Walliserops

Taxobox
name = "Walliserops"
fossil_range = Devonian


image_caption = "W. trifurcatus"
status=Fossil
regnum = Animalia
phylum = Arthropoda
classis = Trilobita
ordo = Phacopida
familia = Acastidae
genus = "Walliserops"
type_species = "W. trifurcatus"
type_species_authority = Morzadec, 2001
subdivision_ranks = Species
subdivision =
* "W. trifurcatus"
* "W. hammii"
* "W. tridens"
* + 3 undescribed species

"Walliserops" (named after Prof. O. Walliser of Göttingen) is a genus of spinose phacopid (acastid) trilobites found in Lower to Middle Devonian age rocks from Morocco. All species of "Walliserops" share, and are famous for, the spectacular three-pronged "trident" that rises from the glabella. "Walliserops" is most closely related to the genus "Comura" (Brett & Chatterton, 2001, Chatterton "et al"., 2006).

"Walliserops" was originally erected for a single species, "Walliserops trifurcatus" (Morzadec, 2001). Later, two other species were assigned: "Walliserops hammii" & "Walliserops tridens" (Chatterton "et al", 2006). All three currently described species come from the same strata near Foum Zguid in southern Morocco, three as yet un-described species are recorded from other locations (Chatterton "et al.",2006).

Early reports of "trident" trilobites (Brett & Chatterton, 2001) and placement within the proposed new genus "Parabolops" ("Parabola Face") - long tridents being placed within "P. neptunis", short tridents placed within "P. hammi." - were pre-empted by the publication of the detailed analysis of "Walliserops" (Morzadec, 2001).

Departures from bilateral symmetry are an unusual feature within "Walliserops" species, most clearly shown by the curved occipital spine of "W. hammii" taking a noticeable curl to one side. The regular development of these features in multiple specimens suggest a genetically controlled feature of the genus and not mutations or pathology. Most of the exceptions to bilateral symmetry noted (and also the absence of spines on the first two thoracic segments) can be explained by adaptations allowing the trident to be held off the sea floor while walking. Between the species there are variations in the extent of departure from bilateral symmetry: "W. trifurcatus", with a long trident that is curved away from the seabed, has less obvious departures from bilateral symmetry than "W. hammii", with a short trident close to the seabed (Chatterton "et al.", 2006).

The function of the trident itself is poorly understood. With the amount of energy and nutrients expended in growing such a large adornment (probably multiple times as the trilobite shed its skin) its function was clearly important (Chatterton "et al.", 2006). Although a number of suggestions have been made (e.g. sensory apparatus, disguise or protection), the most satisfactory current explanation is that the trident served as "horns" similar to present day beetles (Knell & Fortey, 2005).

Sexual dimorphism was an intriguing prospect (longer trident forms as jousting males) when only two species (or possible dimorphs) were known (Brett & Chatterton, 2001). With the description of three species from the same location, polymorphism (e.g. caste system in ants and bees) was another prospect but seemed unlikely (Chatterton "et al.", 2006). Although the presence of horns strongly suggests sexual dimorphism (Knell & Fortey, 2005), lack of data on numerous fronts currently prevents firm conclusions from being drawn (Chatterton "et al.", 2006).

Gallery

External links

*Natural History Museum (London) virtual tour of "W. hammii" [http://www.nhm.ac.uk/nature-online/virtual-wonders/vrtrilo3.html]
*Example of a "W. trifurcatus" from the University of Alberta [http://associations.eas.ualberta.ca/palaeo/]
* "Earliest combatants in sexual contests revealed" from "New Scientist" magazine [http://www.newscientist.com/channel/life/mg18625015.100/] .
*Fossil Mall entry about "W. trifurcatus" [http://www.fossilmall.com/EDCOPE_Enterprises/trilobites/MTrilo5/MTrilo5.htm]

References

*Brett, K., and Chatterton, N. 2001. "Parabolops", a new asteropygine trilobite from southern Morocco with an unusual trident-like anterior cephalic frontal process (Abstract). "3rd International Conference on Trilobites and their Relatives". University of Oxford (UK).
*Chatterton, B., Fortey, R., Brett, K., Gibb, S., and McKellar, R. 2006. Trilobites form the Lower to Middle Devonian Timrhanrhart Formation, Jbel Gara el Zquilma, southern Morocco. "Palaeontographica Canadiana" 25. ISBN 1-897095-15-5
*Whittington, H.B., 1997. Mode of Life, Habits and Occurrence. In: Kaesler, R. L., ed. Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, revised. Volume 1: Introduction, Order Agnostida, Order Redlichiida. xxiv + 530 pp., 309 figs. The Geological Society of America, Inc. & The University of Kansas. Boulder, Colorado & Lawrence, Kansas. ISBN 0-8137-3115-1
* [http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1626209 Knell R.J. and Fortey R.A. 2005. Trilobite spines and beetle horns: sexual selection in the Palaeozoic?] "Biology Letters" 1, 196-199. Link courtesy of The Royal Society.
*Morzadec, P. 2001. Les Trilobites Asteropyginae du Dévonien de l'Anti Atlas (Maroc). "Palaeontographica Abt." A, 244:53-85