Australia: The Land Where Time Began
Cambrian Explosion - Branches that Link Lobopoda and Crown Arthropoda
There is a wide array of fossils that appear to have affinities with arthropods between the lobopods of Cambrian age and crown-group arthropods; that have what appear to be segmented exoskeletons and some sort of jointed appendages in most, the crown group being called the Euarthropoda to distinguish it from such stems. Some of the synapomorphies of crown groups are missing from more basal groups, though they have synapomorphies of their own. There are 2 forms near the base of this assemblage, and close to the lobopodians, Kerygmachela and Pambdelurion, that have been interpreted as having flexible, as opposed to jointed, sclerotised appendages that can be seen as having derived from lobopods (see Budd, 1999). These forms have gills, and Pambdelurion has a radial mouth which is surrounded by sclerotised plates resembling the mouths of anomalocaridids. These forms differ from canonical lobopodians in these, as well as other respects. These “gilled lobopods” have been commonly cited as evidence that the lobopodians gave rise to the arthropods.
Another step towards arthropodisation may be represented by Opabinia. This form, about 5 cm long, has an anterior appendage on a stalk, multiple eyes and odd posterior blades or flaps. Opabinia is believed to have been mostly a swimming form, and it is suggested by the authors1 that it is probably allied with the anomalocaridids, known from several genera that have been grouped into the clade Radiodonta. The anomalocaridids were fairly large animals, some possibly reaching up to 1 m in length. Anomalocaridids have a radial mouth, as does Pambdelurion that has been preserved as a circlet of sclerotised plates, some forms having 32 plates, around the mouth opening. It had paired claws flanking the mouth anteriorly. Anomalocaridids persisted into the Ordovician, at least, and it was specimens, that the authors1 describe as ‘beautiful (and gigantic) specimens’ from the Ordovician that have recently been recovered (Van Roy & Briggs, 2011). It has been that the jointed claws could presumably be flexed to capture prey and transfer it to the mouth circlet, which the authors1 suggest was probably specialised for crushing exoskeletons. Swimming is suggested to probably have been accomplished by the movement of the lateral flaps, to which gills were attached, that were along the sides of the body. Both radiodontids and Opabinia belong to the paraphyletic stem group assemblage between the lobopodians and Euarthropoda.
To understand the evolution of arthropod limbs, and therefore the origin and early diversification of arthropods it is critical to resolve the phylogenetic relations among these groups (Budd & Telford, 2009; Edgecombe, 2010). There are a number of problems facing researchers in this area. An example is that lobopodians all have relatively simple, unspecialised legs, though in Opabinia and anomalocaridids there are paired lateral flaps instead of legs and these flaps have gills along the upper aspect, particularly in Opabinia (X.L. Zhang & Briggs, 2007). Jointed appendages that are well sclerotised reappear beyond the Radiodonta. Some of the complexities involved in understanding the evolutionary pathways among these groups are illustrated by the ongoing debate regarding a number of questions, such as:
1. Are the appendages of arthropods homologous to those of lobopods, as argued by Budd?
2. Are they homologous to the lateral flaps of the Radiodonta?
3. Or are they completely novel structures?
According to the authors1 this debate is not close to settlement (Budd, 1996; Budd & Daley, 2012; Waloszek et al., 2007; X.L. Zhang & Briggs, 2007), illustrating some of the complexities of understanding the evolutionary pathways among these groups. There are still other stem arthropods that are almost certainly closer to crown arthropods than are the radiodontids, which have been recovered from assemblages of Burgess Shale type. There are 2 such groups, that could possibly be closely related, the Canadaspids, bivalves that have sometimes been assigned to crown Crustacea, and Fuxianhuia and related taxa. They differ from crown groups mainly in the details of their limbs (see Edgecombe, 2010).
The Megacheira, the “short great appendage arthropods” such as Fortiforceps, are another stem group that are well represented among the Lower Cambrian lagerstätten. Relatively large, paired appendages arising on the anterior portion of the head, characterise this group, tough it is often difficult to be sure precisely where they insert as the heads are partially hidden beneath a shield and it is not evident where the boundaries of the head segments are. Anterior head appendages are innervated from the brain, which has 3 lobes, in living arthropods: in anterior-posterior order they are the procerebrum (bearing eyes), the deuterocerebrum, and the tritocerebrum. Arthropods have their eyes on their most anterior head segment, the ocular segment, and they are innervated from the most anterior lobe. The antennae in crustaceans and the chelicerae in chelicerates, are on the second segment only in both groups, and are innervated from the middle lobe. Appendages arising on the ocular segment are not present on any extant arthropods, though extant onychophorans have 2 lobes to their brains and their antennae arise on their ocular segment, the eyes being situated at the base of the antennae and both are innervated from the anterior brain lobe. Therefore, if crown onychophorans and arthropods are both derived from among the lobopodians, then at some point there was significant evolution of the brain and ocular appendages to produce their different derived features.
