Chacaicosaurus

Chacaicosaurus Temporal range: Middle Jurassic, Early
Restoration
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	†Ichthyosauria
Node:	†Neoichthyosauria
Genus:	†Chacaicosaurus Fernández, 1994
Species:	†C. cayi
Binomial name
†Chacaicosaurus cayi Fernández, 1994
Synonyms
Stenopterygius cayi (Fernández, 1994)

Chacaicosaurus is an extinct genus of long-snouted thunnosaur ichthyosaur known from the northwestern Patagonia area of Argentina.

History of research

Chacaicosaurus is known only from the holotype MOZ 5803, an articulated partial skeleton which preserved a partial skull and almost complete forefin. It was collected in the Chacaico Sur locality from the Emileia giebeli ammonoid zone of the Los Molles Formation, Cuyo Group, dating to the early part of the Bajocian, a stage of the Middle Jurassic that lasted from around 170.3 to 168.3 million years ago. The specimen was found near Zapala city of the Neuquén Basin.

Chacaicosaurus was named by Marta S. Fernández in 1994 and the type species is Chacaicosaurus cayi. The generic name is derived from the name of the Chacaico Sur locality, in which the holotype was collected, and sauros, Greek for "lizard". The name of the species refers to the Mapuche sea god Cay. Chacaicosaurus, along with Mollesaurus periallus, which was found at the same locality, are the only diagnostic ichthyosaur specimens from the Aalenian-Bathonian interval.

Description

Skull

The skull of the holotype measures 98 centimetres (3.22 ft) long, while its mandible (lower jaw bone) is 99 centimetres (3.25 ft) long. The skull bears a narrow snout which is, characteristically, heavily elongated, making up 80% of the skull’s length, gradually sloping to a point at its front. Uniquely among ichthyosaurs, the snout bears elongate, rounded ridges that run longitudinally along the premaxillae and nasals, with one ridge on each side. The cranial proportions of Chacaicosaurus are similar to those of other long-snouted ichthyosaurs, such as Eurhinosaurus, though unlike that genus Chacaicosaurus does not have an overbite. Unusually, adult Chacaicosaurus appear to have either had very reduced teeth or been toothless.

The enlarged external nares (openings for the nostrils) of Chacaicosaurus each measure about 10 centimetres (3.9 in) long, and are not positioned especially close to the orbits (eye openings). The orbits of Chacaicosaurus are very poorly preserved, so their size is unclear. In the description of the genus, Fernández estimated the sclerotic ring (the ring of bony plates supporting the eyeball) to have had a diameter of around 13 centimetres (5.1 in). She considered this to also be a reasonable rough approximation of the width of the orbit, thus concluding that Chacaicosaurus had especially small orbits. While Ryosuke Motani also stated that the orbits were small in 1999, Michael Maisch and Andreas Matzke instead considered the orbits to be especially large.

The wide basioccipital (rear lower braincase bone) of Chacaicosaurus lacks a peg-like projection on its front end. The surfaces on the basioccipital for articulation with two other pairs of braincase bones, the opisthotics and stapedes, are both angled towards the top of the skull, with the latter pair reclined. The occipital condyle, the knob on the back of the skull for the articulation of the spinal column, is clearly demarcated from the rest of the basioccipital. The occipital condyle is not especially large, taking up relatively little of the rear face of the basioccipital, with additional bone surface extending outwards and beneath it.

Postcranial skeleton

The first cervical (neck) vertebra, the atlas, bears a triangular site for articulation with the skull, and has a prominent keel running along the middle of its underside. The cervical centra (vertebral bodies) bear deep depressions where they articulated with the neural arches, with the diapophyses (upper pairs of processes articulating with the ribs) are positioned at the same height as these facets. The transverse processes (sideways projections on the vertebrae) are extensive. The interclavicle (a shoulder bone positioned between the collarbones) is very wide at its center, from where the sideways and backwards projections originate.

The narrow forelimbs of Chacaicosaurus are rather small compared to its skull, and closely resemble those of Stenopterygius. The radius (front lower arm bone) bears an incision on its front edge, as do the seven uppermost bones in the digit beneath it. The radius is comparable in size to the ulna (rear lower arm bone), in addition to the middle upper carpal (wrist bone), the intermedium. While the boundary between the lower arm bones is short, they are in contact across its entire length, with no gap between them. Distinctively, each forelimb of Chacaicosaurus contains four primary digits, the longest of which contains at least 14 elements. The foremost of these digits arises from the front upper carpal, the radiale, the second from the intermedium, and the rear two from the rear upper carpal, the ulnare. While the phalanges (digit bones) start out polygonal, they become increasingly small and rounded towards the tip of the flipper, where the digits are less tightly packed. The phalanges are also very thick and boxy. In addition to the four primary digits, there is also an accessory digit behind them, a digit which terminates before reaching the wrist.

Classification

In the original 1994 description, Fernández did not find Chacaicosaurus to compare favorably to any other Jurassic ichthyosaur, therefore refraining from assigning Chacaicosaurus to a particular family. A study by Motani, published in 1999, sought to better understand ichthyosaur relationships through a cladistic analysis. While he did not include Chacaicosaurus in this analysis, he assigned it to the group Thunnosauria, citing similarities between its snout and forelimb and those of Stenopterygius acutirostris (a species since reassigned to Temnodontosaurus). A year later, Maisch and Matzke also conducted a phylogenetic analysis of ichthyosaur relationships, though again did not include Chacaicosaurus in the analysis. They considered the skull of Chacaicosaurus to resemble that of Ophthalmosaurus but its forelimbs to be much more like those of Stenopterygius. Noting that Chacaicosaurus lived after Stenopterygius but before Ophthalmosaurus, the researchers considered it possible that Chacaicosaurus was an intermediate between the two genera. Due to its similarity to Stenopterygius, Maisch and Matzke placed it in the family Stenopterygiidae within Thunnosauria. They also considered it possible that rather than belonging to its own genus, C. cayi could be a derived member of Stenopterygius, but considered the evidence insufficient to combine the two genera. However, in 2007, Fernández considered the differences between Chacaicosaurus and Stenopterygius to be insufficient, and synonymized the two genera, with C. cayi becoming Stenopterygius cayi as a result of the reassignment. In 2010, Maisch retained Chacaicosaurus as a separate genus, assigning it to Stenopterygiidae alongside Stenopterygius and Hauffiopteryx.

In 2011, Valentin Fischer and colleagues conducted a cladistic analysis of thunnosaurian relationships, marking the first inclusion of Chacaicosaurus in such an analysis. They found Chacaicosaurus to be the sister taxon (closest relative) of Ophthalmosauridae, with Stenopterygius as the closest relative of this grouping, with another analysis in a 2012 study, also led by Fischer, finding a similar placement. However, Erin Maxwell and colleagues criticized this placement in 2012, noting that the traits Fischer and colleagues had used in the 2011 and 2012 studies to link Chacaicosaurus to Ophthalmosauridae were also seen in Stenopterygius, and maintained Chacaicosaurus as a synonym of that genus.

See also

• List of ichthyosaurs
• Timeline of ichthyosaur research