METRIORHYNCHOIDEA, Fitzinger, 1843

Young, Mark T., Brusatte, Stephen L., Ruta, Marcello & Andrade, Marco Brandalise De, 2010, The evolution of Metriorhynchoidea (mesoeucrocodylia, thalattosuchia): an integrated approach using geometric morphometrics, analysis of disparity, and biomechanics, Zoological Journal of the Linnean Society (Zool. J. Linn. Soc.) 158 (4), pp. 801-859 : 816

DIVERSITY OF METRIORHYNCHOIDEA

METHOD

Counts of taxonomic diversity (taxic diversity) have come under increasing criticism, as biases in the rock record invariably engender an underestimate of palaeobiodiversity (see Lane, Janis & Sepkoski, 2005 and the references therein). To correct for this bias, phylogeny-based approaches (which take into account ‘ghost lineages’ and ‘range extensions’ implied by the phylogeny) have been introduced (e.g. Norell, 1992; Smith, 1994). As a comprehensive cladistic treatment for Metriorhynchoidea is now available, diversity can be investigated using phylogenetic interpolation. A new compendium of metriorhynchoid taxic diversity (see Appendix) was compiled based upon an exhaustive literature search and specimen examination. Phylogenetic diversity was compiled using the phylogeny of Young & Andrade (2009) to correct for ghost ranges and range extensions. Taxic and phylogenetically estimated diversity measures were then plotted against time (Bajocian–Valanginian; based upon Ogg, Ogg & Gradstein, 2008).

RESULTS

Both observed and inferred curves of diversity track each other well during the Bajocian to the middle Callovian, and during the Tithonian to the late Valanginian ( Fig. 9 ). During the Bajocian–Bathonian, metriorhynchoid diversity was comparatively low (three or four species when phylogenetically corrected). However, during the Callovian there is a sharp rise in diversity, reaching seven species by the middle Callovian. From the late Callovian to late Kimmeridgian, the observed diversity departs considerably from the inferred diversity. In particular, the greatest underestimate is observed during the late Oxfordian (a fourfold underestimation). This is largely in agreement with Bardet (1994), who found that the fossil record of marine reptiles during the Oxfordian is only 44% complete. There is a sharp decline in metriorhynchid diversity beginning in the late Tithonian, which continues throughout the Berriasian, and is then followed by a slight increase in the Valanginian.

Based upon the divergence between the taxic and phylogenetic diversity curves, we conclude that there are many more species of metriorhynchoids still to be discovered. The Kimmeridgian–Berriasian deposits of La Casita and La Caja formations of Mexico are a good example of the rich metriorhynchid fauna yet to be fully unearthed (see Buchy, 2007, 2008a, b; Buchy et al., 2007).

Figures

Figure 9. Species diversity of Metriorhynchoidea (both taxic and phylogenetically corrected) for each stage subdivision.