3.4.3. Triassic-Jurassic Boundary

The Triassic-Jurassic boundary is marked by a major extinction in the marine realm. This event, one of the five largest extinctions during the Phanerozoic Eon, is especially well documented in the European Alps, where there are massive losses of species of bivalves and gastropods. Ammonoid cephalopods almost became extinct. It has often been suggested that there was also a major biotic crisis in the continental realm. The Late Triassic is characterized by the appearance in the fossil record of many of the "modern" clades of continental tetrapods (dinosaurs, mammals, turtles, lepidosaurs, frogs and salamanders) or their proximate sister-taxa, which then co-existed with various groups persisting from earlier stages of the Triassic (such as crurotarsan archosaurs) or even the preceding Permian period (such as various taxa of therapsids). Near or at the Triassic-Jurassic boundary, the latter disappeared or experienced major losses in diversity. Surprisingly, dinosaurs did not show any losses of lineages at this transition (Fig. 3.4.3.1). Among pollen and spores of terrestrial plants, there was a dramatic change from high-diversity palynofloras to assemblages almost entirely made up of cheirolepidaceous conifers (Corollina or Classopollis).

Figure 3.4.3.1 (above): Ranges of families tetrapods through the Triassic and Early Jurassic depicted at the stage level. The darker blue bars ranges are from Olsen (1986) and the lighter blue bars are range extensions since that time. Note that the largest concentrations of extinctions occurs at the Triassic-Jurassic boundary. [Note: Rhaetian is her included with the Norian]. Click on the image for a higher resolution figue.
 

Currently, a major obstacle to analyzing the extinction event on land is the virtual absence of detailed studies of continuous sequences of continental strata ranging from the Norian-Rhaetian to the basal Jurassic. For example, in the Germanic Basin, the continental strata of the Late Triassic (Norian-Rhaetian) are overlain by marine rocks of the Liassic transgression. At present, the best prospects for tracing the biotic changes in continental settings are offered by the well-dated strata of the Newark Supergroup and in Greenland. A recently discovered tetrapod assemblage from the Jacksonwald syncline of the Newark basin in Pennsylvania appears to predate the palynologically identifiable Triassic-Jurassic boundary (Fig. 3.4.3.2) by only 800,000 years (Olsen et al., in prep.) (Fig. 3.4.3.3).
Figure 3.4.3.2 (right): Palynologically identified Triassic-Jurassic boundary exposed in the Jacksonwald syncline of the Newark basin about 10 m below the lowest CAMP basalt (Orange Mountain Basalt). A thin coal or carbonaceous shale containing virtually only fern spores (fs, fern spike) separates strata with typical Late Triassic (Tr) from typical Early Jurassic (Ju) pollen and spore assemblages (Fowell et al., 1993).

	What is to be done? First, we need to establish the pattern of biotic change in continental settings on a much finer temporal and geographic scale. This requires the identification and systematic study of additional continuous, fossiliferous sequences of continental strata. Second, we need to correlate any changes in continental biotas with the well-established biotic crisis in marine communities. Magnetostratigraphy now offers a powerful tool for such correlations. Are the observed biotic changes synchronous? Are they worldwide in scope or is there a geographic pattern? Only after addressing these questions will it become meaningful to look for potential causes for the biological changes (climatic changes, large-scale volcanism [CAMP], or bolide impact).
	Figure 3.2.3.3 (left): Lower jaw of procolophonid reptile Hypsognathus fenneri shown in medial (A) and lateral (B) views. From the Jacksonwald syncline, about 800 ky older than the Triassic-Jurassic boundary.  This is a representative of a typical Triassic family of reptiles.