Australia: The Land Where Time Began

A biography of the Australian continent 

Cambrian Explosion - Lobopodians and Onychophora

Extant Onychophora are morphologically similar, having paired antennae and lobopodal walking legs, (legs that are stubby, rounded and annulated), and all extant forms are terrestrial and the extinct forms are marine. The Cambrian fossils with lobopodial appendages are much more diverse and have generated considerable controversy concerning their relationship to crown panarthropods (Hou & Bergström, 1995; Liu et al., 2008; Ma, Hou & Bergström, 2009; Ramskold & Chen, 1998). The first onychophoran to be recognised from the Burgess Shale was Aysheaia. Many lobopodians from the Cambrian Explosion were “armoured”, with a diverse array of plates and spines. The earliest confirmed body fossils of lobopods were recovered from deposits of Cambrian Stage 2 age, and it has been argued (J. Liu et al., 2008) that Zhijinites, a small shelly fossil from Cambrian Stage 1 is likely to be a lobopodian sclerite, and there have been reports from rocks of similar age in Siberia of Microdictyon plates (Kouchinsky et al., 2012). All forms from the Cambrian have elongated bodies and have lobopod-limbs that are circled by putative haemal channels, though the only exception is that the location of the mouth is different from extant onychophorans, the variable nature of the morphology of the limbs, the presence of terminal claws on limbs, the variety of spines and plaques on the trunk, and the general absence of prominent paired, muscular antennae, though small antennae have been reconstructed on some species from the Cambrian, which differ markedly from the muscular antennae of extant onychophorans.

Antennacanthopodia gracilis was an exceptional form from the Cambrian Chengjiang Fauna, with 2 pairs of antennae (Ou et al., 2011), the larger, more anterior of which, resemble the antennae of crown onychophorans. There are also possible paired eye spots on A. gracilis, a cuticle that are lightly sclerotised, and on the limbs there are terminal walking pads instead of claws, though these are other features that are more similar to extant onychophorans than to those of the lobopods. A. gracilis had body spaces that are relatively large, which are presumed to be haemocoels, that are arranged in a similar way to those of onychophorans. It has reasonably been speculated that (Ou et al., 2011) A. gracilis may be related to a branch of the lobopodians that gave rise to crown Onychophora.

Among those arguing for a lobopodian origin for arthropods Graham Budd, Uppsala University, has been prominent, citing anatomical comparisons between crown groups and fossil morphologies interpreted by him as having features that link onychophorans with stem plus crown arthropods (Budd, 1996, 1999). The description of Diania cactiformis, a spectacular new form from the Chengjiang Fauna (J. Liu et al., 2011), had added a new dimension to the question of the origins of arthropods. Diania is heavily armoured, which includes spiked elements that encase its legs, which suggests jointing and at least a degree of incipient segmentation. The armour on the trunk, the elements of which are quite narrow, suggests a different type of flexibility. It is easy to see how the jointing characterising the carapace of arthropods could have evolved from such integumental armour, while integrating within the jointed skeletal architecture internal segmental features to produce the body plan of arthropods (J. Liu et al., 2011), Though there has been a considerable amount of debate over the phylogenetic placement  of Diania. Diania was proposed to be truly intermediate between Cambrian age lobopods and Euarthropods. The authors¹ suggest that this hypothesis seems to be unlikely because of the discovery by several new phylogenetic analyses that have placed Diania well within other lobopods of the Cambrian (Mounce et al., 2011). This emphasises both the difficulty of resolving phylogenetic relationships among the Panarthropoda and the importance of phylogenies that are rigorously developed.

Many of the known fossils from the Cambrian are poorly preserved and incomplete, with key morphological features being difficult to discern. Therefore advances in knowledge of morphological details and their interrelationships is dependent on the continued discovery of new fossils revealing more features, though the general impression of the variety of body shapes and ornamentation of lobopods from the Cambrian is correct. There have already been challenges by subsequent study to the early phylogenetic hypothesis. A panarthropod phylogeny has been presented with a series of paraphyletic lineages in which the tardigrades and the onychophorans are extant representatives of a once more disparate assemblage of forms. The authors¹ suspect that phylogenetic studies in the future may alter the picture, possibly significantly, of the topology shown in Fig. 6.27 in their book¹.

Sources & Further reading

1. Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado