Toretocnemus (meaning 'perforated tibia') is an extinct genus of ichthyosaurs that lived during the Carnian stage of the Upper Triassic in what is now North America. Two species are known, T. californicus and T. zitelli, first described in 1903 by John Campbell Merriam from fossils discovered in the Hosselkus Limestone, Shasta County. The second species was first seen by Merriam as belonging to a distinct genus, but in 1999 it was reclassified into the original taxon. Toretocnemus fossils are primarily known from California, although some specimens are also reported from Alaska and Mexico. With Qianichthyosaurus, the taxon is part of the Toretocnemidae, of which it is also the type genus.
History of research
The specimen that would eventually be cataloged as UCMP 8100 was found along the Pit River in Shasta County, California, by Annie Montague Alexander. While much of the specimen had been lost to erosion, many elements were still preserved, namely some skull fragments, around 30 vertebrae, including some dorsal (trunk) vertebrae, but being mostly represented by frontwards caudal (tail) vertebrae, a large number of ribs, the pelvis, the right forelimb, and both the left and right hindlimbs. Another specimen, UCMP 8099, was also found by Alexander in Shasta County. This one was uncovered on a ranch and consisted of a partial skull, various incomplete vertebrae, poorly-preserved ribs, gastralia (belly ribs), the shoulder girdle, forelimbs, and a partial hindlimb. Both of these specimens came from the Trachyceras zone of the Hosselkus Limestone. Alexander gave them, in addition to multiple other fossil reptiles, to the University of California around 1903.
It was John Campbell Merriam who described the genus and its type species Toretocnemus californicus in an article published the same year the fossils were received, also designating specimen UCMP 8100 as the holotype of this taxon. The genus name is derived from Ancient Greek , ("torētós", "perforated") and , ("knḗmē", "shin bone" or "tibia"), all meaning "perforated tibia", while the specific name refers to the state of its discovery. In the same publication, Merriam named another genus and species, Leptocheirus zitteli, for UCMP 8099. For a reason not explained by Merriam, the specific name of this taxon honors the German paleontologist Karl Alfred von Zittel. However, the name Leptocheirus was preoccupied (already in use for something else), and George Albert Boulenger renamed it Merriamia in 1904. In 1908, Merriam noted that there were additional specimens from the same area as the holotype that likely pertained to T. californicus, although these had yet to be prepared.
Noting the multiple anatomical similarities between Toretocnemus and Merriamia and the closeness of their sites of discovery, Ryosuke Motani synonymized the two genera, with the older name Toretocnemus taking precedence. Restudying the specimens, he found that their preservation did not allow for the distinguishing traits proposed by Merriam to be observed properly. However, he tentatively maintained both species (T. californicus and T. zitteli) as valid, as there was little overlapping material between the specimens, complicating comparisons. In 2000, Michael Maisch and Andreas Matzke concurred with this synonymy, also while keeping T. californicus and T. zitteli as separate species.
In 2001, T. californicus vertebrae were found in the Antimonio Formation near the mining town of El Antimonio in Sonora, Mexico. Although ichthyosaur vertebrae are quite commonly found here, most of these were larger shastasaurid vertebrae. The ones found in the Antimonio Formation were much smaller, ranging in diameter from . Neural spines were not present, but there were clear grooves for their articulation.
In a 2019 dissertation, Katherine Anderson, together with Patrick Druckenmiller and Jim Baichtal described two specimens from Alaska which they assigned to Toretocnemus. One of these, UAMES 3599, came from the Nehenta Formation on the coastline of Gravina Island. It was found in 1969 and extracted in 2004. The other specimen, UAMES 34994, was found in the Hound Island Volcanics Formation on Hound Island.
Description Toretocnemus is classified within Ichthyosauria, a group of marine reptiles well-adapted to life underwater. The eyes of ichthyosaurs were large, and their skulls generally bear elongated snouts. Their bodies were streamlined and smooth-skinned, either bearing very small scales or devoid of scales entirely. The limbs of ichthyosaurs are heavily modified into rigid flippers. Additionally, ichthyosaurs are also known to have dorsal fins as well as caudal fins, supported by a downward bend in the tail. Toretocnemus probably did not grow longer than , with some length estimates putting both species at only about , making it a smaller member of the group. In 2019, Anderson and colleagues estimated a total length of under for the Alaskan specimen UAMES 3599.
Skull
The orbits (eye sockets) of Toretocnemus are very large, with the jugals (a pair of bones which form the underside of each orbit) being narrow. The bony plates making up the scleral ring, a structure housed within the orbit, are very large. The exposure of the angulars (lower rear exterior lower jaw bone) on the outside of the mandible is very limited in size in Toretocnemus, with most of the outer surface of the rear part taken up by the surangular. Both the splenials (a pair of inner mandibular bones) and the articulars (mandibular bones involved in the jaw joint) are enlarged in Toretocnemus, especially in the case of the latter. The teeth of Toretocnemus are all the same shape and erupt from a groove.
