Many decapod crustaceans have slightly asymmetrical pincers, often with one claw being chunkier and specialized for “crushing” while the other is more slender and used for “cutting”.
But fiddler crabs take this sort of asymmetry to the extreme as part of their sexual dimorphism – males have one massively oversized claw, which is used for both visual display to potential mates and for physical fights against rivals.
Some of the earliest fiddler crabs are known from the Miocene of what is now northern Brazil. Although the fossils have been given several different taxonomic names since their discovery in the 1970s (including Uca maracoani antiqua, Uca antiqua, and Uca inaciobritoi) they’re currently considered to be indistinguishable from the modern Brazilian fiddler crab, Uca maracoani, meaning that these crabs have remained externally unchanged for the last 16 million years.
Up to about 4cm in carapace width (~1.6″), modern Uca maracoani are found in coastal mangrove swamps and tidal mudflats around the northern and eastern coasts of South America – and some of these environments have also undergone little change since the Miocene. Males of the species can develop their enlarged pincer on either side of their bodies, with lefties and righties seeming to occur in equal numbers.
Palaeopagurus vandenengeli lived in what is now northern England during the Early Cretaceous, about 130 million years ago. Around 4-5cm long (~1.6-2″), it was found preserved inside the shell of the ammonite species Simbirskites gottschei.
Its left claw was much larger than its right, and together they would have been used to block the shell opening when it was hiding away inside. And while the exact shape of its abdomen isn’t known, it probably asymmetrically coiled to the side to accomodate the spiralling shape of the host shell.
Hermit crabs seem to have switched over to using gastropod shells by the Late Cretaceous, around 90-80 million years ago, possibly due to marine snails developing much stronger sturdier shells during this period in response to the increasing prevalence of specialized shell-crushing predators. The more upright snail shells would also have been much easier to drag around the seafloor than ammonite shells – and meant that they were ultimately less affected by the total disappearance of ammonites during end-Cretaceous mass extinction.
The genus Walliserops was one of the weirdest-looking trilobites, covered in numerous pointy spines and sporting a large three-pronged “trident” on the front of its face.
They also had some degree of asymmetry in their bodies. Their tridents often didn’t fork evenly, and their long forehead spines curved off to one side – possibly so they could lift their heads up without stabbing themselves in the back.
Walliserops hammii lived in what is now Morocco during the early-to-mid Devonian, about 403-392 million years ago. Around 5cm long (~2″) It was one of the “short trident” species of Walliserops, and its chunky forehead spine curved particularly strongly to the right.
The function of these trilobites’ elaborate tridents is still poorly understood. But an unusual individual of the long-tridented species Walliserops trifurcatushas been found with a lopsided four-pronged trident, and since it was able to grow to full maturity the shape of the structure probably wasn’t absolutely vital for survival, suggesting it wasn’t used for feeding or sensory purposes.
The tridents may instead have been used for combat with each other similar to the horns of some modern beetles. However, these sorts of features are usually only seen in males, and there’s currently no definite evidence for any significant sexual dimorphism in trilobites.
(Although perhaps like ceratopsid dinosaurs their ornaments were just present in both males and females, being also useful for species recognition, visual display, and defense against predators.)
Lobopodians were some of the earliest known panarthropods, closely related to velvet worms, tardigrades, and the ancestors of all the true arthropods. They were small soft-bodied worm-like animals with multiple pairs of fleshy legs, and some species also bore elaborate spikes, armor plates, and fleshy bumps all over their bodies – with the spiny Hallucigenia being the most famous example.
But unlike its more charismatic relative Paucipodia inermis here didn’t seem to have any ornamentation at all.
Known from the Chinese Chengjiang fossil deposits, dating to about 518 million years ago, Paucipodia lived in what was then a shallow tropical sea. Its 13cm long (~5″) tubular body had nine pairs of legs, with each foot tipped with a pair of hooked claws, and the inside of its mouth was ringed with tiny sharp teeth.
Several specimens have been found preserved in association with the weird gummy-disc animal Eldonia, which may indicate Paucipodia either preyed on them or scavenged on their carcasses.
Some Paucipodia fossils also have enigmatic tiny “cup-like” organisms attached to their legs. It’s currently unknown what exactly these were, or whether they were parasitic in nature or simply opportunistically “hitching a ride” similar to the Inquicus found on armored palaeoscolecid worms in the same fossil beds.
The Ptychopariida were some of the earliest known trilobites, first appearing in the early Cambrian about 521 million years ago and surviving until the end of the Ordovician about 444 million years ago. They included some of the most numerous and common trilobite species, and were probably ancestral to multiple other major lineages – including the very last trilobites at the end of the Permian – making them incredibly important in understanding the overall evolution of trilobites as a whole.
