Biologyzing a Preserved Horseshoe Crab (Xiphosura)

Some people like to use their money for drugs, gambling, or whatever illicit vice. I used my money to buy a preserved horseshoe crab specimen from a company that specializes in providing teaching materials for biology lab courses.
I am absolutely intrigued by these types of organisms; not just because they are living fossils with a stubborn phenotype that dates back 450 million years, but also because they share synapomorphies with familiar modern arthropods today. That's why I made this video: to show you some of those synapomorphies.

Hopefully you enjoy!

[JohnnnyJohn]

Growing up in Florida, we used to find baby Horseshoe Crabs all the time. We would let them crawl around on our hands and they would lightly pinch us.

[collicou]

Huh, I did not know that simple eyes evolved from compound eyes, I had also assumed the opposite. Very cool!
We all aspire to be living fossils at the end of the day 😅

[johnproctor]

Not going to lie. When you were ripping the packaging off with your hands and nothing else, I was horrified😂

[titanomachy]

I find it interesting that its species name is Polyphemus. I guess it was named that because of the ocelli in the middle of its carapace, reminiscent of the single eye in the middle of the brow of the cyclops in the Odyssey, who is often depicted with eye sockets as well as a large eye on his forehead, kind of similar to the arrangement of two smaller ocelli and one larger ocellus in the middle. I also noticed how similar book gills of the Xiphosura look to the pleopods of crustaceans. I wonder if it is actually a homologous structure, I mean look at the book gills/lungs of isopods, the terrestrial woodlice have a similar respiratory strategy as the arachnids except their book gills are exposed. On the underside, isopods look like small horseshoe crabs with twelve or fourteen (though some species have less, in fact some even have no pleopods) walking limbs rather than ten. It is pretty understandable that the horseshoe crab was considered a crab, not just because of the way it looks but also because it has ten legs like a decapod, followed by what look fairly like swimmerets. They serve the same purpose in both groups: respiration and propulsion through water or kicking up sand. The main difference is that pleopods are biramous limbs (in most crustaceans) while book gills are not biramous appendages. This causes me to wonder: is the uniramous walking limbs followed by uniramous swimming and respiration limbs general body plan the ancestral configuration of the euarthropod stem group that would be evolutionary predecessors to both chelicerates and mandibulates? Or did it have biramous pleopods? I know the earliest panarthropods had uniramous limbs, but it seems like biramous limbs became kind of the standard later in the Cambrian. Like, were the ancestors of both xiphosurans and pancrustaceans (and arachnids and sea spiders and eurypterids) arthropods with a body plan similar to this? Or did biramous limbs come first for the whole clade that would contain the mandibulates and chelicerates but not more primitive lobopodians? It seems like uniramous limbs came first for arthropods in general, then some group of euarthropods evolved biramous limbs, then later horseshoe crabs (and eurypterids, pantopods, and arachnids) lost their biramous limbs and they developed a body plan of uniramous walking limbs followed by uniramous gills (unlike the crustaceans which have biramous pleopods), therefore exhibiting an atavism since it is thought that uniramous limbs were the ancestral form of arthropods limbs, as seen in lobopodian worms.

I just looked it up and there is a paper on a Proterozoic xiphosuran species they found fairly recently that exhibits biramous appendages, so it seems like yeah, arthropods started with uniramous limbs, then some euarthropods that are ancestral to both mandibulates and chelicerates evolved biramous limbs, then later some groups like all living chelicerates lost their biramous appendages. This is also interesting in that it indicates that there were at least two seperate genetic alterations that led to two different types of chelicerates losing biramous limbs, since the earliest horseshoe crabs had biramous appendages and arachnids and pantopods have uniramous appendages and whenever I see a euryperid fossil it has uniramous limbs. And speaking of eurypterids, aren't arachnids technically descendants of eurypterids? So far as I can tell, the most basal arachnids are scorpions, and there are some eurypterids that are sometimes classified as actual huge, amphibious scorpions like Brontoscorpio, so aren't arachnids eurypterids in the same way that hexapods are actually crustaceans? I also wonder if Brontoscorpio was already capable of stinging. Xiphosurans could never sting anything with their tail, but as far as I know the closest living group of arthropods to the arachnids while being outside the arachnids is the horseshoe crabs. The pycnogida/pantopoda (sea spiders) are especially enigmatic, given how they don't have conventional chelicerae but rather a proboscis made up of two weird chelicerae that form a tube together, similar to the proboscis or rostrum of a lepidopteran or true bug. I was surprised to learn they are popularly grouped with the Chelicerata, either as a true chelicerate or a stem offshoot, as I saw them as weird enough to be their own subphylum.

Anyways, great video, very informative! I will have to give your other videos a watch.