#chelicerates

[a-book-of-creatures]

Chelicerate anatomy, showing where the "head" is in each.

From Storer, Usinger, Stebbins, and Nybakken (1972) General Zoology (5th Ed.).

[have-you-seen-this-animal]

Have you seen the Atewa hooded spider (Ricinoides atewa)?

I have now

Yes, in photos/videos

Yes, irl

I'm not sure

Voting ended onSep 7, 2025

The first image is of an adult, the second is of a nymph. This species is about 9.63 mm long, one of the largest ricinuleids.

[moonysm]

My girlfriend, watching a documentary about spiders: do you want my pedipalps all over you?

Me, half asleep 'cause it's the middle of the night: yes of course baby

[jayrockin]

Day 3 of Inktober is my new obsession, horseshoe crab paws. I can’t believe nobody told me.

PATREON | STORE

[rattyexplores]

S. violacea, a very round species of jumping spider.

14/01/24 - Simaethula violacea

QLD:WET

[wikipediapictures]

Marengo batheryensis

[the-overthinktank]

Beautiful women named callobuis severus are appearing in my sink

[dropsofsciencenews]

The Spiders That Came From The Sea

ESP version ITA version

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If you were asked: what is the oldest scientifically recognised arachnid? Your answer would probably be Parioscorpio venator, which lived during the early Silurian period, between 437.5 and 436.5 million years ago. The previous record holder was the Scottish Dolichophonus loudonensis, which at 434 million years old is 1 to 3 million years younger than P. venator.

Until a few days ago, that would have been a perfectly acceptable answer. However, a recent discovery has pushed the origin of arachnids—encompassing not only scorpions, but also spiders and ticks—even further back in time and, surprisingly, placed it in the ocean.

The turning point came from the study of an ancient fossil: Mollisonia symmetrica. Initially considered a primitive marine chelicerate, the Mollisonia fossil, dated approximately 500 million years ago and preserved at the Harvard Museum of Comparative Zoology, was examined using advanced optical microscopy. Its segmented body, similar to that of a scorpion, with a rounded carapace and six pairs of appendages, already suggested a relationship with modern arachnids, but the real secret lay in its nervous system.

Through the analysis of neuropils (regions of nervous tissue between neurons), researchers identified a remarkable similarity to those of today’s spiders and scorpions. M. symmetrica, which lived in an aquatic environment, had a prosoma—the anterior part of the body—with segmental ganglia arranged in a radial pattern, and an unsegmented brain sending nerves to appendages resembling chelicerae. Most astonishingly, it exhibited an antero-posterior inversion of the brain compared to insects, crustaceans, and horseshoe crabs, as if it were “flipped”—just like in modern spiders. This unique arrangement, found only in arachnids, provides modern species with superior control over their many appendages, which is fundamental for the agility, speed, and predatory skills that still define them.

The discovery therefore suggests that the earliest arachnids evolved in a marine environment, not exclusively on land as previously believed. Traditionally, it was thought that the first creatures to conquer land were myriapod-like arthropods and some primitive insects. Now, thanks to this discovery, researchers suggest that an ancestor of arachnids, similar to Mollisonia, may also have made the transition to terrestrial life, preying upon other ancient creatures that already inhabited the emerging continents.

See You Soon and Good Science!

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