An hypothesis has been proposed to overcome this evolutionary problem (Budd, 2002) according to which the “great appendages” could be interpreted as arising from the ocular segment, as is the case of extant Onychophora. He suggested that if this interpretation is correct the appendages on the ocular segment on those forms from the Cambrian then the appendages would be innervated from the procerebrum, which differs from the condition in the crown arthropods, i.e., the switch from anterior innervation of anterior appendages took place after the Megacheira, or other arthropods with similar appendage origins, had branched from the stem arthropods. It has been found by study of more recent fossil finds that in at least some forms the great appendages arise from just behind the eyes, so they may not have been on the ocular segment. The simplest explanation of evolution of these features is that the condition present in crown arthropods arose prior to the branching that led to the Megacheira. The question then remains as to when the brain style of crown arthropods and anterior innervation arose. The authors1pose the question of could it simply have been after the LCA of Onychophora and Arthropoda, within the lineage, though not known as fossils, that gave rise to the basal arthropods? Also, in what form was the brain in lobopods? The austhors1 suggest that the answer may never be known for certain.
Trilobites were the best studied and most widely known arthropods of the Cambrian prior to the redescription of the Burgess Shale Fauna that began in the mid-1960s. They are among the best-studied taxa from the Palaeozoic and remain the most numerous by far, outnumbering all other groups in terms of the number of species and genera known from the Cambrian. The base of Cambrian Stage 3 is defined by the earliest-known trilobites. The carbonate exoskeletons of trilobites are more readily preserved than those of most arthropods from the Cambrian that have organic integuments that are tough but not mineralised, and they are commonly found in the many fossil faunas where only skeletons that are mineralised are preserved. The obvious correlation between the ability to preserve well and variability leads to speculation that the groups with unmineralised integuments that have been recovered from the Burgess Shale, Chengjiang, Emu Bay, and other similar faunas may have been far more diverse than it is possible to document. The Naraoiidae are a group with unmineralised integuments that are a closely related sister group to trilobites that have been found only in the Chengjiang and Burgess Shale assemblages (Hou & Bergström, 1997). It is uncertain whether trilobites branched before or after the LCA of crown arthropods.
Among the larger, better-known fossils that have been described from the assemblages of the Chengjiang and Burgess Shale crustaceans don’t appear to be represented, though the group may be represented by fragmentary mouth parts in drill cores of Stage 3 or Stage 4 from the Northwest Territories, Canada (T.H. Harvey & Butterfield, 2008). These fragments indicate a mandibular feeding apparatus that is more similar to the apparatuses that have been found in crown-group crustaceans than to those of known stem-group taxa. An animal that may have been as large as 5 cm that is believed to have likely fed by scraping particles from benthic surfaces, that were then filtered and eventually comminuting them on mandibular molars in a sophisticated feeding system. Also, in the Stage 4 rocks from south China a crustacean larva has been identified (X.G. Zhang et al., 2010). Therefore, crown crustaceans, and therefore crown arthropods, are likely to be represented among fossils from the Early Cambrian.
In summary, among these spectacular though problematic ecdysozoans groups from the Early Cambrian, there appears to have been a radiation of scalidophorans, lobopods and arthropods in the latest Neoproterozoic and Early Cambrian. The scalidophorans of the Cambrian appear to be stem lineages displaying more morphological diversity than their crown groups. Lobopodians from the Cambrian are a very disparate, paraphyletic stem clades including ancestral groups of the arthropods, tardigrades and onychophorans. In the Cambrian the burgeoning lobopod and arthropod faunas, almost all clades of which are extinct, were clearly major elements in the economy of Cambrian communities, and according to the authors1 their ecological effects must have been felt in most corners of the marine biosphere, judging from their great variety of morphology.
Erwin, Douglas H., & Valentine, James W., 2013, The Cambrian Explosion: The Construction of Animal Biodiversity, Roberts & Co., Greenwood Village, Colorado
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