This review summarizes information on the behavioural ecology of mixed-species troops (interspecific associations) formed by different species of callitrichines, small New World monkeys, in western and central Amazonia. The formation of mixed-species troops is an integral part of the biology of several species of this subfamily. Niche separation between associated species is obtained through vertical segregation which results in differences in the prey spectrum. The degree of niche separation is a predictor for the stability of mixed-species troops. Individuals may benefit from the formation of mixed-species troops through increased safety from predators, increased foraging efficiency, and/or increased resource defence. Costs of mixed-species troop formation are probably very low and mainly relate to patterns of interspecific behavioural interactions. We point to gaps in our knowledge and suggest pathways for future research into mixed-species troops.

Current information on the conodonts Clydagnathus windsorensis (Globensky) and Promissum pulchrum Kovács–Endrödy, together with the latest interpretations of conodont hard tissues, are reviewed and it is concluded that sufficient evidence exists to justify interpretation of the conodonts on a chordate model. A new phylogenetic analysis is undertaken, consisting of 17 chordate taxa and 103 morphological, physiological and biochemical characters; conodonts are included as a primary taxon. Various experiments with character coding, taxon deletion and the use of constraint trees are carried out. We conclude that conodonts are cladistically more derived than either hagfishes or lampreys because they possess a mineralised dermal skeleton and that they are the most plesiomorphic member of the total group Gnathostomata. We discuss the evolution of the nervous and sensory systems and the skeleton in the context of our optimal phylogenetic tree. There appears to be no simple evolution of free to canal-enclosed neuromasts; organised neuromasts within canals appear to have arisen at least three times from free neuromasts or neuromasts arranged within grooves. The mineralised vertebrate skeleton first appeared as odontodes of dentine or dentine plus enamel in the paraconodont/euconodont feeding apparatus. Bone appeared later, co-ordinate with the development of a dermal skeleton, and it appears to have been primitively acellular. Atubular dentine is more primitive than tubular dentine. However, the subsequent distribution of the different types of dentine (e.g. mesodentine, orthodentine), suggests that these tissue types are homoplastic. The topology of relationships and known stratigraphic ranges of taxa in our phylogeny predict the existence of myxinoids and petromyzontids in the Cambrian.

It has long been assumed that the extant bilaterian phyla generally have their origin in the Cambrian explosion, when they appear in an essentially modern form. Both these assumptions are questionable. A strict application of stem- and crown-group concepts to phyla shows that although the branching points of many clades may have occurred in the Early Cambrian or before, the appearance of the modern body plans was in most cases later: very few bilaterian phyla sensu stricto have demonstrable representatives in the earliest Cambrian. Given that the early branching points of major clades is an inevitable result of the geometry of clade diversification, the alleged phenomenon of phyla appearing early and remaining morphologically static is seen not to require particular explanation. Confusion in the definition of a phylum has thus led to attempts to explain (especially from a developmental perspective) a feature that is partly inevitable, partly illusory. We critically discuss models for Proterozoic diversification based on small body size, limited developmental capacity and poor preservation and cryptic habits, and show that the prospect of lineage diversification occurring early in the Proterozoic can be seen to be unlikely on grounds of both parsimony and functional morphology. Indeed, the combination of the body and trace fossil record demonstrates a progressive diversification through the end of the Proterozoic well into the Cambrian and beyond, a picture consistent with body plans being assembled during this time. Body-plan characters are likely to have been acquired monophyletically in the history of the bilaterians, and a model explaining the diversity in just one of them, the coelom, is presented. This analysis points to the requirement for a careful application of systematic methodology before explanations are sought for alleged patterns of constraint and flexibility.