A PHYLOGENETIC INVESTIGATION OF THE MECHANISMS REGULATING DIVERSITY AND RATE OF EVOLUTION IN POST-DEVONIAN LUNGFISH (OSTEICHTHYES: DIPNOI)

Significant changes in evolutionary success in the history a clade have been described in a number of taxa; a variety of hypotheses have been forwarded to explain this phenomenon both on a universal and a case-by-case basis. Lungfishes (Osteichthyes: Dipnoi) demonstrate a notable change in evolutionary success, experiencing an initial phase of rapid speciation and trophic diversification in the Devonian and a subsequent phase of apparent stagnation characterized by low rates of speciation and limited trophic diversity. To investigate this trend further, a phylogenetic analysis of both Devonian and post-Devonian taxa was conducted using a comprehensive set of discrete morphological characters. Characters were assigned to subsets (bins) representing type of character change (meristic and developmental) and anatomical region. Character evolution history was recovered in PAUP*4.0b and temporal distributions of character evolution data were compiled for both total character evolution and for each bin. These data suggest a higher rate of character evolution in Devonian lineages than in post-Devonian lineages; more detailed analyses suggest that part of this signal reflects a taphonomic bias, as numerous skeletal elements of post-Devonian taxa are highly cartilaginous, limiting inference of character evolution in unmineralized elements. However, the dental character suite preserves meaningful biological signal. Characters related to initiation of odontode development and histology of mineralized tissues experience very high rates of evolution in the Devonian; subsequent evolution of dental characters is restricted to meristic variation. This suggests that initial trophic and taxonomic diversity was facilitated by developmental plasticity, but canalization of dental development and high species turnover in the Late Devonian resulted in a highly specialized dentition with low plasticity. The highly stereotypical dentition of post-Devonian Dipnoi consistently constrained trophic diversification and led to the subsequent restriction of the clade.