Nectocaris

Nectocaris pteryx
Temporal range: Cambrian Stage 3–Cambrian Stage 5
Current reconstruction showing the large and small morphs of Nectocaris
(from Smith, 2013[1])
Scientific classification
Kingdom: Animalia
Phylum: Mollusca (?)
Stem-group: Cephalopoda (?)
Family: †Nectocarididae
Genus: †Nectocaris
Conway Morris, 1976
Species: † N. pteryx
Binomial name
Nectocaris pteryx
Conway Morris, 1976

Nectocaris pteryx is a species of possible cephalopod[2] affinity, known from the "early Cambrian" (Series 2) Emu Bay Shale and Chengjiang biota, and the "middle Cambrian" (Series 3, Stage 5) Burgess Shale.

Nectocaris was a free-swimming, predatory or scavenging organism. This lifestyle is reflected in its binomial name: Nectocaris means "swimming shrimp" (from the Ancient Greek νηκτόν, nekton, meaning "swimmer" and καρίς, karis, "shrimp"; πτέρυξ, pteryx, means "wing"). Two morphs are known: a small morph, about an inch long, and a large morph, anatomically identical but around four times longer.[3]

Anatomy

Nectocaris pteryx from the Burgess Shale; funnel is visible folded to left of specimen. Image from Smith (2013).

Nectocaris pteryx from the Burgess Shale; funnel is visible folded to left of specimen. Image from Smith (2013).[1]

Nectocaris had a flattened, kite-shaped body with a fleshy fin running along the length of each side.[2] The small head had two stalked eyes, a single pair of tentacles, and a flexible funnel opening out to the underside of the body.[2] The funnel often gets wider away from the head. Internally, a chamber runs along the body axis, containing a pair of gills; the gills comprise blades emerging from a zig-zag axis. Muscle blocks surrounded the axial cavity, and are now preserved as dark blocks in the lateral body.[3] The fins also show dark blocks, with fine striations superimposed over them. These striations often stand in high relief above the rock surface itself.[3]

Diversity

Although Nectocaris is known from Canada, China and Australia, in rocks spanning some 20 million years, there does not seem to be much diversity; size excepted, all specimens are anatomically very similar. Historically, three genera have been erected for nectocaridid taxa from different localities, but these 'species' – Petalilium latus and Vetustovermis planus – likely belong to the same genus or even the same species as N. pteryx. Within N. pteryx, there seem to be two discrete morphs, one large (~10 cm in length), one small (~3 cm long). These perhaps represent separate male and female forms.[3]

Ecology

The unusual shape of the nectocaridid funnel has led to its interpretation as an eversible proboscis, but this is difficult to reconcile with the fossil evidence. Instead, its fluid dynamics seem optimal for a role in jet propulsion, where it would have maintained an efficient slow flow of water over the large internal gills.[3] The eyes of Nectocaris would have had a similar visual acuity to modern Nautilus (if they lacked a lens) or squid (if they did not).[3]

Affinity

The affinity of Nectocaris has been a controversial subject, and some scientists feel that it is still uncertain.[4][5] The balance of evidence seems to suggest a relationship with the cephalopods, whether in an ancestral or derived position.[3] The most pleasing interpretation – which is not without problems – is to interpret some of the features of Nectocaris as shared with the earliest cephalopods. Features that seem to be shared with the modern coleoids (which evolved much later) can then be attributed to convergence – but perhaps using the same underlying genetic machinery as modern coleoids do today.[3]

History of study

Original reconstruction based on a single, incompletely preserved, lateral specimen. The author of this reconstruction, based on the material then available, considered Nectocaris to bear arthropod and chordate-like features.

Nectocaris has a long and convoluted history of study. Charles Doolittle Walcott, the discoverer of the Burgess Shale, had photographed the one specimen he had collected in the 1910s, but never had time to investigate it further. As such, it was not until 1976 that Nectocaris was formally described, by Simon Conway Morris.[6]

Because the genus was originally known from a single, incomplete specimen and with no counterpart,[7] Conway Morris was unable to deduce its affinity. It had some features which were reminiscent of arthropods, but these could well have been convergently derived.[6][8] Its fins were very unlike those of arthropods.[6]

Working from photographs, the Italian palaeontologist Alberto Simonetta believed he could classify Nectocaris within the chordates.[9] He focussed mainly on the tail and fin morphology, interpreting Conway Morris's 'gut' as a notochord – a distinctive chordate feature.[9]

