Monday, March 11, 2019

Vaderlimulus: Beast of the Week

Today we will be looking at an invertebrate who's lineage is ancient even compared to the dinosaurs.  Let's check out Vaderlimulus tricki!

Vaderlimulus was a prehistoric horseshoe crab that lived in what is now Idaho, USA, during the early Triassic Period, about 245 million years ago. (But I should note that horseshoe crabs in general go back over 400 million years!) The specimen on the fossil record measured about four inches long, and like horseshoe crabs today, was likely a scavenger, feeding on dead organic material that sinks to the bottom of whatever watery habitat it was living in.  The genus name translates to "Vader horsehoe crab" after Darth Vader from the Star Wars franchise.  It was named this way because the unique shape of the creature's shell resembles the sides of Darth Vader's famous helmet.

Vaderlimulus tricki life reconstruction by Christopher DiPiazza.

Vaderlimulus' claim to fame is its amazingly-shaped front portion of its shell, which was extremely wide and tapered into a backwards-facing point on each side.  It's uncertain why the animal evolved this way?  Horseshoe crabs today use their wide, rounded front shells to help stabilize themselves as they trek muddy or gravelly bottoms of the brackish and salt water habitats they live in.  The wide shells make them less likely to flip over if a strong current hits them or if they bump into a rock or something too hard.  Even if they do get flipped, they can arch their backs and use their stiff tails (which are not weapons by the way) to help them flip back over.  We know Vaderlimulus lived in what was at the time a partially freshwater environment, so maybe there was a more consistent or stronger current that would have required it to be more stabilized with the wider shellgear.

Like all horseshoe crabs, Vadelimulus had two compound eyes on the top of its shell, and likely had a series of tiny, barely noticeable eyes around the perimeter of the front of its shell.  Under that first body segment, it had ten legs, armed with small pinchers for picking up tiny morsels to eat.  In the center of those legs it would have had a small mouth.  The segment following the front part housed the shingle-like gills on the ventral side, followed by the stiff tail.  Horsehoe crab tails, like stated earlier, are primarily for stabilization, and are not dangerous at all.

Specimen of Vaderlimulus tricki currenly housed at the New Mexico Museum of Natural History and Science.

Like its modern relatives, Vaderlimulus, probably walked onto land to breed.  When modern horseshoe crabs do this today, the females lay their tiny green eggs in the sand and the males fertilize them.  Males often grab hold of the back end of the females with specialized front legs that look like boxing gloves with hooks, in an effort to fertilize the eggs as soon as they are released.  Because of this, females are larger than the males so they can be strong enough to carry their smaller partners as they move around in the surf.  It also helps to have a larger body to produce as many eggs as possible.

That is it for this week!  As always feel free to comment below or on our facebook page!


Allan J. Lerner et al. 2017. First fossil horseshoe crab (Xiphosurida) from the Triassic of North America. Neues Jahrbuch für Geologie und Paläontologie 286 (3): 289-302; doi: 10.1127/njgpa/2017/0702

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