Monday, 10 November 2014

An enigmatic animal from the Australian continental shelf, with possible similarities to some members of the Ediacaran Fauna.

The Ediacaran Fauna comprises a group of fossils from the Late Ediacaran Period, found at sites around the world and pre-dating the Cambrian Explosion, which is considered to indicate the origin of the majority of modern animal groups, and in particular those with mineralized skeletons. Some biologists have suggested that these organisms represent an entirely separate experiment in multicellular organization (referred to as Vendozoans), and that they are not ancestral to any modern animals (collectively referred to as Metazoans). Others see similarities between some members of the Ediacaran Fauna and extant animal groups, and suggest that they fossil assemblages contain members of the Phyla Porifera (Sponges), Cnidaria (Jellyfish, Corals, Sea Anemones etc.) and even groups such as Molluscs and Echinoderms. Under this second hypothesis the Ediacaran fossils represent true Metazoan Animals, although some of them probably belong to groups now extinct.

In a paper published in the journal PLoS One on 3 September 2014, Jean Just, Reinhardt Møbjerg Kristensen and Jørgen Olesen of the Section ofBiosystematics at the Natural History Museum of Denmark (Zoological Museum) at the University of Copenhagen, describe two enigmatic animals from the Australian continental shelf, which do not appear to belong to any known animal group, but which do show some resemblance to some members of the Ediacaran Fauna.

The specimens were collected by the Australian National Facility Research Vessel ORV Franklin in 1986, from two site on the Australian continental shelf at depths of 1000 and 400 metres, as parts of bulk samples containing benthic bathyal invertebrates with a sled which collected both sediments and organisms from the seafloor. Larger specimens were removed by hand, then the samples were passed through a number of sieves, before washing and storing in 80% ethanol for later analysis. The specimens were taken to Canberra for examination, where they were inadvertently treated with absolute alcohol instead of 80% ethanol, resulting in some damage to the specimens (strong shrinkage and rendering the specimens glassy and brittle), and furthermore making them unsuitable for genetic analysis. Once the uniqueness of the specimens was realised, the collection sites were revisited in the hope of collecting more, but both locations were found to be bleached (denuded of epibenthic invertebrates)

The specimens are considered to belong to a new genus, named Dendrogramma in reference to its branching gastric system, and this is placed within a new family, named Dendrogrammatidae. It is considered highly likely that these animals belong to a completely new Phylum, but due to the damage caused to the specimens, the lack of genetic data and the inability to locate any further specimens, Just et al. refrain from establishing any higher taxonomy.

The first new species is named Dendrogramma enigmatica, in reference to its enigmatic nature. The species is described from 13 specimens collected from a depth of 1000 m off Point Hicks and 1 specimen collected from a depth of 400 m off the Freycinet Peninsula on Tasmania. These are roughly mushroom-shaped, with a disk-shaped body and a stalk. The mouth is at the end of the stalk, connected to the body by a gastrovascular canal or pharynx within the stem, then branches numerous times within the body disk. The disk has a distinct notch and the mouth has two lobes. The largest specimen is 11 mm in diameter and has a 7.8 mm stem.

Dendrogramma enigmatica. (A, B) Lateral views; (C) aboral view, (D) adoral view. Photographs taken after shrinkage. Just et al. (2014).

The second species is named Dendrogramma discoides, in reference to the shape of its body-disk, which lacks the notch seen in Dendrogramma enigmatica. This species is described from four specimens collected from a depth of 1000 m off Point Hicks. The largest is about 17 mm in diameter, with a 4.5 mm stalk. This species has three lobes to the mouth rather than two.

Dendrogramma discoides, (A) aboral view. (B) Enlargement of (A) showing gastrovascular canal (stippled) of stalk (pharynx) and point of connection to the first branching node of gastrovascular system of the disc. (C) Oblique oral view of trilobed mouth-field with mouth opening in centre; entire pharyngeal part of the gastrovascular system is shown. Just et al. (2014).