The classification of Nectocaris was revisited in 2010, when Martin Smith and Jean-Bernard Caron described 91 additional specimens, many of them better preserved than the type. These allowed them to reinterpret Nectocaris as a primitive cephalopod, with two tentacles instead of the 8 or 10 of modern cephalopods. The structure previous researchers had identified as an oval carapace or shield behind the eyes[10] was suggested to be a soft funnel, similar to the ones used for propulsion by modern cephalopods. The interpretation would push back the origin of cephalopods by at least 30 million years, much closer to the first appearance of complex animals, in the Cambrian explosion, and implied that – against the widespread expectation – cephalopods evolved from non-mineralized ancestors.[2]

A later analysis claimed to undermine the cephalopod interpretation, stating that it did not square with the established theory of cephalopod evolution.[11] According to these authors, Nectocaris is best treated as a member incertae sedis of the arthropod group Dinocaridida (which includes the anomalocaridids), but they stopped short of formally changing the classification.[11] However, it is straightforward to demonstrate that an anomalocaridid affinity is not supported.[12][4] Other studies have yet to put forward a more plausible affinity,[5][4] and alternatives to the cephalopod affinity are fraught with difficulties.[3] Whether Nectocaris represents a derived or basal cephalopod, and whether it belongs in the stem or crown group, it is more parsimonious to interpret its distinctive features as homologues of their equivalent in cephalopods – even if the absence of a shell in Nectocaris is an independent feature.[3]

References

  1. 1 2 Smith, M. R. (2013). "Data from: Affinity, ecology and diversity of the early 'cephalopod' Nectocaris". Dryad Digital Repository. doi:10.5061/dryad.7m6kg.
  2. 1 2 3 4 Smith, M. R.; Caron, J. B. (2010). "Primitive soft-bodied cephalopods from the Cambrian". Nature 465 (7297): 469–472. Bibcode:2010Natur.465..469S. doi:10.1038/nature09068. PMID 20505727.
  3. 1 2 3 4 5 6 7 8 9 10 Smith, M. R. (2013). "Nectocaridid ecology, diversity and affinity: early origin of a cephalopod-like body plan". Paleobiology 39 (2): 291–321. doi:10.1666/12029.
  4. 1 2 3 Runnegar, B. (2011). "Once again: Is Nectocaris pteryx a stem-group cephalopod?". Lethaia 44 (4): 373. doi:10.1111/j.1502-3931.2011.00296.x.
  5. 1 2 Kröger, B. R.; Vinther, J.; Fuchs, D. (2011). "Cephalopod origin and evolution: A congruent picture emerging from fossils, development and molecules". BioEssays 33 (8): 602–613. doi:10.1002/bies.201100001. PMID 21681989.
  6. 1 2 3 Morris, S.C. (1976). " Nectocaris pteryx, a new organism from the Middle Cambrian Burgess Shale of British Columbia". Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 12: 703–713.
  7. ↑ Gould, S.J. (1989). Wonderful Life: The Burgess Shale and the Nature of History. Hutchison Radius. ISBN 0-09-174271-4.
  8. ↑ Waggoner, B. M. (1996). "Phylogenetic Hypotheses of the Relationships of Arthropods to Precambrian and Cambrian Problematic Fossil Taxa". Systematic Biology 45 (2): 190–222. doi:10.2307/2413615. JSTOR 2413615.
  9. 1 2 Simonetta, A.M. (1988). "Is Nectocaris pteryx a chordate?". Bollettino di Zoologia 55 (1–2): 63–68. doi:10.1080/11250008809386601.
  10. ↑ Morris, S. C. (1989). "Burgess Shale Faunas and the Cambrian Explosion". Science 246 (4928): 339–346. Bibcode:1989Sci...246..339C. doi:10.1126/science.246.4928.339. PMID 17747916.
  11. 1 2 Mazurek, D.; Zatoń, M. (2011). "Is Nectocaris pteryx a cephalopod?". Lethaia 44: 2–4. doi:10.1111/j.1502-3931.2010.00253.x.
  12. ↑ Smith, M. R.; Caron, J. -B. (2011). "Nectocaris and early cephalopod evolution: Reply to Mazurek & Zatoń". Lethaia 44 (4): 369–372. doi:10.1111/j.1502-3931.2011.00295.x.

Further reading

External links

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