Trilobite Paleogeography
last revised 24 June 2008 by S. M. Gon III
Biogeography involves the mapping and study of the patterns of distribution of organisms within and between the world's regions. The biogeography of plants and animals reflects their ecological requirements and the habitat niches they occupy. Some species are widespread, while others are restricted to certain regions of the globe. Paleogeography offers some of the same approach, but must take into account the fact of plate tectonics, and the changing of continental and ocean basin patterns over the course of hundreds of millions of years. The paleogeography of trilobites is particularly important because they were extremely diverse, were distributed all over the globe, and offer much insight on paleoenvironments and biostratigraphy.
Trilobites occupied marine environments from tropical equatorial to polar paleolatitudes. Some families of trilobites were narrow in their requirements. For example, the family Bathyuridae (Proetida:Bathyuroidea) was found only in paleoequatorial regions. Trilobite marine niches ranged from intertidal and nearshore to deep continental slopes. Because there was very significant continental movement during the Paleozoic Era, with continents drifting apart, as well as converging and joining, the distribution and evolution of trilobites over the nearly 300 million years of their existence reflects a complex paleogeography. For example, the locations of two famous trilobite-bearing sites, Burgess Shale (Canada) and Chengjiang (China) are in temperate zones now, but in the Cambrian, were tropical. 
Early Paleozoic Animation (left) by C.R. Scotese ©2003, PALEOMAP Project.
Why are the trilobites from Oklahoma and Morocco so similar? One interesting example of trilobite paleogeography involves the Devonian trilobite sites of Oklahoma (such as the famous Haragan Formation), and those of Morocco (such as the prolific Hamar Laghdad Formation of the Atlas Mountains). Although on different continents today (as well as different paleocontinental systems during the Devonian), the similarities of the taxa from the two regions are amazing. Here, for example, are three of the parallel developments between the Oklahoma trilobite assemblage (top) and that of Morocco (bottom). In some cases, the same genera are found in both locations.

Taxon similarities between Devonian Oklahoma and Morocco Trilobite Assemblies

Family Dalmanitidae
Huntoniatonia oklahomae (Oklahoma)		 Family Odontopleuridae
Dicranurus hamatus (Oklahoma)		 Family Phacopidae
Reedops deckeri (Oklahoma)
Family Dalmanitidae
Odontochile hausmanni (Morocco)		 Family Odontopleuridae
Dicranurus monstrosus (Morocco)		 Family Phacopidae
Reedops maurulus (Morocco)

What explains the similarities? While the Oklahoma and Morocco trilobite sites are well-separated on today's globe (top figure below), Gondwana and Euramerica were in much closer proximity during the Devonian, with their continental shelfs nearly contiguous. The ancient Moroccan site and Oklahoma site were also at similar paleolatitudes, leading to the development of extremely similar ecosystems, trilobite species, and fossil assemblages. 

Huntoniatonia
oklahomae
Haragan
Oklahoma
Locations of the Oklahoma and Moroccan trilobite deposits today

Above: Although Oklahoma and Morocco are widely separated today, their trilobite assemblies are strikingly similar.		 Odontochile
hausmanni
Hamar Laghdad
Morocco
Huntoniatonia 
oklahomae
Haragan
Oklahoma
Paleozoic (Devonian) continental configuration shows the connection
The Devonian map shows that the corresponding points were at similar Paleolatitudes and close enough
to have sprung from shared ancestral taxa that developed separately on the different continental shelf systems.
 Odontochile 
hausmanni
Hamar Laghdad
Morocco

Deducing the ancient habitats of trilobites from their surrounding matrix.
Today, trilobite fossils have been found on every continent and subcontinent, and judging from the type of sediment in which they are found, we can deduce whether species were nearshore (e.g., in limestones formed from fossil fringing reefs) or from deeper habitats (e.g., marked by atheloptic species found in continental slope sediments). While it seems that the earliest Cambrian trilobites lived largely in shallower habitats, by Mid-Cambrian times the whole inshore to deep water range of biofacies was occupied by trilobites. These developed regional diagnostic genera that occupied concentric bands away from inferred shorelines, as shown in the example, taken from the Devonian shores of the Old World (Gondwana) Province (the origin of today's Moroccan trilobite beds).

©2000 by S. M. Gon, adapted after a figure in the 1997 Treatise

Species patterns such as the one above have been found in many post-Cambrian trilobite sites in every region of the world. In many cases, the regional endemic species were found in the nearshore and carbonate mound habitats, while wider-ranging taxa were found in deeper waters, reflecting a pelagic life history and distribution, such as shown in Carolinites (below).


Carolinites genacinaca
has the widest global distri-
bution of any species of trilobite
 Paleogeography vs modern fossil distribution
The patterns of trilobite fossil distribution on today's continents can be bewildering unless the movements of continents during the Paleozoic is taken into account.  Indeed, the consistent patterns and correlation of certain trilobite genera and families with certain regional biofacies over time helped deduce the positions of the paleocontinents, such as Gondwana, Laurentia, Paleosiberia, etc. 
For example, the trilobite Carolinites genacinaca (Proetida:Telephinidae) is today found in such far-flung and geographically disparate places as subtropical Australia, the western US, the northern arctic, eastern Siberia, and southeast China (see top map below). However, when we look at the global configuration at the time that Carolinites was alive, we see that all of those locations were tropical equatorial, that Carolinites was a widespread epipelagic species of the time (bottom map below). As can be seen from the image of the species (left), it had large eyes and a streamlined body form diagnostic of free-swimming pelagic species (a reconstruction of Carolinites in a venter-up swimming posture is presented here, immediately below). Indeed, it is thought to be the trilobite with the widest global distribution, past or present.
A reconstruction of a swimming Carolinites
This image depicts Carolinites in a possible swimming posture. The inverted swimming position, with limb gills serving as paddles, is typical of many aquatic or marine arthropods today (for example, fairy shrimp). No limbs of Carolinites have ever been recorded. The presence of long antennae and pygidial cerci is also conjectural. Note that the large eyes would offer an excellent field of view in either this position or one with limbs oriented downward.		 We're off to find more Proetida! See you there!
This image ©2000 by S. M. Gon, created in Macromedia Freehand

Distribution of fossils of Carolinites genacinaca today

The sites where Carolinites genacinaca is found today range from north arctic to south subtropical.

Paleozoic biogeography of Carolinites genacinaca

When the continents are adjusted for Lower Ordovician configuration, the sites are all tropical-equatorial.
(both figures adapted from McCormick & Fortey 1999. J. Paleontol. 73(2):202-18.)


Walking Trilobite animation ©2000 by S. M. Gon III