The new animals have a simple diploblasic bodyplan, with two dermal layers, the epidermis (outer skin) and the gastrodermis (lining of the gastrovascular system) separated by a gelatinous mesoglea (tissue thought to have a structural role, helping the animal keep its shape, but no other function). This is seen in two other groups of living animals, the Cnidarians (Jellyfish etc.) and the Ctenophores (Comb Jellies). However they have no other anatomical features seen in either of these groups.

Traditional classifications of modern animals have always regarded the Porifera (Sponges) as the sister group of all other animals, due to their lack of a permanent body structure or tissue (a Sponge squeezed through a sieve and broken into its component cells can simply reassemble itself) and resemblance to colonies of the single-celled Choanoflagellates. The Cnidaria and Ctenaphora, having similar levels of organisation are considered to be ancient branches on the animal family tree, predating the origin of the Bilateria (the group including all familiar animals other than Cnidarians, Sponges and Comb Jellies). However recent genetic studies have suggested that the Ctenaphores might be the most ancient branch on the animal family tree, diverging before the Sponges and reaching their current level of organization through parallel evolution rather than a close relationship with the Cnidarians.

This clearly has implications for the phylogenetic position of Dendrogramma. If the Cnidaria and Ctenaphora are closely related then Dendrogrammai s likely to be closely related to both, but if they are only distant cousins, then the position of Dendrogramma on the animal family tree is harder to determine.

Possible positions of Dendrogramma in a simplified phylogeny showing the deepest splits in the metazoan Tree of Life. The position of Ctenophora is controversial so two possibilities have been shown with dashed lines, one as sister group to the remaining metazoans (the ‘Ctenophora-first’ hypothesis), and one as sister group to Cnidaria (Coelenterata hypothesis). Just et al. suggest that Dendrogramma most likely is related to Ctenophora and/Cnidaria (red arrows) due to general similarities in body organisation. However, depending on the position of Ctenophora and on whether certain aspects of Dendrogramma (e.g., mesoglea and gastrovascular system) are ancestral for Metazoa or modified, Dendrogramma can be positioned in a variety of ways below Bilateria (yellow oval). Just et al. (2014).

Dendrogramma also shows a resemblance to some fossils from the Ediacaran fauna, notably Albumares brunsae, Anfesta stankovskii, and Rugoconites. These fossils share a similar morphology to Dendrogramma, with a disc-shaped body containing a system of branching canals similar to the gastrovascular system of Dendrogramma, although all are considerably larger. In addition Albumaresbrunsae and Anfestastankovskii have tri-lobed mouths, similar to those seen in Dendrogramma discoides. The branching system of Rugoconites tenuirugosus appears initially branch three times, while that of Dendrogramma always branches twice (giving its branches a ‘Y’ shape), but Just et al. estimate that if Dendrogramma was also preserved in the same position, then it would also appear to have an initial three-way branch, suggesting that this could also be an artefact in Rugoconites tenuirugosus.

(1)Albumares brunsae, (2) Anfesta stankovskii, (3) Rugoconites enigmaticus. Just et al. (2014).

This means that if the phylogenetic position of Dendrogramma can be resolved, then this can be used to infer that of Albumares, Anfesta, and Rugoconiteste, providing further insight into the relationships between Ediacaran and modern faunas, and helping to resolve the Vendazoan hypothesis (since if Dendrogramma can be shown to be related to a modern animal group, then presumably the Ediacaran fossils that resemble it also do so).

See also…

The fossils of the Ediacaran Period record the first widespread macrofossils in the rock-record. Many of these fossils do not appear to belong to any modern group, but instead are thought to belong to an extinct taxa (sometimes known as ‘Vendobionts’), which may-or-may-not be related to modern Animals, though some fossils have been linked to Sponges (a group which...

Most modern animal groups appear abruptly at, or very shortly after, the beginning of the Cambrian period, 542 million years ago. The Cambrian starts abruptly with a layer of small shelly fossils that are hard to assign to any group, which are then replaced abruptly by fossils belonging to more familiar groups; arthropods, molluscs, brachiopods, etc., which then persist...

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