''Skeemella'' is a genus of elongate animal from the

Middle Cambrian Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek (di ...

Wheeler Shale The Wheeler Shale (named by Charles Walcott) is a Cambrian ( 507 Ma) fossil locality world-famous for prolific agnostid and ''Elrathia kingii'' trilobite remains (even though many areas are barren of fossils) and represents a Konzent ...

and Marjum

lagerstätte A Fossil-Lagerstätte (, from ''Lager'' 'storage, lair' '' Stätte'' 'place'; plural ''Lagerstätten'') is a sedimentary deposit that preserves an exceptionally high amount of palaeontological information. ''Konzentrat-Lagerstätten'' preserv ...

Utah Utah is a landlocked state in the Mountain states, Mountain West subregion of the Western United States. It is one of the Four Corners states, sharing a border with Arizona, Colorado, and New Mexico. It also borders Wyoming to the northea ...

. It has been classified with the banffozoan vetulicolians.

Description

''Skeemella'', which was first described in 2005, is diagnosed as having a body in two sections, covered in

cuticle A cuticle (), or cuticula, is any of a variety of tough but flexible, non-mineral outer coverings of an organism, or parts of an organism, that provide protection. Various types of "cuticle" are non- homologous, differing in their origin, structu ...

. The anterior section is short and wide, has a straight dorsal margin and a curving ventral margin, and is divided longitudinally in a way that makes it resemble a head shield. The anterior region is interpreted as being made of nine segments separated by thinner membranes (rather than as a single unit with multiple openings). ''Skeemella'' has a narrow, worm-shaped rear section with 43 segments in holotype specimen, identified as tergites separated by flexible membranes. The rear section terminates in what appears to be an arthropod telson, an elongate, unsegmented flattened structure that ends in two backward-pointing spines. In 2020, two more specimens of ''S. clavula'' were found in

Drumian The Drumian is a stage of the Miaolingian Series of the Cambrian. It succeeds the Wuliuan and precedes the Guzhangian. The base is defined as the first appearance of the trilobite '' Ptychagnostus atavus'' around million years ago. The top is de ...

sediments of 'Middle' Marjum Formation, '' Ptychagnostus punctuosus'' Biozone. They are less well preserved than the holotype but one of them apparently has the outer part of the cuticle preserved, whereas the holotype has the inner part. The Marjum specimen has individual posterior body segments that are shorter and wider than those of the holotype. This most likely illustrates individual rather than interspecies differences.

Taxonomy

''Skeemella'' shows typical vetulicolian features, such as a body divided into two distinct parts: a wider torpedo-shaped front end and a segmented rear section interpreted as the muscular driver for an active swimming lifestyle. Vetulicolians were originally described as relatives of arthropods, but their classification is debated; the discovery of new genera with a row of front-section openings interpreted as

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

slits has shifted their interpretation to stem-group deuterostomes or chordates, or perhaps even crown-group chordates related to tunicates. Newer reconstructions of vetulicolians often resemble tunicate larvae or simple cephalochordates, with the front section as a pharynx used for breathing and ramjet style filter-feeding and the rear section as muscle blocks. However, ''Skeemella'' is an unlikely candidate for this interpretation; the rear section segments bear clear affinities to arthropods. Either ''Skeemella'' is not a vetulicolian, researchers do not yet have enough data to correctly interpret ''Skeemella,'' or vetulicolians are not deuterostomes. ''Skeemella'' is one of a number of unrelated "platypus problems"; early lagerstätten fossils that have seemed to mix features of chordates and arthropods, two animal clades considered very distant from each other. Additional evidence will need to be discovered to fully resolve the relationships.

References

{{Taxonbar, from=Q7534258 Vetulicolia Enigmatic prehistoric animal genera Cambrian animals of North America Wheeler Shale Fossil taxa described in 2005