Lobopodia

For a type of cell projection see Pseudopod.
Lobopodia
Fossil range: Cambrian Series 2–Early Pennsylvanian[1]

The taxa Onychophora and Tardigrada survive to Recent


Reconstruction of the lobopod Aysheaia
Scientific classification
Kingdom: Animalia
Superphylum: Ecdysozoa (in part)
Included groups
Excluded groups

Arthropoda

The lobopodians, members of the informal group Lobopodia, (from the Greek, meaning "blunt feet") is a group of poorly understood animals, which mostly fall into a stem group of arthropods. Their fossil range dates back to the Early Cambrian. Lobopodians are segmented and typically bear legs with hooked claws on their ends.[2]

The oldest near-complete fossil lobopodians date to the Lower Cambrian; some are also known from a Silurian Lagerstätte.[3] They resemble the modern onychophorans (velvet worms) in their worm-like body shape and numerous stub-legs, which are termed lobopods (the term 'lobopodian' refers to the entire animal). They differ in their possession of numerous dorsal armour plates, "sclerites", which often cover the entire body and head. Since they taper off into long, pointed spikes, these probably served a role in defence against predators. Individual sclerites are found among the so-called "small shelly fossils" (SSF) and Small carbonaceous fossils from the early Cambrian period.[2][4] The "lobopodia" group is considered to include these Cambrian forms in addition to the extant onychophorans.

Representative taxa

The better-known genera include, for example, Aysheaia, which was discovered in the Canadian Burgess Shale and which is the most similar of the Lobopoda in appearance to the modern velvet worms; a pair of appendages on the head have been considered precursors of today's antennae. Xenusion was apparently able to roll itself up, spines outward, giving insight into the defensive strategies of the Lobopoda.

By far the most famous of the lobopodian genera, however, is Hallucigenia, which, like Aysheaia, was discovered in the Burgess Shale. Its name refers to its bizarre appearance. It was originally reconstructed with long, stilt-like legs and mysterious fleshy dorsal protuberances, and was long considered a prime example of the way in which nature experimented with the most diverse and bizarre body designs during the Cambrian.[5] However, further discoveries showed that this reconstruction had placed the animal upside-down: interpreting the "stilts" as dorsal spines made it clear that the fleshy "dorsal" protuberances were actually legs. This second reconstruction also exchanged the front and rear ends of the creature, which further investigation showed to be erroneous.[6] A study published in 2014 concluded that Hallucigenia is an ancestor of modern onychophora based on its spiny claws.[7]

The armoured lobopodian Diania is significant for having the most arthropod-like appendages. A long-legged taxon, Carbotubulus, is known from the Carboniferous Mazon Creek.[1]

Morphology

Most lobopodians are in the order of inches in length, though tardigrades are ~0.1 to 1.5 millimeters long. They are annulated, although the annulation may be difficult to discern on account of their close spacing (~0.2mm) and low relief.[8] Lobopodia and their legs are circular in cross-section.[8] Their legs, technically called lobopods, are loosely conical in shape, tapering from the body to their clawed tips.[8] The longest and most robust legs are at the middle of the trunk, with those nearer the head and tail more spindly.[8] The claws are slightly curved. Their length is loosely proportional to the length of the leg to which they are attached.[8] The eyes are similar to those of modern arthropods as has been shown in Miraluolishania haikouensis (Liu et al., 2004).

The gut of lobopodians is unusual in that it is straight, undifferentiated, and sometimes preserved in the fossil record in three dimensions. In some specimens the gut is found to be filled with sediment.[8] The gut consists of a central tube occupying the full length of the lobopodian's trunk,[9] which doesn't change much in width - at least not in a systematic fashion. This may be surrounded by serially repeated[9] kidney-shaped diverticulae.[2] In some specimens, parts of the lobopodian gut can be preserved in three dimensions. This cannot result from phosphatisation, which is usually responsible for 3-D gut preservation,[10] for the phosphate content of the guts is under 1%; the contents comprise quartz and muscovite.[8] The gut of the representative Paucipodia is variable in width, being widest at the centre of the body. Its position in the body cavity is only loosely fixed, so flexibility is possible.

Ecology

The way of life of the Cambrian Lobopodia is to a large extent unknown; some species were apparently carnivorous, feeding on other animals such as, for example, sponges (Porifera). Others apparently lived in close association with the enigmatic genus Eldonia.

