Chacaicosaurus

In 1990, Zulma Gasparini led an excavation team from the Museo Olsacher and the Museo de La Plata that operated in the Neuquén Basin of Argentina in Patagonia. They recovered a partial skeleton of an ichthyosaur from the Chacaico Sur locality of the Los Molles Formation, in Zapala Department, Neuquén Province. This specimen, given the number MOZ (standing for Museo Olsacher) 5803, includes a complete but damaged skull, six vertebrae, one bone from the shoulder girdle, Marta S. Fernández named the new genus and species Chacaicosaurus cayi in 1994 to contain MOZ 5803, which serves as its holotype and is the only known specimen of the genus. The name of the genus is derived from the name of the Chacaico Sur locality and sauros, Greek for "lizard", while that of the species refers to the Mapuche sea god Cay. ==Description==

Chacaicosaurus is a medium-sized ichthyosaur based on the size of its skull; Skull The skull of the holotype measures long, while its mandible (lower jaw bones) is long. The skull bears a narrow snout which, characteristically, is 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. Michael Maisch and Andreas Matzke in 2000 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 pairs of other 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) 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 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 family. However, in 2007, Fernández considered the differences between Chacaicosaurus and Stenopterygius to be insufficient, and synonymized them, with C. cayi becoming Stenopterygius cayi as a result. 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, including Chacaicosaurus in their analysis. They found it 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 these studies to link Chacaicosaurus to Ophthalmosauridae were also seen in Stenopterygius, and maintained Chacaicosaurus as a synonym of that genus. Further reiterations of this analysis continued to find such a placement, and another analysis conducted by Dirley Cortés and colleagues in 2021 also found Chacaicosaurus to be closer to ophthalmosaurs than S. quadriscissus. However, not all analyses have found such a placement. A study by Benjamin Moon in 2017 did not find it to be a member of Thunnosauria, though still recovered it within Neoichthyosauria, the group that includes all post-Triassic ichthyosaurs. Additionally, analyses based on the 2016 dataset of Cheng Ji and colleagues also found Chacaicosaurus to be outside Thunnosauria. It was instead found to be the sister taxon of Hauffiopteryx, with the next-closest genus being Leptonectes. Topology from Zverkov and Jacobs, 2021. }} Topology from Bindellini and colleagues, 2021. }} }} ==Paleobiology==

Ichthyosaurs were well-adapted to life in the water, though still needed to breathe air. While some of the earlier ichthyosaurs would have swum by undulating like eels, Jurassic ichthyosaurs were less elongate and more streamlined, and instead would have employed carangiform or thunniform swimming, where the animal was propelled forward by side to side movement of the tail, similar to modern tuna. As these ichthyosaurs would have been fast swimmers, they could chase down their prey instead of lying in wait for it, finding it mainly by sight using their massive eyes. Ichthyosaurs did not lay eggs, as they could not crawl onto land; instead, they gave birth to live young underwater. As ichthyosaurs grew, their bone anatomy changed. Two features of the holotype of Chacaicosaurus, the strong incisions on the leading edge of the forelimb and the rounded upper end of its humerus, indicate maturity, so Fernández considered the skeleton to have belonged to an adult. ==Paleoenvironment==

of the Los Molles and Lajas Formations in the region of Chacaico Sur The Los Molles Formation, the rock unit in which Chacaicosaurus was found, is a member of the Cuyo Group. While this formation, reaching up to thick in some places, consists of rock laid down from the Pliensbachian stage of the Early to the Callovian stage of the Middle Jurassic, Chacaicosaurus is only known from the zone containing the ammonite Emileia giebeli, dating to the early part of the Bajocian, a stage which lasted from around 170.3 to 168.3 million years ago. The rocks in this region are mostly marl and dark shale, though there is also some sandstone. Turbidites, formed by sediment deposited by currents deep underwater, are present. These strata are thought to have been laid down in an open ocean environment, with the sea level lowering at the time. In addition to Chacaicosaurus, three other marine reptiles have been found in the Los Molles Formation: the ophthalmosaurid plesiosaur Maresaurus, and a metriorhynchid thalattosuchian. Like Chacaicosaurus, these three marine reptiles also lived during the early Bajocian, and all of them were well-adapted to offshore life. Chacaicosaurus lived shortly after a faunal turnover in the preceding Aalenian stage, affecting both plesiosaurs and ichthyosaurs. Among the ichthyosaurs, ophthalmosaurids became the dominant group while the non-ophthalmosaurid neoichthyosaurs, the group to which Chacaicosaurus belongs, went into a steep decline. Invertebrates of the time include ammonites, bivalves, brachiopods, and ostracods, and the microscopic foraminifers were also present. All of these groups were abundant and diverse at the time, ==See also==