arachnid – Nix Illustration

Tachypleus syriacus

Tachypleus syriacus was a horseshoe crab from the late Cretaceous (~100-95 million years ago) of what is now Lebanon.

Closely related to modern tri-spine horseshoe crabs, it displayed a similar level of sexual dimorphism. Females grew to at least 25cm long (~10″), with rounded front edges to their carapaces and shorter rear spines, while males were around 30% smaller with a scalloped shape to the front of their carapaces.

One recently described female specimen also preserves distinctive nodules around the rim of its carapace, which may represent some sort of sensory structure.

This particular specimen is also unique for preserving a coprolite in the process of being expelled from the horseshoe crab’s body – that’s right, it died while pooping.

Spectember/Spectober 2023 #11: A Large Spider

An anonymous submission requested a “spider the size of a coconut crab”:

A shaded sketch of a speculative giant herbivorous descendant of jumping spiders. It's a big stocky spider covered in fuzz, with thick legs ending in cloven-hoof-like claws. It has the characteristic large main pair of eyes of jumping spiders, with the other three pairs more spread out around the front and sides of its wide head, and it also has two pairs of "horns". Its abdomen is wide and round, somewhat flattened on the top, with several spiky structures around the edge.

Ceratohispidus aspectus is a distant descendant of jumping spiders living on an Aotearoa-like landmass, isolated with no mammalian predators.

This particular lineage is notable for both their extreme gigantism (with their larger size and weight causing them to lose the ability to jump) and for having taken up herbivory in a similar manner to one modern species. Most of these big plant-eating spiders are around the size of wētāpunga, and occupy a similar ecological niche, but Ceratohispidus is the largest of them by far – rivalling the modern coconut crab with a body length of up to 40cm (~1’4″) and a legspan of almost 1m (~3’3″).

After reaching sexual maturity at 5-10 years old, adults grow very slowly, molting only once every year or two and taking several decades to actually get anywhere close to their maximum size.

Ceratohispidus’ thick legs end in hoof-like claws, and it selectively browses on vegetation by snipping off pieces with its pincer-like palps. A gizzard-like structure in its digestive system helps to grind up fibrous plant material with small gastroliths, and its wide abdomen houses both large book lungs and a tracheal system with air sacs that can contract and expand to provide a small amount of active ventilation.

While the “horns” and spikes ornamenting its body may provide some defense from the few avian and reptilian predators in its habitat, they’re mainly used as part of highly elaborate visual displays between individuals.

Austrolimulus

Horseshoe crabs are famous examples of “living fossils“, having changed their external appearance very little over hundreds of millions of years. But some fossil species were much more varied in shape than their morphologically conservative modern relatives, such as Austrolimulus fletcheri here.

Living in freshwater environments in what is now New South Wales, Australia, during the Middle Triassic (~247-242 million years ago), Austrolimulus had incredibly long spines on each side of its head, reaching a span of around 18cm (7″) – wider than its total body length!

The function of these spines is unclear, but they may have acted like a hydrofoil in fast-moving currents, or they may have served a defensive purpose by making Austrolimulus‘ carapace too wide and unwieldy for some predators to deal with.