FIRST CARBONIFEROUS PROTASPID LARVAE (TRILOBITA)

Journal of Paleontology, Jul 2005 by Lerosey-Aubril, Rudy, Feist, Raimund

Hypostomes.-Hypostomes described in this work are devoid of spinose margins and are generally more adultlike compared to hypostomes known from other trilobites. This apparently confirms the idea that this character is diagnostic for the order Proetida (see Fortey, 1990, p. 559-561). Moreover, they are particularly similar to anaprotaspid hypostomes of D. talenti (Chatterton, 1971), S. astinii (Edgecombe et al., 1997), and Dechenella sp. (Chatterton et al., 1999), except for the fact that they are devoid of the three pairs of submarginal spines. Hence, a particularly large, lens-shaped hypostome with a posteromedian spine may be a diagnostic character for the Proetoidea.

Chatterton (1971) illustrated the supposed in situ position of this hypostome (fig. 20). Although no protaspid hypostomes in life position were found, there might be an alternative interpretation that contradicts Chatterton's view. Firstly, in his reconstruction the anterior border of the hypostome replaces the rostral plate. Edgecombe et al. (1997) described the rostral plate of S. astinii being of equal width all along it extension (fig. 8.3). Likewise, though no rostral plates have been discovered in our sample, several specimens possess librigenae that have fallen into the ventral cavity of the larvae (Fig. 3.2). They are almost identical to those of S. astinii, suggesting that the cephalic doublure may have been similar in proetoid protaspides and thus a rostral plate may have been present. secondly, there are usually marked anterior pits. These are presumed to be homologous with the adult fossulae (Fortey, 1990, p. 545), i.e., the area where hypostomal anterior wings connected with the cranidium. If we consider the position of these pits, the anterior margin of the hypostome must have been situated in a sufficiently posterior position to allow enough room for a rostral plate. Hence, we believe that the hypostome should be placed into a slightly more posterior position than given in Chatterton's (1971, fig. 20) model. Finally, the posteromedial spine does not extend beyond the inner edge of the doublure on Chatterton's reconstruction whereas we believe that it does. Indeed, we have found a particular anaprotaspid specimen (unfortunately broken during preparation for scanning microscopy) that seemed to possess a seventh spine posteromedially. In fact, it was the posterior part of a broken hypostome that has fused with the doublure during the fossilization processes. Thus, the posteromedial spine of an in situ hypostome would actually rest on the small notch that is usually present in the doublure (Fig. 3.12) and project beyond the larval margin (see our reconstruction, Fig. 3.14). An anaprotaspis figured as "of uncertain attribution" ("possibly of an Illaenina") in the revised treatise (Chatterton and Speyer, 1997, p. 212) confirms our model. Considering 1) the number and position of marginal spines and 2) the lenticular hypostome with its posteromedial spine, this specimen obviously belongs to a proetoid. This view is reinforced by the fact that at least four proetoid species occur in the same bed (Chatterton, personal commun., 2003). This specimen is the only Silurian proetoid anaprotaspis so far figured and the only specimen displaying a hypostome in connection. It shows clearly that the hypostome is situated just behind the rostral plate and that the posteromedial spine of the hypostome rests on the doublure as illustrated in our model.