Meeting Abstract
P1.140 Tuesday, Jan. 4 Fossil Insights on Notostracan Limb Differentiation & Evolution HEGNA, T. A.; Yale Univ., New Haven, CT thomas.hegna@yale.edu
Though branchiopod crustaceans are often thought of as possessing primitively homonymous trunk limbs, the typical notostracan has a unique pattern of trunk limb differentiation. This differentiation is manifest in the first thoracic limb by a much-reduced endopod and greatly elongated, pseudoannulated 4th and 5th endites. Anterior thoracic limbs also show some limited differentiation, but by the 5th thoracic limb, the endites and endopod are roughly equant. Only one, poorly-known living species, ‘Lepidurus’ batesoni from Kazakhstan lacks this anterior limb differentiation. Furthermore, all fossil notostracans lack this thoracic limb differentiation. This suggests that virtually the entire fauna of extant notostracans had a relatively recent (≥65 mya) origin. Perhaps more interesting is the fact that this limb differentiation suggests a significant change in body patterning—perhaps related to a shift in Hox gene expression domains, or another downstream regulatory network. Notostracans are also characterized by another unique feature of their limb differentiation: after the 11th thoracic limb, the correspondence between dorsal and ventral segmentation is lost. Dorsally, the trunk maintains its even, ring-like segmentation, but ventrally, the limbs decrease in size and increase in number per segment. This means that there are more limbs per segment posteriorly along the body axis. The genetic mechanisms of this segmental mismatch are not yet understood, but it is suggestive of a divergent body axis. No other branchiopod clade possesses this feature. However, all fossil notostracans with preserved limbs display this feature—going back as far as the Devonian (≤360 mya). These two examples show that the integration of fossil forms into studies of arthropod development can help identify previously unrecognized problems in body patterning and add an extra temporal dimension to their study.