Diversity

During the Cambrian lobopodians displayed a substantial degree of biodiversity. However, only one species is known from each of the Ordovician and Silurian periods,[3][11] with a few more known from the Carboniferous (Mazon Creek).

Phylogeny

The lobopodians are thought to be closely related to the arthropods; indeed, the arthropods may well have arisen from within the lobopodians. They may be less closely related to the tardigrades; precise classification is still in flux.[2]

Budd sees the Lobopodia as representing a basal grade from which the phyla Onychophora and Arthropoda arose, with Aysheaia comparable to the ancestral plan, and with forms like Kerygmachaela and Pambdeleurion representing a transition that, via dinocaridids to arthropods, would lead to an arthropod body plan.[12] Aysheaia's surface ornamentation, if homologous with palaeoscolecid sclerites, may represent a deeper link connecting it with cycloneuralian-like outgroups.[12]

Budd sees the origin of armature in the armoured lobopodians as (originally) associated with muscle attachment points — as an adaptation to allow finer muscular control of the longer limbs seen in this group.[12] In Budd's view, armoured lobopodians represent a clade.[12]

See also

The name Lobopodia is also used for lobose pseudopods, found among certain amoeboids.

References

  1. ↑ 1.0 1.1 Haug, J. T.; Mayer, G.; Haug, C.; Briggs, D. E. G. (2012). "A Carboniferous Non-Onychophoran Lobopodian Reveals Long-Term Survival of a Cambrian Morphotype". Current Biology. doi:10.1016/j.cub.2012.06.066.
  2. ↑ 2.0 2.1 2.2 2.3 Liu; Shu, Degan; Han, Jian; Zhang, Zhifei; Zhang, Xingliang (2007). "Origin, diversification, and relationships of Cambrian lobopods". Gondwana Research 14 (1–2): 277. doi:10.1016/j.gr.2007.10.001.
  3. ↑ 3.0 3.1 Von Bitter, P. H.; Purnell, M. A.; Tetreault, D. K.; Stott, C. A. (2007). "Eramosa Lagerstätte—Exceptionally preserved soft-bodied biotas with shallow-marine shelly and bioturbating organisms (Silurian, Ontario, Canada)". Geology 35 (10): 879. doi:10.1130/G23894A.1.
  4. ↑ Caron, J. -B.; Smith, M. R.; Harvey, T. H. P. (2013). "Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians.". Proceedings of the Royal Society B: Biological Sciences 280 (1767): 20131613. doi:10.1098/rspb.2013.1613. PMID 23902914.
  5. ↑ Gould, S.J. (1989). Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton & Company.
  6. ↑ Further information and references: See Hallucigenia
  7. ↑ Science, vol. 345, p. 856.
  8. ↑ 8.0 8.1 8.2 8.3 8.4 8.5 8.6 Hou, Xian-Guang; Ma, Xiao-Ya; Zhao, Jie; Bergström, Jan (2004). "The lobopodian Paucipodia inermis from the Lower Cambrian Chengjiang fauna, Yunnan, China". Lethaia 37 (3): 235. doi:10.1080/00241160410006555.
  9. ↑ 9.0 9.1 Jianni Liu, Degan Shu, Jian Han, Zhifei Zhang, and Xingliang Zhang (2006). "A large xenusiid lobopod with complex appendages from the Lower Cambrian Chengjiang Lagerstätte" (PDF). Acta Palaeontol. Pol. 51 (2): 215–222. Retrieved 9 February 2011.
  10. ↑ Butterfield, N. J. (2002). "Leanchoilia guts and the interpretation of three-dimensional structures in Burgess Shale-type fossils". Paleobiology 28: 155–171. doi:10.1666/0094-8373(2002)028<0155:LGATIO>2.0.CO;2. ISSN 0094-8373.
  11. ↑ Whittle, R. J.; Gabbott, S. E.; Aldridge, R. J.; Theron, J. (2009). "An Ordovician Lobopodian from the Soom Shale Lagerstätte, South Africa". Palaeontology 52 (3): 561–567. doi:10.1111/j.1475-4983.2009.00860.x.
  12. ↑ 12.0 12.1 12.2 12.3 Budd, G. E. (2001). "Why are arthropods segmented?". Evolution and Development 3 (5): 332–42. doi:10.1046/j.1525-142X.2001.01041.x. PMID 11710765.