| Popis: |
The osteology of Steurbautichthys ("Pholidophorus") aequatorialis gen. nov. from the Middle Jurassic of Kisangani (Stanleyville Formation, Songa Limestones) in the Democratic Republic of Congo is studied in detail. This fish, about 25 cm long, is marked by a large head, a rather aquiline snout and a small oval median rostral, which does not reach to the tip of the snout. The broad nasals meet on the mid-line anteriorly, but are separated by the frontals posteriorly. The anterior extremity of the frontals remains broad and is partly covered by the nasals. The parietals are large quadrangular bones meeting medially and bearing a small pit-line, prolonging the supraorbital sensory canal. The orbitosphenoid joins the lateral ethmoids anteriorly. The interorbital opening between the orbitosphenoid, the pleurosphenoids and the parasphenoid is well developed. The parasphenoid is toothless, and shows well-developed basipterygoid processes. The foramen for the efferent pseudobranchial artery is perforated in the parasphenoid at the root of the basipterygoid process. The jaws bear small conical teeth. The toothed narrow anterior parts of the laterodermethmoids compose the tip of the upper jaw and separate the two premaxillae from each other. The nasal processes of the laterodermethmoids are enlarged and cover the greatest part of the underlying mesethmoid. The premaxillary is small and devoid of an ascending process. The maxillary is long and exhibits a slightly curved oral border. There are two supramaxillaries. The lower jaw is elongated and articulates with the quadrate at the level of the orbital posterior border. The dentary increases in height posteriorly but its oral border is almost rectilinear with a small notch just behind the toothed region. The long and stout meckelian bone rests in a groove of the internal face of the lower jaw. The surangular forms a weakly pronounced coronoid process. The entopterygoid is broad and partly toothed. The ectopterygoid is longer, narrower and toothless. The quadrate exhibits a well-developed quadratic process. The preopercle possesses a long dorsal branch and a broad but very short ventral branch. The preopercular sensory canal bears only a few secondary tubules. The caudal skeleton is nearly homocercal. The two ventral hypurals are narrow. The first dorsal hypurals are fused in a plate. There are at least 3 uroneurals. There are only very few fringing fulcra on the fins. The scales are of lepisosteid shape, with a peg and socket articulation. The surface of the ganoin is feebly ornamented and the posterior border of the scale is smooth. Comparisons are made with Pholidophorus sensu strict, some species of Pholidophorus sensu lato, other pholidophorid genera and other pholidophoriform families. The generic validity of Steurbautichthys is demonstrated. A new pholidophoriform phylogeny is proposed in which Steurbautichthys is referred to the Ankylophoridae, a family of primitive Pholidophoriformes characterized by its laterodermethmoids with an enlarged nasal process and a smaller anterior toothed or tuberculated tip, rejecting laterally the premaxillae. The genera Eurycormus, Ankylophorus, Lehmanophorus, Pholidophoristion, Neopholidophoropsis, Siemensichthys, the species "Pholidophorus" germanicus and probably the genera Pholidorhyncodon and Eopholidophorus also belong to this family. |
