''Beckwithia'' is an extinct genus of average to large size ( to ), soft bodied

artiopoda The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilo ...

arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

s that lived during the late Middle and Upper

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million ...

on the former paleocontinent Laurentia, in what are now Utah and Wisconsin. It has an inverted egg-shape, with a semi-circular head, 11 abdominal segments and a tail spine. It is a typical member of the group

Aglaspidida Aglaspidida is an extinct order of aquatic arthropods that were once regarded as primitive chelicerates. However, anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within the chelicerates, and that they lie instead w ...

Etymology

''Beckwithia '' was named after Frank Beckwith, editor and publisher of the Millard County Chronicle of Delta, Utah in the early to middle 1900s, who had a passion for trilobites.

Distribution

''Beckwithia'' has been collected from the Middle and Upper Cambrian of Utah (Weeks Formation,

House Range The House Range is a north-south trending mountain range in Millard County, of west-central Utah. The House Range was named in 1859 by James H. Simpson. It was named by Simpson because "...of its well-defined stratification and the resemblance ...

Millard County Millard County ( ) is a county in the U.S. state of Utah. As of the 2010 United States Census, the population was 12,503. Its county seat is Fillmore, and the largest city is Delta. History The Utah Territory legislature created the county o ...

) and from the Franconian (in North-America called Sunwaptan) of Wisconsin.

Taxonomic history

''Beckwithia'' was previously placed in a separate family, the Beckwithiidae, but it has now been established that the defining character (that the terminal segments appeared to be fused) is actually an artifact.

Description

The outline of the

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton (endoskeleton) in for example, a human. In usage, some of the ...

of ''Beckwithia'' is an inverted egg-shape with a stout sword-like tail spine or

telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on accou ...

. The dorsal shield covering the cephalothorax (or

prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...

) is semi-circular, with the margin curving smoothly from the front to an acute, but rather small genal spine at the outside and the back. The front and side margin is bordered by a flatted and undecorated zone of almost constant length/width. The area within that band is densely covered in pustules. Two distinctly raised eyes that are circular to oval are located at the outer rim of what could be compared to a trilobite

glabella The glabella, in humans, is the area of skin between the eyebrows and above the nose. The term also refers to the underlying bone that is slightly depressed, and joins the two brow ridges. It is a cephalometric landmark that is just superior to ...

, with a strongly effaced outline and lobes. The abdomen (or

opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects to a ...

) consists of 11 segments (or possibly sometimes 10 or 12) perpendicular to the midline, that terminate to the sides in small spines that angle to the back. Each segment carries a spine at midline too that is directed backward, a character that distinguishes ''Beckwithia'' from other Aglaspidid genera. Only one species, ''Beckwithia typa'', is currently recognized.

References

External links

Description of the Aglaspididae with emphasis on ''Beckwithia'' (in Italian) with many illustrations
{{Taxonbar, from=Q4878791 Aglaspidida Fossils of the United States Fossil taxa described in 1931