Ebrachosuchus neukami, O. Kuhn, 1936

Butler, Richard J., Rauhut, Oliver W. M., Stocker, Michelle R. & Bronowicz, Robert, 2014, Redescription of the phytosaurs Paleorhinus (‘ Francosuchus’) angustifrons and Ebrachosuchus neukami from Germany, with implications for Late Triassic biochronology, Zoological Journal of the Linnean Society 170 (1), pp. 155-208 : 202-204

ANGUSTIFRONS AND EBRACHOSUCHUS NEUKAMI

Our re-examination of the Ebrach phytosaur specimens has revealed a large number of substantial anatomical features that differ between Paleorhinus angustifrons and Ebrachosuchus neukami , including:

1. The external nares of Ebrachosuchus are proportionally shorter and broader than those of Paleorhinus angustifrons , with a relatively broader internarial septum.

2. The posterior rims of the external nares of Ebrachosuchus are not raised into thickened, rugose ridges, unlike the condition in Paleorhinus angustifrons , and the lateral rims of the external nares lack the distinct profile in lateral view that is present in Paleorhinus angustifrons (see above).

3. The external nares of Ebrachosuchus are relatively more posteriorly positioned than those of Paleorhinus angustifrons , being separated by only a very short distance from the anterior corner of the antorbital fenestra.

4. The internal antorbital fenestra is proportionately smaller in Ebrachosuchus (anteroposterior length equal to that of the orbit) relative to the condition in Paleorhinus angustifrons (anteroposterior length exceeds that of the orbit).

5. The antorbital fossa is reduced in Ebrachosuchus , being limited to a small lacrimal antorbital fossa at the posteroventral corner of the internal antorbital fenestra. By contrast, Paleorhinus angustifrons retains an extensive maxillary antorbital fossa and a proportionately larger lacrimal antorbital fossa.

6. The infratemporal fenestra of Ebrachosuchus is strongly expanded anteroposteriorly relative to that of Paleorhinus angustifrons , terminating ventral to the midpoint of the orbit.

7. Alveolar ridges are only weakly developed on the premaxilla and maxilla of Ebrachosuchus , but are well developed in Paleorhinus angustifrons and other phytosaurs.

8. Weaker heterodonty is present in the maxilla of Ebrachosuchus than in Paleorhinus angustifrons (inferred from variation in alveolar diameter).

9. The postorbital–jugal bar is twisted along its length to face dorsally and is expanded transversely and overhangs the infratemporal fenestra in Ebrachosuchus , whereas this morphology is absent in Paleorhinus angustifrons .

10. The posterior process of the jugal is shallower dorsoventrally relative to its length in Ebrachosuchus than in Paleorhinus angustifrons (probably correlated with difference 6).

11. A strong anteroposteriorly extending ridge and nodular row is present on the lateral surface of the anterior process of the jugal of Paleorhinus angustifrons but is absent in Ebrachosuchus .

12. The nasals of Ebrachosuchus are transversely expanded, forming a broad platform between the orbits and external nares, whereas the nasals of Paleorhinus angustifrons are much narrower.

13. The combined transverse width of the frontals of Paleorhinus angustifrons is substantially greater than the combined width of the parietals, whereas the parietals are proportionately broader in Ebrachosuchus .

14. The concavity on the dorsal surface of the squamosal is much more weakly developed in Ebrachosuchus than in Paleorhinus angustifrons .

15. The ridge forming the lateral margin of the postorbital–squamosal bar bifurcates posteriorly in Paleorhinus angustifrons but not in Ebrachosuchus .

16. The posterolateral corner of the squamosal is rounded in dorsal view in Ebrachosuchus but pointed in Paleorhinus angustifrons .

17. The base of the dorsal process of the quadratojugal is relatively much broader anteroposteriorly in Paleorhinus angustifrons than in Ebrachosuchus .

18. The lateral surface of the anterior process of the quadratojugal is dorsoventrally concave in Ebrachosuchus but is flat in Paleorhinus angustifrons .

19. The quadrate foramen of Ebrachosuchus is greatly enlarged in size relative to that of Paleorhinus angustifrons , and is approximately two-thirds of the width of the foramen magnum.

20. There is a large foramen at the junction between the parietal and supraoccipital in Paleorhinus angustifrons that is absent in Ebrachosuchus .

21. The basisphenoid is anteroposteriorly compressed and the basitubera enlarged and anteroposteriorly and transversely expanded in Ebrachosuchus relative to the morphology of these elements in Paleorhinus angustifrons .

22. The suborbital foramen is boomerang shaped in Paleorhinus angustifrons and slit-like in Ebrachosuchus .

23. A large, subcircular ectopterygoid foramen is present in Paleorhinus angustifrons , as opposed to a slit-like foramen in Ebrachosuchus .

24. A pronounced medial ridge on the palatine separates the horizontal main body of the bone from the dorsomedial wing and narrows the palatal vault medially in Ebrachosuchus .

The skulls essentially are identical in size and are unlikely to represent individuals of radically different ontogenetic stages, so these substantial observed differences are unlikely to be attributable to ontogeny. The skulls were recovered within 10 cm of one another, within the same bedding plane ( Kuhn, 1936), and it is worth considering the possibility of sexual dimorphism. Sexual dimorphism has been proposed for the phytosaur Machaeroprosopus on the basis of morphological differences in the robustness of the premaxilla and the development of a narial crest ( Zeigler, Lucas & Heckert, 2003). Major differences between the proposed female and male individuals of Machaeroprosopus are, however, limited to the preorbital region of the skull ( Zeigler et al., 2003). By contrast, substantial differences between Paleorhinus angustifrons and Ebrachosuchus are not obvious in the premaxilla (at least, those parts preserved), but numerous and substantial differences do occur in the remaining parts of the preserved skulls. A number of these differences, particularly the reduction of the antorbital fossa and the anterior extension of the infratemporal fenestra (as well as potentially the more posterior position of the external nares) in Ebrachosuchus may be phylogenetically significant. Moreover, while Paleorhinus angustifrons closely resembles Paleorhinus bransoni and the Krasiejów phytosaur specimens, no Ebrachosuchus -like phytosaur has yet been described from North America or Krasiejów. Although sexual dimorphism cannot be excluded, we believe it is considerably more likely that Paleorhinus angustifrons and Ebrachosuchus neukami represent generically distinct taxa. These species may have occupied distinct ecological niches, based upon the substantial elongation of the rostrum of Ebrachosuchus . Other examples of coexisting but morphologically distinct genera of phytosaurs are known, such as the co-occurrence of Nicrosaurus and Mystriosuchus in deposits of the Löwenstein Formation of south-west Germany (see above), so nichepartitioning by different genera of phytosaurs may have occurred widely during the Late Triassic.