Walcott, Trilobites, and the Cambrian Radiation  

Notice: For family reasons Dr.  Nigel C. Hughes will not be able to give the below talk, originally scheduled on Tuesday Aug.  4^th at 1:30 pm.  The documentary “On the Trail of Primitive Life (The Cambrian Period)” will be displayed instead, alongside a poster session.  The original abstract is reproduced below.

Charles Walcott’s early interest in palaeontology was fuelled through extensive fossil collection in his native New York Sate, where he developed a particular fascination with trilobites.  Both these attributes were critical to his later successes as a professional palaeontologist, including his work on the Burgess Shale fauna.  Walcott’s talents as a collector were honed as his private collection grew in both scientific and monetary value and, as with many early “amateur” palaeontologists, served as a key to his later appointment as a professional scientist.  Walcott understood the value of specific items within his collection.  Although trilobites had been described scientifically for over an hundred years, their possession of limbs during life remained a matter of active debate as late as 1876.  In that year Walcott published his second paper, which was an account of limbs in Ceraurus pleurexanthemus.  That work demonstrated not only the careful observation of those who first recognized the limbs, but also Walcott’s technical ability in devising how to analyze and present his results.  As early as 1878, Walcott also published the first ontogenetic sequence for a North American trilobite that included multiple developmental stages, and went on to describe scores of new trilobite taxa and the rocks that contain them.  Throughout his career he maintained his active interest in trilobite soft tissues and his conclusions on the form of the “spiral” trilobite limb, which were first presented in his earliest investigations.  He published a seminal review paper on trilobite appendages in 1918, and a long description of Olenoides serratus from the Burgess Shale in 1924. 

Walcott’s views on the unique structure of the trilobite limb stand in contrast to his interpretations of other arthropods in the Burgess Shale fauna.  A recent view holds that although trilobites had a series of unique defining characteristics that distinguish them from other living or fossil arthropod groups, their extensive history may in some ways be representative of the evolutionary history of “trilobitomorph” arthropods as a whole.  This is important because the trilobite fossil record is far richer in terms of taxonomic, ecological, and temporal diversity, and in ontogenetic coverage, than that of non-biomineralized fossil arthropods.  The trilobites and their allies appear to be quite phylogenetically basal within the Euarthropoda, the group to which all familiar living arthropods belong.  If so, trilobites may throw light on the developmental and body patterning condition of early euarthropods, and the record of their evolution might serve as a proxy for early euarthropod evolution.  Studies of trilobite and other early arthropod development hint that early euarthropods developed through a prolonged series of moult stages, each accompanied by relatively modest morphological change.   As expected, there appear to have been limits to the amount of size change possible per moult.  During the Cambrian, early members of the group displayed wide variety in the number, size, and pattern of articulation of individual segments in the trunk.  Later evolutionary radiations commonly focused on the morphological differentiation of numerically standardized sets of segments.  With respect to segmentation, we see contrasting sets of morphological character evolution, with different patterns of character variability represented at different times in phyletic history – just as Darwin predicted.