WHAT DOES IT MEAN TO BE TETHYAN?

Paleogeographic interpretations of tectonostratigraphic terranes in western North America frequently use the term “Tethyan”, a word that relates back to the Tethys and Paleo-Tethys oceans of the Mesozoic and Paleozoic. In paleontologic interpretations of terrane paleogeography, “Tethyan” has taken on several different, and often contradictory, meanings.

Strictly speaking, a Tethyan fossil would be the fossil of an organism that once existed in a faunal province that occupied the Tethys Ocean or its precursors. In terrane paleogeography, however, “Tethyan” is contrarily used to describe latitudinally- and/or longitudinally-restricted fossil distributions (i.e. tropical) that may or may not occur in the Tethys, as well as fossils originally known from the Tethys but subsequently found elsewhere in the Panthalassan Ocean. Non-specific usage of the term “Tethyan” has led to misinterpretations of terrane paleogeography that are in conflict with known fossil distributions.

The paleobiogeographic distribution of a given fossil must be determined by multiple geographically disparate samples spanning the time range of the fossil. Accurate reconstruction of this paleobiogeographic distribution thus accounts for changes to that distribution through time and requires precise geochronology. Adequate sampling and consideration of various sedimentary facies and depositional environments are required to assert absence of fossils from an area. Paleobiogeographic interpretations of terranes require contemporaneous data for comparison, and in order to mitigate possible discrepancies, the history of the terrane over geologic time must be considered.

Usage of the term “Tethyan” for fossils needs to be restricted to those known to exist only in the Tethys Ocean (or a slightly expanded Tethyan Province). Fossils that appear similar to those in the Tethys are Tethyan-like and do not necessarily have Tethyan paleogeographic affinity. Fossil distributions can be warm-water, tropical, temperate, oceanic, or Panthalassan without being “Tethyan”. Ongoing studies of various fossil groups have recognized changes in fossil distributions over time and show that fossil distributions are far more complicated than “Tethyan” or non-“Tethyan”, proving the need for the aforementioned methods.