Millions of trilobites are easily collected close to Canada's biggest city and yet, until a few year ago, the only geologist to have studied them was a Toronto mineralogist whose chosen field was blowpipe analysis

A carcass and a moulted exoskeleton of Pseudogygites latimarginatus from the Whitby Formation of southern Ontario. Diagram shows moulting scenario. (Photo and diagram by RL (c))

Edward J. Chapman held the chair in geology and mineralogy at the University of Toronto for 42 years starting in 1853. His specialty was mineralogy, in particular, assaying and blowpipe analysis, but he taught the full range of geological courses, including paleontology. He even published on fossils. In the late 1850s, he wrote a series of papers on a single species of trilobite from black shales in southern Ontario that he named Asaphus canadensis. This trilobite had already been given the name Asaphus latimarginatus by the State Paleontologist of New York. Later, similarity of the Ontario species to the European trilobite Ogygites was noted by a few paleontologists, and it was renamed Ogygites latimarginatus. The similarity, however, was superficial and a result of convergent evolution. The Ontario trilobite then became the type species for a new genus, Pseudogygites (that is, false Ogygites). The trilobite that Chapman described is now known as Pseudogygites latimarginatus.

P. latimarginatus occurs in huge numbers at Upper Ordovician exposures of bituminous shale on Lake Ontario and Georgian Bay; mainly as dense accumulations of its distinctive ribbed tail that has the appearance of a petrified butterfly. Complete trilobites are not uncommon and they are found as two types of configurations. 1. Intact specimens without displacement of head, thorax or tail. These represents the carcasses of individuals that died. 2. Specimens with the head separated at its sutures and lying askew the front of the thorax and tail. These represent moulted individuals. Pseudogygites is much rarer in the underlying limestones where the most common asaphid trilobite is Isotelus. We have studied the growth history, or ontogeny, of both trilobites and discovered some interesting relationships. Immature Isotelus individuals are very similar to mature Pseudogygites individuals. This suggests to us that Pseudogygites was derived from Isotelus by paedomorphism -- that is, the retention of the juvenile characters of the ancestor into the mature descendant.

The exoskeleton of some of the trilobites and other fossils in these black shales have been replaced by pyrite (iron sulphide) and these golden trilobites are particularly attractive to collectors. The pyrite and the high organic content of the shale point to the absence of oxygen in the dark muds on the sea bottom when they were deposited in the Late Ordovician, 450 million years ago.

Further reading: