Tuesday 18 March 2014

The first Entoproct?

The Lophotrochozoa are a diverse group of Invertebrate animals indicated to have a common ancestry by genetic analysis. The group includes the Annelida, Mollusca, Bryozoa, Cycliophora, Brachiopoda, Entoprocta and Phoronida. Within this group several groups are united by the presence of a crown of tentacles (the Lophophore) surrounding the mouth, which continuously opens and shuts while feeding, snatching planktonic food items which are then consumed. This group, the Superphylum Lophophorata, comprises the Bryozoa, Cycliophora, Brachiopoda and Entoprocta. Of these to Phyla, the Brozoa and Entoprocta, form colonies made up of large numbers of connected individuals, called zooids (ecologically but not morphologically similar to Corals). While the two groups are superficially similar, they are considered to have been separate since early in their evolutionary history, due to the different arrangements of their digestive tracts, with the anus of the Brozoans located outside the ring of the lophophore, but that of the entoprocts located within.

The Lophophorate Phyla are believed to have differentiated early in the Cambrian (the shelled Brachiopods becoming a distinctive part of the marine fauna early in the fossil record) and the colonial Bryozoans appear by the early Ordovician. Like many soft bodied groups the Entoprocts lack an extensive fossil record, with the oldest known fossils to date coming from the Jurassic of the UK, so while their relationship to the other Lophoporate and Lophotrichozoan groups is supported by the genetic data, the origin of the group and nature of its earliest members remains obscure.

In a paper published in the journal Nature on 17 January 2013, a group of scientists led by Zhifei Zhang of the Early Life Institute at Northwest University and the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences describe previously described animal from the Early Cambrian Chengjiang Lagerst├Ątte of Yunnan Province as a possible early Entoproct.

The animal, Cotyledion tylodes, does not obviously resemble the modern Entoprocts, which are small, typically colonial animals (though some solitary forms are known); it has a cup shaped body with a long stalk by which it is attached to the substrate (typically the shell of another animal), and is covered in sclerites (shell elements), whereas Entoprocts are entirely lacking in mineralized tissue. It is also considerably larger than any known Entoproct, and had previously been considered to be a Carpoid Echinoderm (more closely related to Vertebrates than to Lophotrochozoans).

Sclerites on the calyx and stem of the putative Entoproct Cotyledion tylodes from the Cambrian Chengjiang Fauna of Yunnan, China. (a) Entire specimen, dashed boxes indicate positions of (b) and (d), note the seemingly increased arrays of sclerites; (b), details of (a) as indicated; note the elongate sclerites marked by an arrow; (d), Enlargement of (a) as indicated, showing the merged two sclerites indicated by two arrows. Zhang et al. (2013).

However like the modern Entoprocts Cotyledion tylodes has a U-shaped gut, with both the mouth and anus inside the ring of the lophophore, a trait which is considered to be unique to, and indicative of, the Entoprocts.

(c) A laterally compressed specimen of Cotyledion tylodes attached to the gena of a trilobite, showing U-shaped gut with 3-D buccal cavity and enlongate anal tube enclosed inside an apertural cavity; (d), interpretative drawing of the same. The hemispheric buccal cavity with basal mouth (anterior) and elongate anal papillae (posterior) marked with solid arrows and hollow arrows, respectively. Zhang et al. (2013).

While the presence of a shelly exoskeleton made up of numerous sclerites would not be predicted in a fossil relative of the modern Entoprocts by examination of the animals alone, this is a trait thought to have been present in early members of several other Lophotrochozoan groups, notably the Molluscs, Anelids, Brachiopods and Phoronids, so to find such a covering in an Entoproct is not a complete surprise. Cotyledion tylodes also has a larger body and more complex body-plan than any modern Entoproct, but again this is not completely unexpected. Evolution proceeds by simplification as often as by increased complexity, and many Early Cambrian fossils show unexpected features not seen in their modern relatives.

Reconstruction of Cotyledion tylodes in life position. Zhang et al. (2013).

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