosaurus, Edmontonia, and Animantarx. In a 2001 analysis, Carpenter included the former three genera in a sister clade to a group containing the latter three, although he found that Panoplosaurus could belong to either clade, depending which taxa and characters were chosen. The phylogenetic results of Rivera-Sylva and colleagues in 2018 are below, showing the relationships of Sauropelta to other nodosaurids.

In the early 1930s, famed dinosaur hunter and paleontologist Barnum Brown collected the holotype specimen of Sauropelta (AMNH 3032, a partial skeleton) from the Cloverly Formation in Big Horn County, Montana. The locality is inside the Crow Indian Reservation. Brown also discovered two other specimens (AMNH 3035 and 3036). The latter is one of the best-preserved nodosaurid skeletons known to science, includes a large amount of in situ armor, and is on display in the American Museum of Natural History in New York City. AMNH 3035 preserves the cervical armor and most of a skull, missing only the end of the snout. Expeditions in the 1960s led by the equally renowned John Ostrom of Yale University's Peabody Museum of Natural History recovered additional incomplete specimens from the Cloverly. In 1970, Ostrom coined the genus Sauropelta to include remains discovered by both expeditions. Derived from the Greek σαυρος (sauros) ('lizard') and πελτε (pelte) ('shield'), this name is a reference to its bony armor. Although Ostrom originally named the species S. edwardsi, nomenclaturist George Olshevsky corrected the spelling to S. edwardsorum in 1991 to conform to Latin grammar rules.