Argyrosomus FOSSIL REMAINS
Creators: name: Otero, Olga
name: Béarez, Philippe
name: Argyriou, Thodoris
Publisher: name: Zenodo
Year: 2013
Subjects: Biodiversity
Taxonomy
Animalia
Chordata
Actinopterygii
Perciformes
Sciaenidae
Argyrosomus

description: DESCRIPTION OF ARGYROSOMUS FOSSIL REMAINS
All the fossils have been ascribed to the sciaenid genus Argyrosomus.
Like in many perciforms, the premaxilla of Argyrosomus has a stout articular head and an elongated alveolar process that bears the teeth ventrally (Fig. 2). When preserved, the ascending process is much higher than the leaf-shaped articular process and reaches twice its height in A. regius. Ŋe ascending process and the leaf-shaped articular process separate from each other close to the base, and form a sharp angle. Ŋe ascending process is broken in the fossils but its base and the shape of the articular process are similar than in extant specimens, notably by their relative position, by the opening for venous vessels and for the maxillary nerve branch that opens laterally on the articular process, and also by the rounded dorsal edge of the articular process. Ŋe alveolar process of Argyrosomus has an anterior edge which is squarish in lateral view and projects frontwards the ascending process. Moreover, at the back of the bone, a low posterior process grows gradually from the dorsal edge of the alveolar process and along all (A. regius) or most of its posterior quarter (A. japonicus). In our fossils, only the anteriormost part of the posterior process is preserved as all the premaxillae lack their posterior tip. Ventrally on the alveolar process, a row of enlarged caniniform teeth extends along the lateral margin, the second tooth being the largest, and two to three rows of villiform teeth develop medially, except at the symphysis where the alveolar process is covered by small teeth. Ŋe teeth are not preserved in the fossils but the size and distribution of the tooth sockets correspond in all the features to the Argyrosomus pattern.
A single fossil Argyrosomus maxilla has been recovered, at Sahabi. It is preserved in its anterior half only so that the enlarged posterior dorsal plate, rather common in perciforms, cannot be observed (Fig. 3). As shown between the three specimens figured in front views, the articular head varies in height, direction and extension of the dorsal process where the palatine articulates. Generally, the articular head of the fossil is the most massive element. However, this feature may vary within a same species and especially in relation with the size of the specimen, as observed in other perciform fish (e.g., Lates; Otero 2004). Ŋe maxillary internal process is medio-ventrally directed and surrounded by a groove. In lateral view, a stout dorsal crest allows the attachment of the adductor mandibulae I on a length which is about the same size as the head height.
In Argyrosomus, the ventral plate that develops below the sensory canal of the dentary is vertical so that the bone is narrow in dorsal view (Fig. 4). Ŋere is a strong ventral notch between the symphysis and the ventral plate of the bone so that a prominent chin hangs at the symphysis. It is even noticeable in the fossils where it might have been somewhat weathered during transport, at least in certain specimens. Another peculiar feature is the dorsally projecting “spoiler” at the dorsal posterior branch of the bone but it is observed in modern specimens only as the pos- terior part of fossil dentaries is always broken. Ŋe opening of the various foramina is similar in both fossil and modern specimens, notably between the fossils and A. japonicus. Ŋe upper foramen of the dentary opens in front of a large and deep lateral groove for the lip ligaments and the mandibular sensory canal runs in a bony tube with three openings in front of the upper foramen level. Ŋe fourth elongated opening of the mandibular sensory canal does not reach the posterior border of the bone. Finally, the dentary tooth patch in Argyrosomus develops as a mirror of the premaxilla dentition, with one inner row of enlarged teeth and one outer row of smaller ones. In dorsal view, the symphysis seems slightly enlarge in the fossil.
Like the bones formerly described, fossil anguloarticular (Fig. 5) are fragmentary. Ŋey lack the anterior extremity that inserts in the dentary. Ŋe glenoid cavity is deep so that the articulation with the quadrate must be tight. Ŋe whole bone is also rather deep, with an angle ranging between 30° and 40° between the postero-dorsal pad that projects upward and frontward and the horizontal pad that develops above the sensory canal from the glenoid cavity. Ŋe angulo-articular sensory canal runs in an open deep groove usually covered by a thin bridge at the mid-length of the bone (lacking in the A. regius specimen figured. One of the most characteristic features of Argyrosomus is the shape of the posterior edge of the angulo-articular and the position of the retro-articular. Ŋis bone is only preserved in one fossil, at Arrisdrift, but its position is known by the shape of its suture line with the angulo-articular in some other specimens. In lateral view, the posterior edge of the bone shows a notch below the caudal tip of the glenoid cavity.Ŋ e retro-articular articulates with the angulo-articular below this notch and projects posteriorly, behind the posterior tip of the angulo-articular (Fig. 5). In lateral view, the retro-articular is at least twice as long as high and it extends anteriorly until the front level of the glenoid cavity.
Ŋe single Argyrosomus fossil quadrate is known from Arrisdrift. It was found connected with a posterior fragment of the ectopterygoid. In lateral view it resembles modern quadrates notably by the high outline and the posterior bumped edge (Fig. 6). Ŋe articular condyle and the base of the posterior border of the bone are particularly wide, notably when observed in anterior view because the anterior edge is much thinner than the posterior one. Ŋere is a median expansion of the condyle in the fossil. It exists in a reduced version in A. japonicus but not in the specimen of A. regius.
Ŋe abdominal vertebrae show a delicate striated ornamentation of the bone. Only first vertebrae have been identified in both Arrisdrift and Sahabi (Fig. 7). Ŋeir bean-shaped articulation facets for the exoccipital are medially connected. In lateral views the centra exhibit parallel edges. Ŋe anterior face of the centrum is less wide, almost rounded, and either more or less pointed dorsally (Arrisdrift) or somewhat dorso-ventrally flattened (Sahabi). Ŋe posterior face of the centrum is bigger than the anterior one and somewhat dorso-ventrally flattened.
No other fossil bony remain from Arrisdrift and Sahabi can be confidently attributed to Argyrosomus.
description: Published as part of Otero, Olga, Béarez, Philippe & Argyriou, Thodoris, 2013, First identification of the genus Argyrosomus (Teleostei, Sciaenidae) in Neogene African outcrops, pp. 49-65 in Geodiversitas 35 (1) on pages 55-58, DOI: 10.5252/g2013n1a4, http://zenodo.org/record/4537986