Nicrosaurus

Nicrosaurus

Nicrosaurus Fossil range: Late Triassic
Nicrosaurus skull, from Zittel, 1913
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Sauropsida
Order:	Phytosauria
Family:	Phytosauridae
Subfamily:	Pseudopalatinae
Genus:	Nicrosaurus Fraas, 1866

Nicrosaurus is an extinct genus of phytosaur reptile. Although it looked and probably lived like a crocodile, it was not closely related to these creatures, instead being a good example of parallel evolution. The main difference between Nicrosaurus (and all other phytosaurs) and real crocodiles was the position of the nostrils - Nicrosaurus's nostrils were placed directly in front of the forehead, whereas in crocodiles, the nostrils are positioned on the end of the snout.

A recent phylogenetic analysis found that the genera Pseudopalatus, Mystriosuchus, Redondasaurus and Nicrosaurus were pseudopalatines.

description and paleobiology

Some distinguishing anatomical features of Nicrosaurus are the external nares at the skull roof level, the dorsoventrally compressed and rounded posterior squamosal processes, the broad and heavy rostrum and a strong prenarial crest.[3]

Nicrosaurus may have been more terrestrial than other phytosaurs. Occurring in marginal-lacustrine or outrightly terrestrial settings, it bears longer limb bones, a straighter femur and a deeper pelvis than other phytosaurs. Combined with its unusually deep upper jaw and heterodont teeth, it was most likely a secondarily terrestrial predator, probably not at all dissimilar from terrestrial crocodilymorphs like sebecians.

Teeth

Nicrosaurus dentition is highly heterodont, the tooth shapes varying from wide, laterally compressed blade teeth to cylindrical, recurved caniniform teeth.[5]

In terms of jaw morphology, a full prenarial crest is a distinctive anatomical feature for Nicrosaurus kapffi. In both the upper and lower jaw, the dentition has five morphologically separated arrays of teeth: tip-of-snout set, premaxilla set, maxilla set, tip-of-mandible set, and dentary set. Moving posteriorly in all of these sets, except the tip-of-the-snout and tip-of-mandible sets, tooth morphology starts out relatively simple and undifferentiated and gradually changes, resulting in a morphocline. The upper dentition is considered to be tripartite.

The anteriormost teeth, or carinae, of the premaxilla set in Nicrosaurus kapffi are enlarged and strongly curved. These are usually the largest teeth of the upper jaw. Much of the other anterior teeth in this set, as well as in the maxilla, are difficult to distinguish from one another.

The anteriormost teeth of the premaxilla are firmly anchored and labially vaulted.

The number of tooth positions is highly variable for the premaxilla and maxilla set. However, no studies indicate that there is a direct correlation between tooth count and skull size. Teeth rows usually consist of 40-45 members.

The tripartite dentition, enlarged carinae, and strong terminal members of the premaxilla suggest that Nicrosaurus kapffi (and many other phytosaurs) may be adapted to dismember medium to large-sized prey after killing such prey with a strong, quick blow.[6]

Nicrosaurus in general have massive snouts similar to extanct crocodilians, suggesting they preyed on tetrapods instead of fish.

Distinctive post-cranial features

All derived phytosaurs have an ilium that is characterized by a blade that elongates posteriorly and an anterior process that is short in length. However, compared to all phytosaurs, the ilium of Nicrosaurus kapffi is dorso-ventrally elongated, similar to that of Erythrosuchus africanus. Because of the elongation, the acetabulum is also longer relative to other phytosaurs. Compared to Leptosuchus the angle between the pubis and ischium is also greater. In Nicrosaurus, the proximal end the humerus is flattened.

Semi-aquatic vs Terrestrial

Because of their resemblance to modern crocodiles, it was initially thought that phytosaurs were also semi-aquatic animals. The slender jaws further suggested a diet containing fish. An earlier study explained that more massive Nicrosaurus could have also had a diet consisting of large land reptiles which came near waters or amphibians of streams and ponds.[8]

When it comes to phytosaurs, most inferences on ecology are in comparison with modern day crocodilians. However, genera within phytosaurs may also have had different ecological preferences. Such is the case for Nicrosaurus and Mystriosuchus, the biggest distinguishing factor between the two being the shape of their snouts. The latter had a slender skull with bipartite dentition, suggesting a diet of fish and small tetrapods, while the former had a massive skull with tripartite dentition, suggesting prey were larger animals.

