Cambrian Explosion #61: Crustacea – Little Wigglers

We’re finally at the end of this series, and to finish off let’s look at one of the few types of Cambrian true crustaceans that are known only from fully mature adults: the skaracarids.

These tiny soft-bodied meiofaunal animals are known from late Cambrian areas of “Orsten-type preservation” in Sweden and South China, with a possible additional fragmentary occurrence in Poland – suggesting that they had a global distribution.

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Cambrian Explosion #60: Crustacea – Larvae Larvae Everywhere

One of the characteristic features of the crustacean lineage are their larval forms, passing through various tiny larval stages. They often look nothing like their eventual adult forms and historically weren’t even recognized as being the same species, with their complex lifecycles not being properly recognized until the late 1800s.

A lot of Cambrian crustaceans are only known from their larvae, preserved in exquisite microscopic detail in sites of “Orsten-type preservation”. Only disarticulated fragments of larger-bodied forms have been found in a few places, and it isn’t until much later in the Paleozoic that fossil crustaceans actually seem to become abundant in marine ecosystems.

It’s not clear why there’s such a bias in their early fossil record compared to most other arthropods, but possibly they were just very very rare animals early on. Adult forms may have mostly lived in places where they just didn’t fossilize, while their tiny larvae sometimes dispersed into different environments with a better chance of preservation.

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Cambrian Explosion #59: Stem-Crustacea – Actual Ancient Aliens & Bivalved Buddies

The majority of known fossils of Cambrian crustaceans are in the form of minuscule microfossils with “Orsten-type preservation” – formed in oxygen-poor seafloor mud and exceptionally well-preserved in three-dimensional detail. They can only be discovered and studied after dissolving away the rock around them with acid and picking through the residue under a microscope, then they’re scanned with an electron microscope to see their fine details.

And it turns out some of these tiny early crustaceans looked really weird.

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Thylacares

Thylacocephalans were a bizarre group of extinct marine arthropods that often looked like tiny alien creatures – and whose evolutionary relationships are still uncertain. Despite existing in oceans around the world for at least 350 million years, their fossil record is rather spotty and their internal anatomy is often poorly preserved, making it difficult to figure out anything more specific than “probably some sort of crustacean“.

Possible thylacocephalans are known from as far back as the Cambrian, but one of the earliest definite members of the group was Thylacares brandonensis.

Living during the early Silurian, around 436 million years ago, in the region that today is part of Wisconsin, USA (found in the same fossil formation as last week’s Venustulus), this species measured up to about 7.5cm long (3″). Its body was enclosed by a large bivalved carapace, with protruding stalked eyes and what may have been a pair of antennae, along with smaller raptorial limbs than its later relatives.

While it was less specialized than other thylacocephalans it was probably a similar sort of swimming predator, catching prey with its spiny limbs.

Clausocaris

While this might look like a sci-fi alien design, it was actually a very real Earth animal!

This strange-looking creature was Clausocaris lithographica, a member of a group of unique marine arthropods known as thylacocephalans. Only about 3.5cm long (1.4″), it lived in a shallow tropical lagoon environment during the Late Jurassic of Germany, about 150-145 million years ago.

Like most other thylacocephalans it had a narrow flattened shield-like carapace, three pairs of large grasping limbs, and a battery of swimming appendages further back – along with absolutely enormous bulbous eyes. Based on this anatomy it would have been a highly visual hunter, using its huge eyes to locate prey items and then snagging them with its long spiny limbs.

And we’re not even entirely sure what type of arthropods thylacocephalans actually were. They’re generally thought to be some sort of crustacean, but their highly modified anatomy makes linking up their exact evolutionary affinities very difficult. Whatever they were, they must have been incredibly successful as a group because they first appeared in the early Cambrian (~518 mya) and survived all the way into the Late Cretaceous (~94 mya).

Almost-Living Fossils Month #08 – A Lot Of Lobsters

Hoploparia was a type of clawed lobster that first appeared in the fossil record in the Early Cretaceous about 140 million years ago. Many many different species within this genus have been found all over the world – over 100 of them have been described! – with quite a lot of anatomical diversity between them, showing that these lobsters were very good at adapting to a wide range of habitats and climates.

Although the vast majority of Hoploparia species lived just in the Cretaceous period, a small number of them did survive the end-Cretaceous mass extinction 66 million years ago. Hoploparia stokesi here was one of them, known from both the Late Cretaceous and Early Paleocene of Antarctica (~70-61 mya) – and was actually one of the first fossils ever described from the continent.

Specimens of this species are usually about 13cm long (5″), and show an evolutionary shift over time, developing much stronger claws and jaws, suggesting they were adapting their diet towards hard-shelled prey.

Various species of Hoploparia persisted on in North America, Europe, and Antarctica for the first half of the Cenozoic, but they never recovered to anywhere close to their Cretaceous levels of diversity. By the Early Miocene (~23-16 mya) there was just one known species left hanging on in Antarctica, and then they were gone.

(However, some modern lobster genera may in fact have originated from somewhere within the huge Hoploparia lineage back in the Cretaceous, so they might at least still have some close living relatives!)