Glenn W. Storrs

Conference Abstracts Volume

The fossil record has been used directly to interpret patterns of origination, diversification, extinction, biogeographic dispersal, and other evolutionary processes. All too often, it has also been used to reconstruct phylogenies of organisms. If, however, the fossil record of a group is particularly poor, i.e. consists of numerous stratigraphic or geographic gaps relative to actual group history, macroevolutionary interpretations based upon it may be severely flawed. Nevertheless, rarely do palaeontologists attempt to quantify the accuracy of the record from which they make their interpetations. , It is now generally recognized that the fossil record is often insufficient to accurately reflect the true sequence of cladogenic events in phylogeny. Cladistic analysis of shared derived characters is, therefore, considered to be a more rigorous and powerful indicator of organismal relationships than is mere stratigraphic position. As cladistic hypotheses progress and knowledge of stratigraphic sequence is improved, however, a broad congruence between the two should eventually emerge. A rigorous application of cladistic techniques can be used to test the relative quality of any group's fossil record to the extent that the resulting cladogram reflects actual evolutionary relationships. Recent phylogenetic analyses of Triassic, largely European, sauropterygians (Diapsida, Sauropterygia) indicate that several monophyletic groups of these marine reptiles are relatively well supported by existing fossil data. The most plesiomorphic of these is the Pachypleurosauria, comprising small Anisian-Ladinian lacertiform animals, while the most derived representatives are Pistosaurus (Anisian) and the more typical Plesiosauria (Rhaetian-Maastrichtian). Large, intermediately positioned lacertiforms ('nothosaurids') from the Scythian-Camian are probably paraphyletic. Nothosauria as historically constituted is not monophyletic. The Placodontia are here considered a sauropterygian sister taxon to �nothosaurids' + Plesiosauria. Construction of a dendrogram incorporating relative stratigraphic positions and phylogenetic relationships of Triassic sauropterygians reveals numerous areas of fossil record weakness at the generic level. All 'nothosaurs' which preserve enough character state information for analysis were placed in context. First and last occurrences demonstrate their minimum temporal ranges directly from the fossil record. Placodonts artificially decrease resolution because they have not been rigorously analyzed at the generic level and are, therefore, currently excluded. The most obvious deficiency is the large number of taxa which are known only from single stratigraphic horizons (or single specimensl), thus having no known temporal range. In fact, the "Lagerstéitte Effect" imposed by the particularly productive locality of Monte San Giorgio, for example, undoubtedly biases the occurrence of apparently short-lived taxa in the Ladinian. A simple completeness total for the few Triassic sauropterygian genera with demonstrated stratigraphic range may be estimated at 415 my by adding all minimum range lengths inferred from first and last known occurrences. �This number is uninformative until compared with the total minimum implied gaps (MIGS) inferred from sister group positions. By definition, sister groups originate simultaneously during cladogenesis. Thus, any lineage must have existed at least by the earliest time of appearance of its sister clade. A mimimum time of lineage divergence and a MIG are, therefore, implicit in the relative stratigraphic positions of sister taxa. The pachypleurosaur lineage must have been present at least by the late Scythian when its nothosauriform sister clade first appears. Similarly, a MIC of approximately 31 my separates "Plesiosaurus" from Pistosaurus. Sauropterygians probably originated in the Late Permian. MIGs for Triassic sauropterygian lineages total 65 my in the present analysis. Thus, total inferred ranges equal 106.5 my and absolute completeness for the Triassic fossil record of this group is at best only 39%. This low maximum percentage indicates that record quality is poor and generalized statements of sauropterygian patterns of diversity, dispersal, and extinction in the Triassic must be viewed with caution. For example, does the lack of Norian 'nothosaurs' indicate their extinction at the end of the Carnian, or merely their lack of preservation as is obviously the case for plesiosaurs?

English