MORPHOLOGIC LANDMARK ANALYSIS AND GLABELLULAR FENESTRAE COUNT OF NORTH AMERICAN CRYPTOLITHUS TRILOBITES

Cryptolithid trilobites (Trinucleioidea) are known from deep water mixed carbonate and shale facies of the late Ordovician of Eastern North America. The earliest occurrence (Cryptolithus tesselatus) occurs in the early Chatfieldian adjacent to the Taconic Foreland Basin and intracontinental troughs. Similar precursors are known from west of the transcontinental arch and may have migrated into Eastern North America during high sea-level coincident with the Guttenberg Isotopic Carbon Excursion. Although C. tesselatus is reported to range into the Edenian, C. bellulus was more ubiquitous at that time. Previous workers recognized that Chatfieldian specimens (C. tesselatus) were restricted in the number of glabellular pits. In contrast, C. bellulus are much more variable in the number of pits and several morphotypes are recognized on the number and arrangement of pits. In all cases, the latter have more fenestrae than C. tesselatus.

The increase in the number of glabellular fenestrae is not well understood. Although a more detailed study of this morphologic character is needed, preliminary observations show that most C. bellulus are found in slightly coarser-grained sedimentary rocks (calcisiltites & calcareous siltstones) as opposed to finer-grained calcilutites and shales (C. tesselatus). This suggests that C. bellulus may have preferred a slightly shallower environment. However, basal Cincinnatian strata (dominated by C. bellulus) represent the deepest water facies of the entire Chatfieldian-Edenian interval. Thus although the appearance of cryptolithids are clearly tied to sea level fluctuation and/or tectonic-induced subsidence, the introduction of coarser sediments during the Taconic orogeny may have played a role in the evolution of increased numbers of fringe pit fenestrae. This remains to be tested.

In this preliminary study, in addition to counting fringe pits in cryptolithids from various stratigraphic intervals in NY, KY, and PA, we are using landmark analysis and a computer software package (tpsDig) to quantify morphologic changes through time. If increased siliciclastic sedimentation and grain size contributed to morphologic change, this variability should be observable in the trilobites as the Taconic foreland basin became filled in.