Arthropods dominate the animal biosphere, comprising some 80% of all known animal species. They also dominate in numbers of individuals and in the colonisation of almost all known environments, be it terrestrial, marine, freshwater or in the air.
Despite this current dominance, the fossil record of this group is limited. The primary reason for this is that the unmineralised, but chemically toughened, cuticle possessed by most arthropods, while contributing to the success of the group due to its toughness and flexibility, nevertheless has a poor preservation potential. The fossil record is dominated by organisms which possessed biomineralised tissues, such as bone, shell, etc., in life, and biomineralised arthropods are the exception rather than the rule.
Two questions regarding the current arthropod domination of animal life appear conducive to palaeontological investigation:
- Is this biospheric dominance a recent phenomenon? (Has it built up over geological time or was it instilled very early on in metazoan evolution?)
- What is it about arthropods which has allowed this dominance to develop? (Why are they so 'evolution-friendly'? What is the secret of their success?)
Pointers to answer the first question first came from the Middle Cambrian Burgess Shale in British Columbia, Canada. This deposit produced, not only a large number of 'typical' biomineralised forms, but also a myriad of non-biomineralised or 'soft-bodied' forms. This, by now famous, biota showed that the living assemblage of Middle Cambrian organisms was dominated by arthropods. Questions as to the possibility that this biota was unique were dispelled by similar finds from the Lower Cambrian of Chengjiang, China, which confirmed the dominance of arthropods was indeed well established even at this early stage of metazoan evolution.
Light can be shed on the second question by studying arthropods from the Lower Cambrian Emu Bay Shale.
The Emu Bay Shale outcrops along the north coast of Kangaroo Island and at one locality, Big Gully, contains a restricted assemblage of fossils, dominated by arthropods, and containing biomineralised, lightly biomineralised and non-biomineralised forms, and highlight two aspects of arthropod biology, evolutionary mechanisms, and behaviour.
The assemblage is dominated by two biomineralised arthropods, the trilobites Redlichia takooensis and Hsuaspis bilobata. Comparison of ontogentics suggest that Middle Cambrian trilobite Xystridura evolved from Hsuaspis bilobata by heterochrony (changes in the rate or timing of development of characters during ontogeny). Analysis of juvenile forms show that extension of the juvenile growth period delayed maturation, or hypermorphosis, resulted in exaggeration of normally slow, early holaspid growth patterns within the rapid, late meraspid growth stage, producing a marked progradation of the glabella in Xystridura. Forms from the uppermost Lower Cambrian Cymbric Vale Formation of New South Wales appear to show the beginnings of this hypermophic trend. The arthropod body plan appears conducive to evolution via heterochrony, suggesting that simple changes in the rate of development produces significant evolutionary change and thus high 'evolution potential'.
A specimen of the arthropod Naraoia from the Emu Bay Shale represents the earliest direct body fossil evidence of predation on non-biomineralised individuals. Analysis of arthropod cuticle rheology enables elucidation of the predation method, suggesting the predator was Anomalocaris. Comparison with predation damage from mineralised trilobites and coprolites suggests that some anomalocaridids did prey on mineralised organisms (durophagy). The presence of durophagy in the Early Cambrian has important implications for the role of predation pressure in the acquisition of mineralised cuticles, and the rise of enrollment in trilobites.
The relatively high 'evolution potential' of arthropods, especially via heterochrony, combined with the ability to utilise complex behavioural patterns appears to be, at least part of, the secret to the success of the arthropods.