User:Slate Weasel/Alternate sandbox

=Mixosauridae= Refs:

Locomotion
Mixosaurids would have swam using both their tail and body. The mixosaurid dorsal fin would have helped keep the animal steady. The tail fin would have been used in propulsion, and increased the animal's acceleration. However, the heterocercal tail fin preserved in Mixosaurus would not have been as efficient a propulsive structure as the slender-based, crescentic tail fins of later ichthyosaurs, meaning that M. cornalianus would have been slower. Nevertheless, Mixosaurus shows features, such as its sleek, deep, and rigid trunk, small fins, and transversely flattened tail, that are associated with sustained swimming, and resembles some ground sharks in size and fin placement. Mixosaurus was apparently a competent swimmer, able to catch its prey in open water.

Based on a specimen of Phalarodon atavus, Liu and colleagues noted two major adaptations for sustained swimming in mixosaurids. The streamlined body shape of P. atavus would have reduced drag, slightly more so than that of M. cornalianus. Additionally, their disk-like centra would have made the trunk less felxible, as seen in lamnid sharks.

The tall neural spines of mixosaurids would have likely anchored large epaxial muscles, which Appleby proposed would have allowed greater vertical mobility of the tail. Darren Naish proposed in 2023 that such large muscles could indicate that mixosaurids could reach particularly fast swimming speeds.

Diet and feeding
The enlarged sagittal crests of mixosaurids would have anchored heavily enlarged adductor muscles, which would have covered a large amount of the skull roof. This would have provided mixosaurids with a powerful bite.

As mixosaurids were likely fast, Liu and colleagues considered it more likely that they actively sought out their prey, rather than lying in wait for it.

Stomach contents of M. cornalianus include the remains of small actinistian and putative neoselachian fishes.

Based on the interpretations of Renesto and colleagues in 2020, unlike some reptiles, mixosaurids (and presumably other ichthyosaurs) could pass bone from their stomachs through their intestines.

The variation in dentition between different mixosaurids indicates that different species would have fed on different prey. This would have allowed for the coexistance of multiple mixosaurid species in a single area, as seen in Monte San Giorgio, without the otherwise similar animals competing for food.

Growth and reproduction
Like other ichthyopterygians, mixosaurids gave birth to live young, as evidenced by three gravid specimens from Monte San Giorgio, assigned to Mixosaurus cf. cornalianus. None of these specimens have more than three embryos. Initially, Mixosaurus neonates were thought to be born tail-first to lessen the chances of drowning. While this seems to be the case for two of the specimens, the fetuses are oriented head-first in another, making it unclear whether head-first or tail-first birth, if either, was the norm. Furthermore, Miedema and colleagues in 2023 found no obvious correlation between head-first birth and aquatic existence, leading them to suggest that fetus orientation could be due to the neonate getting stuck in the birth canal, easier to push through the canal, or having less of an effect on the location of the adult's center of mass when oriented a certain way. While not entirely dismissing the possibility, Miedema and colleagues considered it doubful that the fetuses would have been able to turn around, as ichthyosaur fetuses apparently unfurled early on, with the embryos straightening out and potentially stiffening as well.

The skulls of mixosaurids grew at a slower rate than their bodies, meaning that the skull became proportionally smaller as the animal aged.

Description
When viewed from above, the skull of Liopleurodon resembles an isoceles triangle in shape. The skull deepens towards it rear, though it was likely less tall than that of its relative Simolestes. However, due to crushing, it is difficult to reconstruct how tall their skulls were. In a 2001 thesis, Noè estimated the skull of Liopleurodon to be around 4 times longer than tall. The skull is about 2.6 times long as wide. The external nares (openings for the nostrils) are small and located near the skull's midline. Further back on the cranium, the orbits (eye sockets), are located about halfway down the skull's length and placed near to the external nares. The eye sockets of Liopleurodon are large and shaped like keyholes, distinguishing traits of the genus. Behind the orbits are the large temporal fenestrae, which have characteristically rounded front edges. The rear face of the cranium bears large openings. The cranium's underside is formed by a large, flat palate, which is perforated by various openings.