Vertebrae and ribs The hindmost dorsal vertebrae in T. californicus have neural spines that curve backwards and notches in the place of zygapophyses (two pairs of forwards- or backwards-directed projections). The thin neural arches of T. californicus bear thin ridges on their sides. While the dorsal centra have similar lengths and heights in T. zitteli, T. californicus has centra that are much less elongated. The dorsal centra are taller than wide, and both their front and back faces are concave (amphicoelous). The parapophyses and diapophyses on the dorsal vertebrae, projections that articulate with the ribs, are small and separate in T. californicus with considerable space in between them; a characteristic feature of the genus. Both the front and back faces of the dorsal ribs of Toretocnemus bear grooves along much of their length, barring their lower ends. The only well-preserved dorsal rib head of T. zitteli is not bifurcated; conversely, the dorsal rib heads of T. californicus are strongly bifurcated in the middle to rear part of the torso. The rib heads located further forwards on the vertebral column of T. californicus are more prominently bifurcated. T. zitteli has five sets of gastralia.
The neural spines (upwards projections on the vertebrae) of the caudal vertebrae are highly elongate in Toretocnemus. In T. californicus, the caudal vertebrae have straighter neural spines and longer zygapophyses than the dorsal vertebrae; these caudal vertebrae also have very tall diapophyses. All of the caudal centra in this species are amphicoelous. The caudal ribs of T. californicus, unlike the dorsal ribs, do not have bifurcated heads. T. zitteli has long caudal vertebrae. The only known caudal centrum of T. zitteli in good condition is from the front part of the tail, wider than it is tall, and amphicoelous. The sides of the centrum are not bowed outwards, and the vertebral body has a six-sided cross-section. The downward bend in the tail of Toretocnemus is weak, and formed by caudal centra with longer tops than bottoms, giving them a wedge-like shape. In 2019, Anderson and colleagues found the angle of the bend would have been at least 6.45 degrees. The diapophyses extend out far to the sides and the facets for the chevrons are well-demarcated. The elongated chevrons of T. californicus are shaped like the letter Y.
Appendicular skeleton The rounded coracoids of T. zitteli are twice as long as wide and do not have any concavities along their edges. Their inner sides are more robust than their outer sides. A hook-like projection is present on the front edges of the short scapulae (shoulder blades). The surfaces on the scapulae that the coracoids and humeri articulate with are separate. The lower ends of the scapulae shoulder blades are widened. The clavicles (collarbones), which contact each other, are also quite broad. Merriam tentatively identified a triangular bone as the interclavicle of T. zitteli. The putative interclavicle is small and its backwards projection is very short. The long upper hip bone, the ilium, is quite narrow in T. californicus. The pubic bones and ischia (lower hip bones) of T. californicus are flat and each pair are in extensive contact along the midline. The wide pubic bones (front lower hip bones) of T. californicus are each perforated by a small hole known as an obturator foramen, similar to Cymbospondylus. The ischia are not as wide as those of Shastasaurus.
The two species of Toretocnemus have similar limbs. In 1903, Merriam stated that the forelimbs of T. zitteli are longer than its hindlimbs, while the hindlimbs or T. californicus are at least as long as its forelimbs, if not even longer. However, in 1999, Motani pointed out that as both specimens are disarticulated and incomplete, it cannot be confirmed that the limbs assigned to each specimen all came from the same individual, therefore this difference is of dubious utility. The upper and lower limb bones of both species are very elongate. Toretocnemus has a narrow humerus (upper arm bone) with a broad lower end; this bone being twice as long as the radius. The upper end of the humerus is roughly flush with the front of the bone, while the rear edge of the humerus is diagonal. There is a well-defined notch halfway down the front edge of the humerus, a notch is also present on the bone's rear edge but is not as strongly demarcated. The surfaces of the facets for the lower arm bones are bowed inwards. The upper end of the humerus in T. zitteli bears a ridge half the length of the humerus on both its top face and underside, though the one on the underside is larger. Another upraised area of the humerus is complemented by a similar structure on the ulna.
An opening is present between the lower limb bones (epipodials) of both species. Both the inner and outer edges of the radius are concave, but only the inner edge of the ulna is notched. While poorly known, the forelimbs of T. californicus appear similar to those of T. zitteli, though the radius of T. californicus is moderately broader than that of T. zitteli and the notches on the forelimb bones of the former are more pronounced. The carpals (wrist and ankle bones) of T. zitteli are set into two rows of three. Only one carpal connects to the lower edge middle upper wrist bone, the intermedium. The intermedium of contacts the ulna more extensively than the radius, as the radiale is broader than the ulnale (front and back upper wrist bones, respectively). The carpals (digit bones) of Toretocnemus forming the outer margins of the flippers all bear notches on at least their outwards-facing edges. Both the front and back edges of the metacarpal of the frontmost digit bear notches.