Weak trophic interactions and the balance of nature (original) (raw)

of rather generalized, small (20-70 cm in length) fishes, with dentitions lacking anterior fangs, a morphological category represented in the analysis by Kenichthys, Gogonasus, Osteolepis (Fig. 4a), Medoevia, the canowindrids and Tristichopterus (Fig. 4b). Rhizodonts 26 , derived tristichopterids 27 and elpistostegids+ tetrapods 5,16,28 , in contrast, show parallel trends towards a quite different morphology: they increased dramatically in size, reduced or lost their median fins, acquired diphycercal tails with a low aspect ratio, and developed a pair of fangs at the lower jaw symphysis (Fig. 4c-e). Rhizodonts and derived tristichopterids also acquired premaxillary fangs 27,29. Rhizodonts seem to have retained a primitive, short-snouted skull morphology (J. Jeffery, personal communication). However, tristichopterids and elpistos-tegids+tetrapods, having a moderately lengthened snout as a synapomorphy (Fig. 4b), independently developed this character further in parallel (Fig. 4d, e). Derived tristichopterids such as Mandageria 29 (Fig. 4d) have very elpistostegid-like head proportions. These changes seem to have occurred during the Middle/Late Devonian period in all three groups. Elpistostegids originated in the latest Givetian 6 ; the earliest known derived tristichopterid is the Frasnian Platycephalichthys 23,27 ; and the earliest known large rhizodont is the Famennian Sauripteris 15. Our analysis indicates that much of the lower part of the tetrapodomorph stem lineage consisted of 'osteolepiform' fishes. The character attributes of this part of the stem lineage can be reconstructed with precision. Parallel evolution towards the morphology of a large predator, with reduced median fins and elaborate anterior dentition, occurred at about the same time in rhizodonts, tristichopterids, and elpistostegids+tetrapods (Fig. 4). The evolution of two latter clades, having extra synapomorphies, also paralleled each other more closely. The Tetrapoda thus arose out of one of several similar evolutionary 'experiments' with a large aquatic predator role. Closer study of these parallel radiations should cast much new light on the ecological background to the origin of tetrapods.