Arthropod systematics has been a long problem with the interrelationships of major groups providing a lot of interesting discussion. This is also true of the extinct Trilobita, to date there is around 180 major families recognized but with only a handful ever studied phylogenetically there is still little of an understanding of their higher level relationships. My work mainly concerns the study of size patterns in several arthropod groups but has focused on the Trilobita, specifically a couple of families which will be discussed below.
The major family I have been involved with is the Asaphidae, placed in the order Asaphida. The Asaphidae is one of the largest trilobite families with over 170 genera assigned to it. The family ranged from the uppermost Cambrian to the Upper Ordovician, becoming extinct in the Hirnantian glaciation event which saw off a great deal of the group as a whole. They are notable for containing several representatives which reached a large size, including the largest trilobite Isotelus rex, a complete specimen measuring just over 70cm. Over the coarse of the last year I have been building a phylogeny of the family, using around 70 taxa to cover their main morphological variety, this is the first time the group has been treated to phylogenetic analysis.
The other group I have been concerned with is the Paradoxididae, mainly the genus Paradoxides, a well known and described group. A recent account suggested that as many as 100 species and subspecies have been attached to Paradoxides, There has been a significant amount of debate as whether to raise the various subgenera such as Hydrocephalus, Eccaparadoxides, Acadoparadoxides and Plutonides to generic status, also the placement of several species within these groupings are contested. As with the Asaphidae there has yet to be an intensive phylogenetic treatment of the group, this has proved difficult due to the nature of the ontogeny of Paradoxides, whereby after the holaspid phase is reached there can still be a significant amount of change in the shape of certain structures. To solve these various taxonomic problems along with a detailed look at the size variation across the group a phylogenetic analysis is ongoing with the help of Adrian Rushton (NHM, London).
(1) One area I have been mainly concentrating on is the concept of Cope’s Rule, that is the progressive increase in body size/mass through time. Ever since the paper of Stanley, 1974 there has been a huge increase in the amount of studies following these ideas. Using mainly comparisons made across phylogenies I have been attempting to see whether the large forms known are placed at a more derived position. Also whether any change in size seen is an Active (increase in upper and lower size bound) or Passive (diffusion into a larger morphospace).
(2) Secondly I am looking at latitudinal size variation, common in birds, it has been suggested that due to heat conservation size is likely to increase in colder climates. Similar pattern have been noted in benthic amphipods and attributed to the increased oxygen dissolution in colder waters. There has already been several examples of large trilobites from higher palaeolatitudes in both the Asaphidae and Paradoxididae. This project is ongoing.