Much like (slow) Water Striders (Aquarius remigis), (lackadaisical) Coelacanth (Latimeria chalumnae) and (the current winner on torpid evolution) Elephant Sharks (Callorhinchus milii), these fellows have a long history in the fossil record with very few anatomical changes. But their leisurely change provides loads of great information. It makes our new friend, Yunnanolimulus luoingensis, shown here an especially interesting and excellent reference point for how this group evolved. We can examine their genome today and make comparisons all the way back to the Middle Triassic (with this new find) and other specimens from further back in the Ordovician.
The evolution of their exoskeleton is well-documented by fossils, but appendage and soft-tissue preservation is extremely rare.
This new study analyzes details of appendage and soft-tissue preservation in Yunnanolimulus luopingensis, a Middle Triassic (ca. 244 million years old) horseshoe crab from Yunnan Province, SW China.
The remarkable preservation of anatomical details including the chelicerae, five pairs of walking appendages, opisthosomal (rear) appendages with book gills, muscles, and fine setae permits comparison with extant horseshoe crabs. The close anatomical similarity between the Middle Triassic horseshoe crabs and their recent analogues documents anatomical conservatism for over 240 million years, suggesting persistence of lifestyle.
The occurrence of Carcinoscorpius-type claspers on the first and second walking legs in male individuals of Y. luopingensis indicates that simple chelate claspers in males are plesiomorphic for horseshoe crabs, and the bulbous claspers in Tachypleus and Limulus are derived.
As an aside, if you hadn't seen an elephant shark before and were shown a photo, you'd likely say, "that's no freaking shark." You'd be wrong, of course, but it would be a very clever observation. Callorhinchus milii look nothing like our Great White friends and are not true sharks at all. Rather, they are ghostsharks belonging to the subclass Holocephali (chimaera), a group known as ratfish. They diverged from the shark lineage about 400 million years ago.
If you have a moment, do a search for "Callorhinchus milii." The odd looking fellow with the ironic name, "kallos" means beautiful in Greek, sports black blotches on a pale silver elongate body. And their special feature? It's the fishy equivalent of "business in the front, party in the back," with a dangling trunk-like projection at the tip of their snout and well-developed rectal glands near the tail.
Photo: CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=719594
Ref: Hu, Shixue and Zhang, Qiyue and Feldmann, Rodney&Benton, Michael&Schweitzer, Carrie&Huang, Jinyuan&Wen, Wen & Zhou, Changyong & Xie, Tao & Lü, Tao & Hong, Shuigen. (2017). Exceptional appendage and soft-tissue preservation in a Middle Triassic horseshoe crab from SW China. Scientific Reports. 7. 10.1038/s41598-017-13319-x.