Limnoscelis

The type species Limnoscelis paludis was collected by the fossil hunter David Baldwin between 1877 and 1880 from the El Cobre Canyon beds of the Cutler Formation, New Mexico. Baldwin was collecting fossils in service of the paleontologist Othniel Charles Marsh during the bone wars. Although Marsh would describe several specimens from Baldwin's collections, which would later be attributed to the species Limnoscelis dynatis. However, the fossils themselves were not recognized as their own species until paleontologists David S. Berman and Stuart S. Sumida described the fossils in 1990. They designated the new species as Limnoscelis dynatis, with “dynatis” being derived from the Greek meaning “strong” or “powerful”, referencing the genus’ capability as a “formidable predator”. ==Description==

The skeleton of Limnoscelis was relatively large, with Limnoscelis paludis measuring 7 feet (around 2 meters) long. Skull and teeth Limnoscelis had a relatively elongated skull, with a narrow snout and wider posterior region. This “line of weakness” has been proposed to be a precursor to the synapsid temporal fenestra, Axial skeleton Limnoscelis had 26 presacral vertebrae. These vertebrae had swollen neural arches, along with neural spine height. Limnoscelis had a multipartite atlas and axis complex, with a ventral anterior process of the axis intercentrum articulating with that of the atlas. though the second sacral vertebra is reduced compared to the first. The absence of the tibiale has been attributed either to poor preservation (possibly due to being cartilaginous), The scapulocoracoid of L. dynatis was shorter and wider than the scapulocoracoid of L. paludis, while also being thinner and less convex. Similarly, the ilium of L. dynatis was also shorter and wider than that of L. paludis. The proximal limb bones (humerus and femur) of L. dynatis were shorter relative to body size compared to those of L. paludis, while its distal limb bone elements (radius, ulna, tibia, and fibula) were longer. Many of these features appear more derived in L. paludis, leading some to consider it to be the more derived of the two species. ==Classification==

In its earliest descriptions, Limnoscelis was identified as an early reptile, thought to be closely related to the Captorhinidae or Pareiasauridae based on its flat occiput, Relationship with the Diadectomorpha These early descriptions framed Limnoscelis as a member of the paraphyletic group Captorhinomorpha within Cotylosauria, alongside the clades Diadectomorpha and Seymouriamorpha. However, these early authors also noted many similarities between Limnoscelis and the diadectid Diadectes, including the bones forming the orbital border, the presence of a glenoid foramen on the scapula, and having similar pectoral and pelvic girdles. This monophyletic grouping of Diadectomorpha is supported by the anterior processes of the atlas and axis intercentra, and the presence of an external iliac shelf, features that are shared by all diadectomorphs. The below cladogram shows the order Diadectomorpha, modified from Heaton (1980). and arguing that a hypothetical ancestor of all amniotes should be small enough to efficiently produce the amniotic egg, with Limnoscelis having been too big to have been this ancestor. This grouping is based on a variety of shared characters, including the possession of an otic trough, having similar atlas-axis complexes, the possession of small posttemporal fenestrae, the possession of a small parietal foramen, the structure and position of the septomaxillae, and the possession of a tall, broad, and flat ilium. Most recently, a study of the inner ear morphology of diadectomorphs using X-ray microcomputed tomography by Klembara et al. also supported the close relationship between diadectomorphs and synapsids. If this relationship is true, it would make all Diadectomophs, including Limnoscelis, crown amniotes. The placement of Limnoscelis and other diadectomorphs within Amniota is supported by other shared characters, including the loss of the intertemporal bone, absence of the temporal notch, presence of an ossified supraoccipital, shared digital formulas, and the possession of a ventrally displaced, laterally directed paroccipital process. While Limnoscelis itself lacked an astragalus, this feature is present in the diadectidae, which could be further evidence uniting the Diadectomorpha with amniotes. However, this may also be the result of convergent evolution. Other studies question the reliability of the characters allying Diadectomorpha with Synapsida, instead agreeing with Heaton's original placement of the Diadectomorpha outside of Amniota, with the two clades remaining sister taxa. Some also argue that Amniota should be defined by the use of the amniotic egg, and that there is little evidence regarding the potential use of this reproductive strategy by Limnoscelis, making it difficult to determine its placement relative to amniotes. The below cladogram, modified from Laurin and Reisz (1995), showing Limnoscelis and the Diadectomorpha sister to Amniota, agreeing with the original placement from Heaton (1980). }} }} }} }} }} The below cladogram, modified from Berman et al. (1992), depicting the alternative hypothesis placing Limnoscelis and the Diadectomorpha as sister to Synapsida within Amniota. }} }} }} }} }} ==Paleobiology==

showing Limnoscelis as aquatic (lower right) In its earliest descriptions by Williston, Limnoscelis was characterized as a slow but nonetheless powerful animal. Despite its long conical teeth indicating a carnivorous diet, ==Paleoecology==

Limnoscelis paludis Limnoscelis paludis is endemic to the El Cobre Canyon beds of the Cutler Formation, New Mexico. An early Permian age again fell into favor, based on faunal similarities with the Arroyo del Agua beds of the Cutler Formation. However, later studies again confirmed a Late Pennsylvanian age based on biostratigraphy using several new marker fossils, with Limnoscelis paludis belonging to this Late Pennsylvanian assemblage. and the temnospondyls Eryops grandis, Platyhystrix rugosus, Aspidosaurus novomexicanus, and Chenoprosopus milleri. Williston noted a lack of fish and shark fossils from the site, supporting the sites reconstruction as a terrestrial, semi-arid, seasonal floodplain. It is possible, however, that the faunal assemblage at El Cobre Canyon represents two horizons, with species including Limnoscelis paludis and Desmatodon hollandi inhabiting the lower (Late Carboniferous) assemblage, and other species including Edaphosaurus novomexicanus, Platyhystrix rugosus, Sphenacodon ferox, Aspidosaurus novomexicanus, and Ophiacodon navajovicus inhabiting the upper (Early Permian) assemblage. Limnoscelis dynatis Limnoscelis dynatis is known from the Sangre de Cristo Formation in Colorado, which is thought to be stratigraphically equivalent to the Cutler Formation and dated to a similar Late Pennsylvanian age. Limnoscelis dynatis fossils have been found alongside the synapsids Edaphosaurus raymondi, and Xyrospondylus ecordi, the diadectid Desmatodon hesperis, the aïstopod Coloraderpeton brilli, the microsaur Trihecaton howardinus, and labyrinthodont amphibians. The presence of paleoniscoid fish and a xenacanth shark indicate the presence of water, with the site possibly representing an oxbow lake. ==References==