across disparate groups. Consequently, loss of these kernels was likely only to have occurred during the infrequent loss of clades the equivalent of the Linnean rank of phylum or class. For marine animals this loss would have been largely during the Cambrian and again during the end-Permian mass extinctions. The loss of major clades of insects during the end-Permian might have caused a loss of some developmental diversity, but it is less clear whether unique developmental processes at the level of kernels were present. Among vertebrates there are many extinct groups of fish and early tetrapods, such as the armored fish of the Devonian and the mammal-like reptiles of the Permo-Triassic, that could have harbored now vanished developmental strategies. But as with insects, it is far from clear they were unique at the level of kernels.
Our understanding of plant developmental biology, although expanding rapidly, is less advanced than for animals, and we do not know whether a similar highly structured regulatory hierarchy exists within plants. Morphologic evidence has revealed the diversity of tree-like forms that evolved repeatedly, with many now-extinct clades using very different developmental and structural strategies to achieve a similar end. All trees need to solve the same basic problem of providing structural support while distributing nutrients vertically. Both modern pines and other flowering trees such as dogwood or oaks are constructed with an inner, woody, secondary xylem produced by the vascular cambium and surrounded by phellem. But cycads are constructed of an inner pith and an outer cortex, with much of the structural support on the outside from persistent leaf bases. Arborescent lycopsids, horsetails in the Carboniferous, tree palms, and tree ferns each have distinct ways of forming trees. Yet each of these different types of trees was adapted to a particular suite of environmental conditions, which influenced the nature of the resulting communities (Niklas, 1997; Donoghue, 2005). Thus it seems likely that major developmental strategies of plants have disappeared, particularly during the late Paleozoic.
Applying some of these different aspects of diversity to past mass extinctions is difficult because of both lack of data and difficulties in establishing appropriate criteria and reproducible metrics, but identifying these different measures of diversity is the first step toward building a more robust and quantifiable approach. Table 9.1 provides a preliminary, somewhat impressionistic, application of these metrics for marine animals across the five classic mass extinction intervals. In the absence of more comprehensive tools, proxies are used for some categories such as reefs for architectural diversity and trace fossils for behavioral complexity.