| Leninia | |
|---|---|
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | †Ichthyosauria |
| Family: | †Ophthalmosauridae |
| Subfamily: | †Ophthalmosaurinae |
| Genus: | †Leninia Fischeret al.,2013 |
| Type species | |
| †Leninia stellans Fischeret al., 2013 | |
Leninia is anextinctgenus ofbasalophthalmosaurineichthyosaur known from the lateEarly Cretaceous (lowerAptian stage) of westernRussia.Leninia was first named by Valentin Fischer, Maxim S. Arkhangelsky, Gleb N. Uspensky, Ilya M. Stenshin andPascal Godefroit in2013 and thetype species isLeninia stellans.[1] It was named forVladimir Lenin, one of the leaders of theCommunist Revolution in Russia, but not directlу: the museum where fossils is housed is located within the Lenin Memorial and Lenin school complex inUlyanovsk; accordingly, the generic name reflects the geohistorical location of the find.
The specimen was discovered in 2012, on the banks of the riverVolga near the town ofKriushi in a limestone nodule. There are several other fossils from the same time period near it, includingammonites,lamellibranchs and fish remains. Only part of the skull was found.[2]
Theskull is incomplete, and without anyteeth. it is partially crushed transversely, and nothing further forward than thenaris remains, with what seems to be a fairly clean break. It is not known whether this was due toorogeny and rock action or to the ichthyosaur's death. The specimen is around 45 centimetres (18 in; 1.48 ft) long, which suggests the skull was between 65–90 centimetres (26–35 in; 2.13–2.95 ft) long overall.
Unusually, the posterior process of themaxilla extends as far back as theorbit, and about halfway along it, possibly even projecting into the orbit at points. The maxillary dental groove is extremely shallow, only 16 millimetres (0.63 in) deep. Thelacrimal-nasal contact is elongated, so that the lacrimal, rather than theprefrontal, forms the posterior edge of the naris. The anterior process and shaft of thejugal are unusually narrow, but the posterior plate is well-developed and extends as high as the middle of the orbit.
Thepostorbital is small compared to the orbit, with a large dorsal orbital rim extending out from the top of the bone. Thequadratojugal seems more robust than that of other ophthalmosaurines, but has a thin anterior surface which articulates with the postorbital. There is a concave area which a ligament would connect to thequadrate process.
Thesquamosal is present but incomplete. However, it is possible to tell the shape by the marks where it would have articulated with other bones. Above the naris, the nasal bone forms a conspicuous lateral 'wing'. There is no clear evidence for aforamen near the prefrontal-nasal facet.
The prefrontal forms a thick anterior process which has grown over the nasal, unlike in other ophthalmosaurids. Its suture with the lacrimal bone is straight. Thefrontals are roughly triangular and have the 'internasal' foramen between them (not between the nasals). They have long posterior processes which suture with the forked processes of theparietals in a star shape, hence the specific namestellans. Thepostfrontal contacts the nasal over a short distance, and as inOphthalmosaurus, has no Y-shaped anterior process. Uniquely, the postfrontal does not border thesupratemporal fenestra, but still forms a long process in contact with the supratemporal.
The parietals touch the anteriomedial process of thesupratemporals, anotherautapomorphy. This anteromedial process also blocks the postfrontal from bordering the supratemporal fenestra. The parietals are crushed against one another, but there is no evidence of a crest.
The supratemporal is more extensive than in most ophthalmosaurines, but is separated from the postorbital by the postfrontal and squamosal bones. Bothpterygoids are present but cannot really be seen, and are distorted.
Thebasioccipital is not perfectly preserved, but the extracondylar area is reduced and concave. Theoccipital condyle is bulbous, but the floor of theforamen magnum is not well preserved enough to be a diagnostic feature.
The occipital head of thestapes is largely expanded, with a largehyoid process, but the quadrate head is missing. Thequadrate is roughly ear-shaped, with an occipital lamella.
Only the posterior end of thelower jaw is preserved, and this lacks any distinctive features, such as teeth. Thesurangular fossa is present but extremely reduced, and theangular is well exposed laterally. These features, however, are not diagnostic or autapomorphies.
Bothsclerotic rings are preserved, but the left one is much less distorted. It is composed of 14 trapezoidal plates, withcrenulations on the internal edge. The sclerotic aperture is only 11.03% of the orbital area, suggesting that the specimen was fully grown. (All of this section is citing[2])
Leninia is one of the latest-living ophthalmosaurines, and also one of the most basal. Despite this, it still had the very large sclerotic ring and aperture, suggesting that this was one of the most basal characteristics of ophthalmosaurines and that they were all deep-diving, remaining conservative in thisecological niche during the entire time period that they existed. The absolute size ofLeninia's sclerotic ring is among the largest known in ichthyosaurs, with onlyBaptanodon ('Ophthalmosaurus natans') and the giantTemnodontosaurus exceeding it. As all the ophthalmosaurines had this kind of structure, and were more similar in eye structure than theplatypterygines, this suggests that they did not diversify in the same way that the platypterygines did but stayed similar as deep divers, possibly as they were so specialised for deep diving that they were outcompeted in other niches.[2]
Cladogram based in the phylogenetic analysis of Fischeret al., 2013:[1]
| Thunnosauria |
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The following cladogram shows a possible phylogenetic position ofLeninia in Ophthalmosauridae according to the analysis performed by Zverkov and Jacobs (2020).[3]
| Ophthalmosauria |
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