The rostrum (snout) of Liopleurodon is constricted where the premaxillae (the front upper tooth bearing bones) and maxillae (larger upper tooth bearing bones) meet. There are around 28 teeth on each side of the upper jaw, five of which are on each premaxilla. When viewed from the side, the teeth are positioned along a wavy surface, but are arranged in a relatively straight line when seen from below. The gaps between the premaxillary teeth are reasonably wide. The second to fourth and eighth to tenth upper teeth are enlarged and caniniform in shape. The premaxillae bear a ridge along their midline behind the constriction of the rostrum. Until the external nares (nostril openings) are reached, the rear extensions of the premaxillae run roughly parallel to the skull's midline. Between the external nares, the boundary between the two premaxillae is depressed, forming a cleft. Each premaxilla bears a prominent projection on its inner side that extends over the maxilla behind it.

Liopleurodon was said to posesses nasals in Noè's 2001 thesis. Noè stated that these bones form the rear inner rim of the external nares (openings for the nostrils), with the prefrontals overlapping their outer sides and the frontals a small amount of their outer rear edges. However, in 2011, Ketchum and Benson stated that nasals are probably absent in all plesiosaurs.

The rear outer rim of the external nares are formed by the prefrontals, the only bones that the maxillae contact in this region. Likely, there is no foramen behind the external nares. The prefrontals do not extend to the front edges of the orbits. There are no ridges on the maxillae below the external nares.

The lacrimals are located between the maxillae and the front and lower edges of the orbits, preventing the maxillae from reaching the orbits, except possibly for a small part of their front edges. The rest of the borders of the orbits are composed of the prefrontals, jugals, and postorbitals. The small sites of contact between the prefrontals and jugals on each side block the postfrontals from reaching the orbits. The rear end of the frontal articulates with the postfrontal, forming a long boundary that is bowed inwards. The bar between the orbit and the bottom edge of the skull is composed of the lacrimal, jugal, and maxilla. No ridge is present on the maxilla where it extends beneath the orbit. The the front upper surfaces of the postorbitals are concave (they each bear a depression). The postorbitals also bear a small hook-like projection.

The boundary between the maxilla and jugal shows a weak zig-zag pattern. The maxilla extends beneath the jugal for as far as the temporal fenestrae, ending in a hook-shaped structure. The top part of this structure is formed by the jugal.

The jugal sends out a process that extends over part of the inside face of the squamosal.

The bridges of bone extending beneath the temporal fenestrae on each side (known as sub-temporal bars) are long, robust, and somewhat flattened from side to side. Their lower edges are mildly bowed upwards. The gap beneath the sub-temporal bar, known as the sub-temporal embayment, is not very deep. The parietals (bones in the rear skull roof) bear a crest. This crest has no large knob at its front, or a dip immediate behind.

There is no opening between the vomers (frontmost palatal bones). The inner projections of the palatines (palatal bones located towards the front of the palate) meet along the midline of the skull beneath where the vomers and pterygoids (another pair of palatal bones) contact each other. The palatines underlie the maxillae where they meet. Beneath the braincase, at the rear end of the palate, the rims of the pterygoids merge into a midline ridge.

The mandible of Liopleurodon is low and lightly built, with the two sides (rami) fused into a structure known as the mandibular symphysis at the front. When seen from behind, the cross-section of the mandible where the rami diverge is shaped like the letter W. The rami diverge rather gradually, with the mandibular width matching that of the cranium. There are six pairs of alveoli (tooth sockets) located entirely on the symphysis, with a seventh pair partially located on it. While all the teeth entirely on the symphysis are large, the seventh pair is much smaller. Tooth size slowly decreases from the eleventh tooth onwards. Each dentary bears about 28 teeth. The underside of the symphysis slopes gently backwards when viewed from the side and bears a midline ridge. The splenials (lower inner mandibular bones) form part of the lower surface of the symphysis for up to a third of its length. There is no foramen (small opening) between the dentaries and splenials. The coronoid eminence is not very pronounced.