Megarachne servinei is quite a strange critter, even for an Eurypterid. It possesses unusual structures, such as an enlarged and round second tergite, a shovel shaped cephalothorax and blades on its appendages. The function of many of these structures remains unknown up until this day, but I have several hypotheses that might possibly explain what purpose they served and how they might have looked like in life.
The enlarged and round second tergite is perhaps the most peculiar structure of Mycteroptids. The shape and texture caused Hu ̈nicken 1980 to believe that Megarachne was a giant mygalomorph spider. Selden et al. 2005 wasn’t able to figure out what function it could have, stating that “the function of the large, circular second opisthosomal tergite remains a mystery.”
The holotype of Megarachne servinei. The second tergite was mistaken for the opisthosoma of a spider.
However, one thing I noticed at looking at the holotype of Woodwardopterus scabrosus, a close relative of Megarachne (which might actually be a junior synonym of Mycterops), is that the second tergite overlaps the other tergites, creating a small space between them. After some speculation I concluded that this hollow space could be used for storing the eggs or even their newborn young. Many chelicerates (and even crustaceans) use this strategy, and eurypterids might be no exception. If true, this hypothesis might suggest increasing parental care in Mycteroptids.
A vinegaroon carrying her eggs beneath her opisthosoma. The opisthosoma is slightly flexed in order to get a better grip on the egg cluster.
A red king crab, perhaps a better example, carrying her eggs under the abdomen.
In order for this strategy to work, the eggs of Megarachne must be of small size. Upscaled “normal” chelicerate eggs would be too big to fit in the narrow space. This is likely however as Eurypterids, being mostly aquatic creatures, would likely reproduce by spawning. This would require small sized eggs in a large amount.
Selden et al. 2005 also noted blade-like shapes on both the holotype of Woodwardopterus and the second specimen of Megarachne. These shapes are unique to Mycteroptiods and are found in both Mycteroptids and Hibbertopterids. It is hypothesised that these blades would help Megarachne sweep feed, raking the sediments.
Selden et al. 2005 gave us a hypothetical reconstruction of Megarachne servinei. However, there is something quite strange about it. We can see that the reconstruction has speculative brush-like tarsa. I am almost completely sure where the inspiration for these odd legs came from: the pusher legs of horseshoe crabs.
The pusher leg of a horseshoe crab.
Now you might say that this isn’t a big deal. After all, Mycteroptids and horseshoe crabs have quite similar lifestyles. The problem in Selden et al. 2005’s reconstruction however, is that the pushers are also present on the legs Megarachne used for feeding, and would’ve only barely used them for walking.
The reconstruction of Megarachne servinei from Selden et al. 2005.
Therefore, I think it would be more likely if the tarsa on appendages III were more adapted to assist Megarachne in sweep feeding rather than walking. So, I thought it would be more plausible if the blades would extend all the way to the end of appendage III, creating a big blade going all the way from the tibia to the claw.
The claws of a spiny lobster. You can see that it also has blades on its appendages. Even though the function of this blade is different, I took my inspiration from this.
Lastly, we have the odd anteromedian carapace protrusion. It is noticeably broader at the end than at the base, which is quite interesting. Taking this in mind, I think it is quite obvious what the function of this protrusion is. Knowing that Megarachne was a sweep feeder, the protrusion would have likely been used as a shovel to further rake up the sediment, maximizing the surface it can feed on.
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