Burgess
Shale-type faunas are fossil Lagerstätten which provide unique
insights into life in the Early-to-Middle Cambrian. The best known
examples of these are the highly productive Middle Cambrian Burgess
Shale deposits of British Colombia and the Early Cambrian Chengjiang
biota of South China, though other less productive sites are also
known, such as the Middle Cambrian Spence Shale of northern Utah,
which is slightly older than the Burgess Shale and has produced a
wide variety of Algae, Sponges, Brachiopods, Eldoniids (soft-bodies animals of uncertain affinities),
stem-Molluscs (animals of apparent Moluscan affinities, but which
lived or split off from the other Molluscs before the common ancestor
of all living groups), Cycloneuralians (the group that includes Kinorynches, Priapulids and Nematodes), Deuterostomes (the
group that includes Vertebrates and Echinoderms), Lobopodians (the
probable Cambrian ancestors of the Modern Velvet Worms), and a
variety of Arthropods, including Trilobites, Carapace-bearing
Arthropods (which may be related to the later Crustaceans), Megacheirans (an extinct group of probably predatory Arthropods), Xenopods (an extinct group of soft-bodied Arthropods) and
enigmatic forms such as Meristosoma paradoxum
and Utahcaris orion.
In
a paper published in the Geological Magazine on 31 August 2016, DavidLegg of the Oxford University Museum of Natural History and Steve
Pates of the Department of Zoology at the University of Oxford,
redescribe the original specimen of Utahcaris orion
plus a new specimen and discuss the relationship of this species to
other Arthropods, in particular the highly successful Chelicerates
(Arachnids, Horseshoe Crabs, their most recent common ancestor and
everything descended from it).
The
original specimen of Utahcaris orion was
discovered by Benjamin Dattilo in Antimony Canyon, and was described
by Simon Conway Morris and Richard Robinson in a paper published in TheUniversity of Kansas Paleontological Contributions in 1988. It is
preserved as a compression fossil in dorsal view, 87 mm in length.
This specimen has a large, subtriangular cephalic shield, plus eleven
trunk and two abdominal segments. Remarkably this specimen also shows
a section of preserved, phosphatized gut containing the fragmentary
remains of a number of Trilobites.
Utahcaris
orion, Antimony Canyon specimen.
(a) Part, photographed using polarized lighting, and (b) accompanying
camera lucida drawing. Gut content is coloured grey. Abbreviations:
P? – putative podomere; and T1–T11 – trunk tergites 1–11.
Legg & Pates (2016).
The new specimen was discovered in Miners Hollow by Robert and Nancy
Meyers, and is described for the first time, providing new insights
into the species and its biology. This specimen is preserved in
lateral (side) view, and shows a number of features not visible in
the original specimen, notable a compound eye beneath the cephalon
and several pairs of elongate flattened limbs.
Additional specimen of Utahcaris orion from Miners Canyon.
(a) Part, photographed using polarized lighting, and (b) accompanying
camera lucida drawing. Gut content and eyes are coloured grey.
Abbreviations: Ap – appendages; E – eye; and T1–T10 – trunk
tergites 1–10. Legg & Pates (2016).
The original study of Utahcaris orion suggested that it might
be related to Sanctacaris uncata, a fossil from the Burgess
Shale which has been suggested to be the earliest known Chelicerate,
as well as to the Leanchoiliid Megacheirans Actaeus,
Alalcomenaeus, and Leanchoilia. The new specimen of
Utahcaris orion reveals no further similarities to the
Leanchoiliid Megacheirans, indicating that the similarities between
Utahcaris
and this group were a product of the limited nature of the material
available rather than any true close biological relationship, however
the relationship to Sanctacaris
uncata is more
strongly supported, and Legg and Pates suggest that these two
species, along with Wisangocaris
barbarahardyae
(a species from the Ema Shale of South Australia to be described in a paper published on 13 May 2016 in the journal Palaeontology by James Jago, Diego
Garcia-Bellído
and James Gehling) be united as the Family Sanctacarididae, the earliest
known Chelicerate group, and that as the Spence Shale is slightly
older than the Burgess Shale, Utahcaris
orion is
therefore the oldest currently know Chelicerate.
The
mouthparts of Utahcaris
orion
are unknown, however the closely related Sanctacaris
uncata
is known to have robust gnathobases (chewing organs), and the
presence of fragmentary Trilobite remains in the gut of the first
specimen of Utahcaris
orion
suggests that it may have had similar structures. This, combined with
the presence of limbs apparently modified for swimming, strongly
implies that the Chelicerates may have been active predatory animals
from the outset, a lifestyle still seen in the majority of living
species today.
See also...
Oesia disjuncta: Enigmatic Cambrian fossil re-interpreted as a tub-dwelling vermiform Hemichordate. The Hemichordates are Deuterostome Animals. members of the group which
also...
Scathascolex minor: A Palaeoscolecid Worm from the Middle Cambrian Burgess Shale of British Colombia. Palaeoscolecids are a group of vermiform animals (worms) known from
Cambrian to Silurian deposits. They had rings similar to...
Eokinorhynchus rarus: A Kinorhynch from the Early Cambrian of Sichuan Province, China. Kinorhynches are tiny (at most 3 mm) worm like
animals found in marine sediments, with segmented tube- or
barrel-shaped bodies, separate head and neck regions and evertable
pharynxes. They
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