Sunday 21 August 2016

Saccoglossus testa: A new species of Acorn Worm from the Late Carboniferous Francis Creek Shale of northern Illinois.

The Deuterostomes are the group of animals which include the Chordates (the wider group that includes the Vertebrates) and Echinoderms, and therefore are of great interest to palaeontologists and evolutionary biologists studying the origins of these groups. Unfortunately all living (and fossil) Chordates and Echinoderms are thought to be highly modified compared to the last common ancestor of the group, leading scientists to theorize that the earliest members of the group probably resembled most closely the third Chordate group, the Hemichordates. The Hemichordates are also highly derived, and comprise two major groups, the colonial, tube-dwelling Pterobranches and the burrowing Enteropneusts or Acorn Worms. Acorn Worms today are almost entirely infaunal (living within marine sediments) a lifestyle thought to have been impossible at the time when the earliest Deuterostomes lived (due to lower oxygen levels in the oceans), but a surface-dwelling Acorn Worm-like animal is a plausible model for the earliest Deuterostomes. The one group of Acorn Worms that live on the surface today are members of the genus Saccoglossus, which use a highly elongated, mucus-secreting proboscis to scrape nutrients from the sediment surface. Unlike most Acorn Worms, which, being soft bodied, are extremely vulnerable to predation if exposed on the surface, Saccoglossus spp. produce a battery of toxic substances to discourage predation, and advertise this with bright warning colours. However Saccoglossus spp. are not thought to be particularly similar to the earliest Acorn Worms, as they have no fossil record, and are therefore thought likely to be a fairly modern group.

In a paper published in the journal Palaeontology on 29 March 2016, Christopher Cameron of the Département de sciences biologiques at the Université de Montréal, reports the discovery of a fossil Acorn Worm assigned to the genus Saccoglossus from the Late Carboniferous Francis Creek Shale of northern Illinois, which is considered to be part of the Mazon Creek Fauna fossil lagerstätte.

The fossil is described as a new species assigned to the genus Saccoglossus and given the specific name testa in honour of Thomas Testa, an assiduous collector of fossils from the Mazon Creek beds who collected of over 7000 such fossils, including the specimen from which the species Saccoglossus testa is described, that are now found in the collection of the Field Museum in Chicago. 

The specimen is preserved as part and counterpart on a split limestone concretion, and closely resembles modern members of the genus, with an elongate proboscis far longer than it is wide (something not seen in other Acorn Worms), and indeed cannot readily be distinguished from modern members of the genus (most of which could not be told apart from a flattened, colourless impression). It is placed in a new species not because it is distinct from modern species, but because of its age; the Mazon Creek beds were laid down about 300 million years ago, far longer than a single species is expected to endure.

Saccoglossus testa, counterparts, frontal fragments of specimen. Abbreviations: C, collar; P, proboscis; T, trunk. Scale bars represent 1 cm. Cameron (2016).

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 includes Chordates (including Vertebrates) and Echinoderms. As such...

The Vetulicolians are are group of organisms known from Cambrian deposits at a number of sites around the world. They Have segmented bodies superficially resembling those of Arthropods, but lack any visible limbs. Since palaeontologists and biologist theorize that the earliest Arthropods lacked limbs, having...

A possible bilaterally symmetrical Echinoderm from Spain.

Echinoderms (Crinoids, Sea Urchins, Starfish etc.) are placed within a larger group of animals called the Deuterostomes by taxonomists. These are animals sharing a common form of embryonic development, whose affinity has now been confirmed by genetic studies. The other groups of Deuterostomes are...

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