This is Mazon Monday post #248. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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Paleolimulus sp.; modified after Shabica and Hay (1997). Scale bar 5 mm.
There are currently 3 recognized species of horseshoe crab known from the Mazon Creek deposit - Euproops danae, Paleolimulus sp., and Liomesaspis laevis. Euproops danae is the most commonly found species. Until 2022, Paleolimulus was largely undescribed. In 2022, Russel D. C. Bicknell, Serge V. Naugolmukh, and Scott C. McKenzie published the paper "On Paleolimulus from the Mazon Creek Konservat-Lagerstätte" in the journal Comptes Rendus Palevol, in which they describe Paleolimulus mazonensis from multiple specimens.
Abstract
Horseshoe crabs are extant chelicerates with a fossil record extending back to the Ordovician. Despite the documentation of their long evolutionary history, xiphosurids are rarely preserved within fossil assemblages due to their unmineralized cuticular exoskeleton. However, in exceptional circumstances, an abundance of fossil xiphosurid specimens have been documented. The Moscovian-aged Mazon Creek Konservat-Lagerstätte represents one such fossil deposit with a high abundance and diversity of xiphosurids. Although fairly well known, the Paleolimulus specimens from the Mazon Creek have not yet been subject to a thorough taxonomic examination. In the light of recent efforts to organise Paleolimulus, we revisit this undescribed material, erect Paleolimulus mazonensis n. sp., and present a phylogenetic analysis that places P. mazonensis n. sp. as a sister taxon to P. signata (Beecher, 1904). The palaeoecology and possible ontogeny of P. mazonensis n. sp. are presented, as well as a statement on the future directions for understanding this xiphosurid genus.
Jack Wittry discusses Paleolimulus sp. on page 39 of "The Mazon Creek Fossil Fauna".
Paleolimulus sp.
Found in the marine Essex Fauna, Paleolimulus is a small to moderate-sized animal distinguished from other Mazon Creek xiphosaurans by eyes that are more medially located. The prosoma is semicircular in outline, raised or arched, and the abdomen is triangular in shape with the segments fused to form a continuous shield. The abdominal axis is distinctly ringed, and in front of the long, sword-like telson is a moveable segment.
Mazon Creek area Paleolimulus has not been described at the species level and is in need of reexamination. The genus is much less common than Euproops danae, but shows the same sexual dimorphism, with females being much larger than males. Rare examples are known to reach 6 cm or more in width. It is known from locations in the United States and Europe into the Permian, and is a member of the family Paleolimulidae within the superfamily Limulacea-which includes modern horseshoe crabs.
Specimens
Holotype Field Museum FMNH PE 81596, Collected by Tom Testa
Fig. 2. — Holotype of Paleolimulus mazonensis n. sp.: A, FMNH PE 81596a, part; B, FMNH PE 81596b, counterpart, showing moveable spine notches (white arrows). Scale bars: 5 mm.
Specimens from Wittry's "The Mazon Creek Fossil Fauna"
Specimen from the collection of Andrew Young (appears in Wittry's book on page 39)
