Fig. 1. The Early Cambrian stem Aculiferan, Halkieria evangelista. The scale is in millimetres
Jakob Vinther, Department of Geology and Geophysics, Yale University jako...@yale.edu
The fundamental interrelationship between molluscs has been an issue of debate ever since the 19th century with Lankester´s (1883) development of a schematic mollusk that later became the infamous Hypothetical Ancestral Mollusk, HAM. For many years following, the traditional scenarios have been that the aplacophorans (Caudofoveata and Solenogaster) and the chitons (Polyplacophora) represent the ancestral morphology of the molluscs and were early diverging branches in the mollusk tree. Most recent molecular analyses have not been able to confirm this and find very little evidence to support interrelationships among molluskan classes in general. This stems from two problems: 1.most molluskan classes diverged in the Early Cambrian in a very short period of time, and 2. some groups have a much higher molecular substitution rate than others.
Therefore, the molecular analyses have been able to reconstruct the major classes, where crown groups have evolved more steadily to their current diversity, but have problems finding good support for any interrelationship among the classes due to the very few substitutions polarizing these brief evolutionary events of diversification 530 million years ago.
Giribet et al. (2006) resolved, for example, two major clades of molluscs: a clade of gastropods, bivalves, chitons and monoplacophorans that are short branched (slow molecular evolution) and a clade with scaphopods, cephalopods and aplacophorans (long branched and fast evolving).
With the research grant from The Malacological Society of London I have been using a concatenated set of seven nuclear housekeeping genes originally used by Peterson et al. (2004) to estimate divergence times with a molecular clock. They have also demonstrated an ability to accurately predict major animal interrelationships with a good precision and are less prone to Long Branch attraction problems. I wanted to test the interrelationships of chitons and aplacophorans and their position with respect to the Conchifera. I have therefore been sequencing 4 chitons, a species of Chaetoderma and a Solenogaster in addition to a number of additional gastropod and bivalve taxa. Some of the taxa already sequenced (see Peterson et al. 2004 and 2008) were chosen for their well recorded fossil history and thus serve for calibration points to estimate divergence times.
Two hypotheses with respect to the interrelationships of aplacophorans and chitons have been presented. One is that aplacophorans are the earliest diverging group (maybe even paraphyletic), based on the vermiform appearance and lack of shell plates. That is the Testaria hypothesis. The other model suggests that chitons and aplacophorans are sister taxa, sharingmorphological features, such as sclerites, epidermal papillae and a suprarectal commisure. That is the Aculifera model (Scheltema 1993). The fossil record leaves little evidence to suggest that they appear early in the fossil record as hypothesized. The early Cambrian fossil record show members of the gastropod and bivalve stem group along with numerous "monoplacophoran" molluscs that later evolved into scaphopods and cephalopods.
This is not due to a lack of sclerite bearing molluscs in the Cambrian. On the contrary, a plethora of sclerite bearing mollusks has been discovered in various Lagerstätte type localities with exceptional fossil preservation (fig. 1) and from small shelly fossil assemblages (Vinther and Nielsen 2005). These forms seem to be on the stem lineage of aplacophorans and chitons (See cladistic analysis in Vinther et al. 2008). Chitons appear in the Early Ordovician or Late Cambrian (Vendrasco and Runnegar 2004) whereas the appearance of aplacophorans is more problematic. Some chiton-like fossils with dorsal shell plates have been discovered that have the mantle extending all the way to the ventral side, either completely reducing a foot (Sutton et al. 2004) or leaving a groove. These forms appear to suggest that scenarios of deriving aplacophorans from a chiton like ancestor with a foot and dorsal shell plates are compelling. The aplacophorans would have emerged in the upper Ordovician. Therefore, the Aculifera model is supported by fossil evidence.
Interestingly there is some developmental data supporting this scenario as well (Scheltema and Ivanov 2002, Nielsen et al. 2007). Nielsen et al. (2007) described the development of Chaetoderma and show that there appears to be a ventral area that gradually becomes occluded by populations of cells migrating ventrally on either side and eventually fusing together along the midline(reduction of a foot?). On the dorsal side seven ridges of calcium carbonate secreting cells are present, which might be vestiges of dorsal shell fields. All this suggest that at least caudofoveates could have evolved from a chiton.
With my analysis I will be able to test this scenario by reconstructing a phylogeny and using a molecular clock. My taxon sampling within the chitons comprises both the Lepidopleurida and the two major groups within the Chitonida (Chitonina and Acanthochitonina). I should therefore also be able to estimate the divergence of the polyplacophoran crown group (Neoloricata) using more reliable calibration points within the bivalves and gastropods. The fossil record of chitons indicates an appearance in the Permian (Sirenko 2006).
I would like to thank the Malacological Society of London for supporting this research project still in progress. Kevin Peterson and his lab have been extremely helpful, thanks goes also to Erik Sperling, Derek E. G. Briggs, Matthias Obst, Christiane Todt and Thomas Near.
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