274 BIOLOGICAL RADIATIONS AND SPECIATION Figure The cause of a clade radiation has often been identified as the evolution of a key innovation One such example is the appearance of ovicells (brood chambers) in some cheilostome bryozoans (A) Welbertopora mutabilis, upper Albian, Fort Worth Formation, Denton County, Texas Part of the colony of one of the earliest neocheilostomes with ovicells in which larva develop (B) Generic plots of non brooding and brooding cheilostomes showing the dramatic rise in the latter The presence of Jurassic and Lower Cretaceous cheilostomes demonstrates that appearance of brooders is not a taphonomic artifact Images supplied by Dr Paul Taylor, The Natural History Museum, from unpublished data along with environmental differentiation, and this almost certainly influenced the radiation of many marine organisms Origination of an Evolutionary Novelty Leading to Taxic Diversity Often the radiation of a particular group has been linked with the appearance of a particularly significant character (e.g., development of the cleidoic egg, enabling the organisms to become reproductively independent of water, or wings to fly) In many of the usual examples there is very little taphonomic control, so that it is difficult to separate the appearance of an evolutionary novelty and increase in species diversity from the preservation bias that may be present in the fossil record But there are some examples that avoid this problem Cheilostomes are the dominant bryozoans (see Fossil Invertebrates: Bryozoans) living today and they are distinguished from other Bryozoa by the possession of box-shaped zooecia The fossil record of cheilostomes begins in the Upper Jurassic, with a rapid rise in numbers of species and genera in late Lower Cretaceous There are two kinds of cheilostomes: malacostegans (about 150 Recent species) and their species descendents, the neocheilostomes (about 5000 Recent species) Modern malacostegans Figure The history of a clade during phylogenetic diversifi cation may follow one of several patterns, idealized here The horizontal axis represents morphological divergence, the verti cal axis time (A) Morphological evolution constrained, species origination high (B) Morphological evolution rapid in early periods relative to speciation (C) Morphological evolution and speciation approximately in step with one another From Foote M (1993) release planktotrophic larvae that spend up to about month free living in the water column before settling and beginning colony growth During this time there is potential for considerable dispersal as well as associated potential for continued gene flow Neocheilostomes brood their larvae within a specialized brood chamber: – the ovicell The larvae have a very short, free life span before settling to the substrate (often adjacent to the parent) and beginning colony formation Thus, gene flow between populations may be very restricted and as a consequence there is the greater potential for genetic divergence Figure 10 Comparison of morphological and taxic evolution in coelacanth fishes using phylogenetic measures (A) Phylogenetic tree showing time bands selected as sample stages Within each time band two types of comparison were made on a pair wise basis and then averaged (B) The mean number of character changes occurring between taxa in each of the five time bands This is a measure of morphological disparity (C) The mean number of nodes (cladogenetic events) occurring between taxa in each of the five time bands This is a measure of taxic diversity In this case morphological diversity tracks taxic diversity except for a slight insignificant increase in taxic diversity relative to morphological diversity in the early stage (Namurian) Data from Forey PL (1998) History of the coelacanth fishes London: Chapman & Hall