| Orthrozanclus | |
|---|---|
 | |
| Reconstruction ofO. elongata by Zhaoet al. (2017)[2] | |
 | |
| Specimen ofO. elongata, from Zhao et al. (2017)[3] | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Superphylum: | Lophotrochozoa |
| Phylum: | Mollusca |
| Family: | †Halkieriidae |
| Genus: | †Orthrozanclus Conway Morris & Caron, 2007[1] |
| Species | |
Orthrozanclus (from Greekὄρθρος +ζᾰ́γκλον (órthros +zánklon), "dawn scythe") is agenus of sea creatures known from two species,O. reburrus from the Middle Cambrian (~505 million years ago)Burgess shale andO. elongata from Early Cambrian (~518 million years ago)Maotianshan Shales. Animals in this genus were one to two centimeters long, with spikes protruding from their armored bodies. The placement of this genus into a specific family is not universally accepted.[4]
Jean-Bernard Caron and Donald A. Jackson found a specimen in theBurgess Shale and in 2006 referred to it as "scleritomorph C" without a detailed description.[5] In 2007 Caron andSimon Conway Morris published a description and named the fossilOrthrozanclus reburrus. The genus name means "Dawn scythe" and derives from Greek, with the species name meaning "bristling hair" in Latin.[1]
The two known specimens ofO. elongata were discovered fromMaotianshan Shale in 2015 and 2016 and formally described the following year.[2]
Orthrozanclus reburrus was 6–10.3 millimetres (0.24–0.41 in) long including its long spines, and had a fairly slim, roughly oval body that tapered towards the rear, was distinctlyconvex on top but was surrounded by a flatter rim. The underside was soft and unarmored, but the upward-facing surfaces were armored by: a small shell, near the front end; three zones of armor plates called "sclerites", which fitted close to the body and one of which ran all the way round the animal; 16 to 20 long, upwards-curving spines on each side of the body. The sclerites and spines wereunmineralized, and had internal cavities that appear to have been circular incross-section. The shell was convex and shaped like a triangle with rounded corners. It had a bulge at the front, a raised rear edge and a ridge along the middle that flared out towards the rear. It also had finely spaced rings that indicate growth by addition of material round the edges, and coarser ridges which may indicate that the animal wasmetameric, in other words built out of repeated "modules". The function of the shell at the front is unknown.[1]
The animal clearly lived on the sea-floor, and is thought to have had a muscular foot rather like that of a snail.[1]
Orthrozanclus′ sclerites are very similar to those of itsBurgess Shale contemporaryWiwaxia.[5]Its shell is very similar to: one of the two Burgess Shale shell types labelledOikozetetes; the forward shell ofHalkieriids, most of which are dated to the EarlyCambrian; and those of other EarlyCambrian fossils such asOcruranus andEohalobia. These similarities suggest thatOrthrozanclus was an intermediate form betweenWiwaxia and the Halkieriids and that all three of thesetaxa formed amonophyleticclade, in other words a group that consists of a common ancestor andall of its descendants.[6] However this drawsOrthrozanclus into a complex debate that has gone on since 1990 about whetherWiwaxia is more closely related tomolluscs or topolychaete worms, and therefore about the entire "family tree" of theLophotrochozoa, a "super-phylum" that is thought to contain modern molluscs, annelids andbrachiopods as well as some extinct groups.[1] The main opponent of this view, Nicholas Butterfield, proposes thatWiwaxia is more closely related to annelids while Halkieriids are very close to molluscs.[4][7][8]
The authors regard Hypothesis 1 as better supported by the evidence, but not robust.[1] |
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Zhao et al. (2017) propose an alternative - that halkieriids (including orthrozanclus and halkieria) are unrelated toWiwaxia, but are instead related to the camenellantommotiids and thus belong in the brachiopod stem lineage.[2]
