Terminology of glabellar lobes in trilobite larvae based on homology

Abstract

Glabellar lobes of holaspid cranidia of trilobites have been numerically designated, LI, L2, etc. (Jaanusson, 1956; Henningsmoen, 1957, figure 1), or lp, 2p, etc. (Harrington, 1959), forward from the occipital ring; most trilobites have four glabellar lobes and an occipital ring. The description of morphological structures with an identical term implies that the structures are homologous in different organisms of a certain group (Inglis, 1966); this is one of the classical definitions of homology (Patterson, 1982). This also seems to be an initial conjecture of homology (“primary homology” of de Pinna, 1991). Likewise, the numerical notation of glabellar lobes should allow us to recognize homology of the lobes among trilobite taxa. Under the above traditional system, the homology is demonstrably recognized in ontogenetic stages with a distinctively differentiated protopygidium (stage 2 in Figure 1); however, this is not the case for earlier intervals occurring before transverse demarcation at the back of the head (stage 1 in Figure 1). This limitation is because the relationships of the occipital ring of the later stages to the posterior axial lobes of the earlier ones are uncertain, and this can be appreciated, when describing a phacopoid protaspis with four axial lobes (stage 1 of Rossaspis pliomeris in Figure 1). We introduce a new term, “Lp”, for the posteriormost axial lobe to remove this limitation, while keeping the traditional system useful. The previous system using letters “L” (after lobus) and “S” (after sulcus), proposed by Jaanusson (1956, p. 37), is preferred over the system using “p”, because “p” of the latter confusingly refers to lobes or furrows (e.g., Harrington and Leanza, 1957, p. 221)