Nicrosaurus and Mystriosuchus have both been recovered in the first and second Stubensandstein in arkosic sandstones separated by floodplain mudstones and were both buried during flooding events in a freshwater river habitat. Both genera appeared at around the same time in central Europe.[2] Additionally, because Nicrosaurus has also been found lacustrine sandstones, aerially exposed planes with breccia and reworked palaeosols, a terrestrial and swamp-inhabiting lifestyle seems probable.[7]

A more recent analysis on available post-cranial bones has provided results that further support the idea of Nicrosaurus being primarily terrestrial. The illium and femur of Nicrosaurus are similar to those of Archosaurs in comparison to semi-aquatic Archosaurs, like today's crocodiles. The resemblance suggests Nicrosaurus could have had a more upright walking style that is often associated with terrestrial organisms.

special characters

The following are a list of characters for both species of Nicrosaurus following a phylogenetic analysis.[9]

Nicrosaurus kapffi (Meyer, 1860): SMNS 4378, 4379, 5726, 5727; BMNH 43743. Nicrosaurus meyeri (Hungerbühler and Hunt, 2000): SMNS 12598 (holotype), 4059; BMNH 42745.

Nicrosaurus kapffi

• Long snout
• Ventrally convex alveolar rim of maxilla
• Tripartite upper dentition
• Posterior rim of nares behind anterior rim of antorbital fenestra
• Infranasal recess present
• Reduced antorbital fossa
• Convex interorbitonasal fenestra
• Strongly developed medial lamella of postorbito-squamosal bar, supratemporal fenestra reduced to slit
• Medial parts of squamosal processes of parietals overhand supraoccipital shelf
• Moderate depression of parieto-squamosal bar (bar 15-25% of absolute skull height)
• Gently sloping dorsal edge of parieto-squamosal bar
• Squamosal, dorsoventral expansion: strongly thickened, dorsal surface of squamosal raised terminally
• Deep supraoccipital shelf
• Posttemporal fenestra delimited by contact of parietal process of squamosal, ventrolateral border of posstemporal fenestra
• Significantly reduced quadrate foramen
• Interpremaxillary fossa: narrow slit
• Anterior extent of palatine: tip extends forward beyond the posterior rim of choana
• Lateral extent of palatines: extend onto palatal vault and meet along the midline
• Suborbital fenestra: reduced to a singular oval fenestra, or subdivided into two or more small openings
• Length of interpterygoid vacuity: tiny oval indentation at posterior rim of cojoined pterygoids

Nicrosaurus meyeri

• Very long snout
• Bipartite upper dentition
• Anterior extent of septomaxilla: posterior to or at level with anterior tip of nasal
• Position of nares: posterior rim of nares behind anterior rim of antorbital fenestra
• Infranasal recess is present
• Reduced antorbital foss
• Convex interorbitonasal area
• Moderately developedmedial lamella of postorbito-squamosal bar
• Medial parts of squamosal processes of parietals overhang supraoccipital shelf
• Moderate depression of parieto-squamosal bar (15-25% absolute skull height)
• Gently sloping dorsal edge of parieto-squamosal bar
• Medial rim of squamosal along supratemporal fenestra (and posterior process): angular
• Length of posterior process of squamosal: moderate
• Lateral flange of the squamosal is absent and present
• Subsidiary opisthotic process of squamosal is present
• Depth and shape of supraoccipital shelf: deep, axis of shelf with steep slope anteriorly and terminal *horizontal deflection of shelf
• Posttemporal fenestra, lateral border: delimited by the contact of the parietal process of the squamosal (dorsal border) and the paroccipital process (ventral border); lamina of the squamosal extends onto the paroccipital process, forming the ventrolateral border of posttemporal fenestra
• Exoccipital and supraoccipital shelf: broad, overhang foramen magnum and occipital condyle
• Significantly reduced quadrate foramen
• Interpremaxillary fossa: present, narrow slit
• Anterior extent of palatine: tip extends forward beyond the posterior rim of choana
• Lateral extent of palatines: extend onto palatal vault and meet along the midline
• Suborbital fenestra: elongated, slit-like
• Length of interpterygoid vacuity: tiny oval indentation at posterior rim of cojoined pterygoids

References