This is Mazon Monday post #314. What’s your favorite Mazon Creek fossil? Tell us at email:esconi.info@gmail.com.
Smithixerxes juliarum is among the rarest animals found in the Mazon Creek fossil deposit. It belongs to an extinct group of arthropods known as the euthycarcinoids, which ranged from the Cambrian to the Triassic periods. Some researchers believe euthycarcinoids may have been amphibious, capable of venturing between water and land.
A few species of euthycarcinoids are known from Mazon Creek, including:
Pieckoxerxes pieckoae (sometimes called Sottyxerxes pieckoae)
Smithixerxes juliarum
These fossils are exceptionally rare within the Mazon Creek biota. In fact, in the comprehensive 1985 study titled “Biota of a Pennsylvanian muddy coast: habitats within the Mazonian Delta Complex, northeast Illinois”, only two specimens of Kottixerxes were found among 229,979 concretions collected from Pit 11, which is a testament to just how scarce euthycarcinoids are in this remarkable deposit.
Smithixerxes juliarum was described by Frederick Schram and W. D. Ian Rolfe in the paper “New Euthycarcinoid Arthropods from the Upper Pennsylvanian of France and Illinois”, which was published in the Journal of Paleontology in 1982. S. juliarum was named for the specimen collected by Karlene Helmus Ramsdell for her then one-day-old niece Julia Smith; and for Julia Rayer Rolfe, spouse of the junior author of the paper.
Additional material.-PE 16701, 27878, 27879, 27880, 31064. Collection of Mr. and Mrs. A. Hay, one specimen. Collection of L. Rockwell, 5125, 5701. Collection of Mr. and Mrs. Ted Piecko, HTP4056. Collection of M. Helmus Smith, 3701, 3702, 3703, Julia no. 1. Etymology. Named for the specimen collected by Karlene Helmus Ramsdell for her then one-day-old niece Julia Smith; and for Julia Rayer Rolfe, spouse of the junior author.
Description. The animal is moderate in size (Table 2). The cephalon is suboval in outline (Pl. 4, fig. 2) although little detail of it is preserved in the specimens at hand.
The preabdomen length: width ratio is about 1.6:1 (Table 2). Only PE 27878 poorly pre-serves what may be a trace of the monosomite posterior to tergite 1. The five large preabdom-inal tergites have a pair of longitudinal carinae lateral to the midline (Pl. 4, figs. 1, 3 5), and the posterolateral corners of these tergites are acutely rounded (Pl. 4, fig. 1). There appear to be 10 or 11 preabdominal sternites in that region posterior to the first tergite (Helmus Smith, 3703); but essentially only one speci-men preserves this feature, and unfortunately not very clearly.
The first postabdominal segment is very short (Pl. 4, figs. 1, 2, 5) half as long as each of the more distal segments. The third, fourth, and fifth postabdominal somites are progres-sively narrower and longer (Pl. 4, fig. 2) and bear posterior-lateral projections (Pl. 4, fig. 3) of the tergites. These postabdominal segments also bear a lateral pair of faint carinae. The telson is very narrow and elongate (Pl. 4, fig. 4) and is more than half but less than the total length of the preabdominal somites.
There is a short entry about Smithixerxes in the “Richardson’s Guide to the Fossil Fauna of Mazon Creek” on page 211 and 212.
Smithixerxes juliarum Schram and Rolfe, 1982 Figures 15B.3, 15B.4
Preabdomen with four multisomite tergites, each with longitudinal crests; 10 or 11 ventral sternites, first postabdominal segment short, distal segments long and narrow and each with posteriorly directed paratergal prolongations; telson approximately equal to postabdomen in length.
The main difference between this species and Kottixerxes is the fewer number of preabdominal tergites, which would seem to imply that this species has larger numbers of triplosomites rather than diplosomites that have been noted in other Euthycarcinidae. Of all the Mazon Creek euthycarcinoids, Smithixerxes is more common than Pieckoxerxes pieckoae but less common than Kottixerxes gloriosus.
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