Trilobites had 2 eyes on top of the head, antennae projecting from beneath the head and, in some, from under the tail, and 2 rows of paired limbs. Each paired limb consisted of 2 branches joined near the base: a comblike gill branch (exite) for breathing and swimming, and a jointed walking leg (endite).
After hatching, each trilobite secreted a shell, composed mainly of calcite but with some calcium phosphate and organic material. The shell consisted of head and tail pieces (cephalon and pygidium, respectively) separated by several articulated segments (thorax). Adults ranged from less than 1 cm long (eg, genus Scharyia of the Mackenzie Mountains, NWT) to over 60 cm (eg, Terataspis of Ontario).
Specimens from the Canadian Shield are often beautifully preserved. Complete skeletons of Triarthrus eatoni and Pseudogygites latimarginatus from Collingwood, Ontario, and of Phacops rana from southern Ont are particularly well known and are prized by collectors. Specimens of giant Terataspis grandis and Isotelus maximus are also known from Ontario.
Along the Hudson Bay in northern Manitoba the largest-known complete fossil of an Isotelus species was found. At 70 cm this fossil dwarfs the majority of trilobite fossils that range from 3-10 cm long.
The Mackenzie Mountains contain a unique sequence of trilobite faunas, in which quartz has replaced the skeletons. This type of preservation is often so good that all the details are preserved on minute larval trilobites as small as 0.2 mm in length. Entire life cycles, from babies to large adults, are preserved in this fashion. There are more levels of silicified trilobites, spanning a greater amount of time, in this part of northwestern Canada than in any other region of the world. These fossils, which range from 350 to 520 million years of age, are used to study trilobite evolution, ecology and geographic distributions.
Author BRIAN CHATTERTON