This is Mazon Monday post #73. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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For this week, we are going to cover the same species of spider we featured in our recent Fossil Friday #69, Architarbus rotundatus. Paleozoic spider-like arachnids are an interesting group. Most lack the ability to produce silk and so are not classified as true spiders. Mazon Creek features three orders of spider-like animals. They are the Phalangiotarbida, the Trigonotarbida, and the Areneae. A. rotundaats belongs to order Phalangiotarbida.
Architarbus rotundatus was described in 1868 by Samuel Hubbard Scudder. In his lifetime (1837 - 1911), Scudder was a leading figure in entomology and the authority on butterflies (Lepidoptera) and grasshoppers (Orthoptera). For his work on insect paleontology, he is considered the father of insect paleontology in the US. Some of his most notable paleontological works include:
- The Fossil Insects of North America (two volumes, 1890)
- Index to the Known Fossil Insects of the World (1891)
- Adephagous and Clavicorn Coleoptera from the Tertiary Deposits at Florissant, Colorado (1900)
The description of A. rotundatus appears in "Supplement to descriptions of articulates. Descriptions of fossil insects found on Mazon Creek, and near Morris, Grundy Co. Ill", from Geological Survey of Illinois 3:566-572.
George Langford had a page on A. rotundatus. Including a very nice drawing of its body.
A. rotundatus appears in both the "ESCONI Keys to Mazon Creek Animal" and "The Mazon Creek Fossil Fauna" by Jack Wittry. The description in Wittry does a great job describing its fossil form.
Architarbus rotundatus Scudder, 1868
On its dorsal side, Architarbus rotundatus has a rhomboidal carapace (head shield) that is strongly curved down the center of the back. There are ten tergites, (dorsal abdominal plates) the first three of which are partially covered by the head shield and bend posteriorly. The carapace is approximately half of the overall body length. The last tergite comprises one-half of the length of the abdomen.
On its ventral side, there are ten sternites (ventral abdominal plates), with the posterior five occupying over two-thirds of the abdominal length. There are marginal fields on both sides. The abdomen has a decidedly overall triangular shape with the last sternite ending in a gradual arc, being only one-third the width of the middle sternites. The abdomen begins its inward taper at the sixth sternite.
Chapter 11 of "The Richardson's Guide to the Fossil Fauna of Mazon Creek" features Arachnids and Order Phalangiotarbida and A. rotundatus figure prominently. The description is very technical. There is a helpful diagram of spider body parts.
Order Phalangiotarbida
A phalangiotarbid was one of the first arachnids to be described from the Mazon Creek region (Scudder, 1868). At least 150 specimens have been collected, making the Phalangiotarbida the most abundant (61 percent) of Mazon Creek arachnids that can be identified to order.
Morphology. Prosoma semicircular (primitive) to rhomboidal (derived), with pointed anterior projection; eye tubercle trifoliate with three pairs of ocelli along its flanks. Ornamentation consists of the fine pustules (interpreted as setal bases). Ventrally, the basal segment (coxa) of appendages III to VI meet at a sternum composed of up to four sclerites, some of which are medially divided.
Chelicerae unknown. Pedipalps incompletely preserved, but are rather short, antenniform ap pendages with at least four segments; coxae unknown. Third pair of prosomal appendages raptorial (used for grabbing, based on kinematic analyses of limb structure; Beall, 1984, 1985); corresponding coxae abut medially, with medial margins sometimes slightly spinose. Last three pairs pedal, with leg length increasing posteriorly.
Dorsally, abdomen with six anteroposteriorly narrow, slightly overlapping, medially divided tergites, followed by a large posterior tagma that may bear relics of fusion of tergites; all surfaces finely pustulated. Posterior tagma bears a round anal operculum near posterior margin (Kjellesvig-Waering, 1978, in preparation). Ventral surface with nine sternites connected to tergites by pleated, unsclerotized integument. First three sternites fit between fourth coxae. Fourth sternite as wide as abdomen (opisthosoma), divided into three transverse sections. Fourth through ninth sternites each bearing a pair of transversely arranged, shallow depressions that represent the external expression of entapodemes for dorsoventral musculature.
One or two pairs of spiracles present. One pair of relatively large spiracles always situated along the anterior margin of the median sclerite of the fourth sternite. Another pair of smaller openings sometimes present at lateral edges of anterior margin of second sternite.
Taxonomy. Kjellesvig-Waering (in prepara tion) assigned all phalangiotarbids to a single family, Phalangiotarbidae, with characteristics of the order. He recognized the following seven genera among the Mazon Creek phalangiotarbids.
Specimens
From George Langford, Sr.
From Wittry "The Mazon Creek Fossil Fauna"
From Phil Anderson - Fossil Friday #69.