…But this group is also one of the biggest wastebaskets in paleontology.
First established in the early 20th century, the ptychopariids seemed to have some fairly good defining characteristics based on their facial sutures, large thoraxes, and relatively small pygidia. But the group quickly became a dumping ground for a massive amount of Cambrian trilobites, eventually containing numerous different families, hundreds of genera, and many more individual species.
Actually figuring out their internal evolutionary relationships also turned out to be extremely difficult – so much so that some paleontologists working on them just gave up trying and arranged the genera names alphabetically instead!
Even cladistic studies from the 1970s onward struggled to make sense of these highly “problematic” trilobites, and any larger-scale analysis was a daunting task due to how huge and diverse the ptychopariid wastebasket had become over the years. Worse, some of the anatomical features the group had been based around were starting to look more like the result of a lot of convergent evolution across multiple lineages than any actual shared ancestry.
Efforts were still made at breaking up the mess, however, with better-understood sub-groups like the Proetida, Harpida, Asaphida, Trinucleida, and Olenida being gradually split off into their own separate orders over the course of the last few decades.
Ptychoparia striata
But even by the early 2010s what remained of the Ptychopariida was still paraphyletic at best, more of an “evolutionary grade” of early trilobites than a single lineage, with most of its constituent families also rather poorly defined. There was even a proposal to abandon the group entirely, stating that “it serves no scientific purpose” and that its orphaned contents should be considered “order uncertain” until their actual relationships can be untangled.
Today the “ptychopariids” are in dire need of a full revision – since they were the ancestors of many other major groups they’re still crucial for understanding early trilobite evolution. There may be a salvageable single lineage somewhere in the remains of this wastebasket, even if it’s restricted to just close relatives of the genus Ptychoparia, but until somebody tackles them properly they’re stuck in taxonomic limbo with their name only being used in a loose sense.
Protorthoptera was a group of fossil insects created in the early 20th century to categorize “primitive” neopterans – some of the earliest insects to have evolved the ability to fold their wings down over their backs. Known mostly from just fossilized forewings, they first appeared around 320 million years ago in the late Carboniferous, and after heavy losses during the Great Dying mass extinction they eventually disappeared in the mid-Triassic about 240 million years ago.
As early as the mid-20th century the protorthopterans were recognized as being a general taxonomic dumping ground, containing a mixture of early members of multiple different “orthopteroid” insect lineages. But invertebrate paleontologists at the time considered this collection of “primitive” insects to lack enough distinctive features to confidently separate them out from each other, and so the highly paraphyletic grouping continued to be used well into the 1990s.
Ctenoptilus elongatus
But in the early 2000s this situation finally changed. Proper cladistic analysis of protorthopteran fossils identified defining features of the wing vein patterns, and many species were reclassified into various lineages within the Archaeorthoptera – which includes modern grasshoppers, crickets, and locusts along with several closely related fossil groups like the titanopterans and caloneurodeans.
“Protorthoptera” is still sometimes used in a loose sense for fossil neopteran insects that still can’t be confidently classified anywhere else, so the wastebasket isn’t entirely cleared here.
And there are some alternate classification systems (mainly proposed by Russian paleontologists) that instead consider many protorthopterans to be notopterans closely related to modern ice-crawlers, and place others as part of other modern neopteran lineages such as webspinners and true bugs.
Hopefully better fossil discoveries and future studies will eventually help clear things up, and give us a better overall picture of the evolution of these insects.
Radiodonts were early arthropods with specialized frontal appendages, disc-like mouths, complex compound eyes, and swimming flaps along the sides of their bodies. Once considered to be bizarre “weird wonders” of the Cambrian Explosion that represented a failed evolutionary experiment, we now know that they were actually a highly diverse and successful lineage that lasted for at least 120 million years.
While some radiodonts were the largest animals of their time periods, Stanleycaris hirpex here was one of the smallest known members of the group – although at around 10cm long (~4″) it was still respectably big compared to most other Cambrian animals.
Discovered in the Canadian Burgess Shale deposits (~508 million years ago), it was originally known only from isolated frontal appendages and mouthparts, and had been assumed to be a fairly typical member of the hurdiid family. But the recent discovery of over 200 new fossils, including some exceptionally well-preserved full body specimens, has catapulted it directly from being poorly-known into now being one of the most completely known of all radiodonts.
And it had a very big surprise for us, right in the middle of its face.
It turns out that Stanleycaris had a huge third eye, unlike anything ever seen in a radiodont before. A large unpaired eye was also part of the five-eyed arrangement in opabiniids and Kylinxia, and finding a similar example in radiodonts too raises the possibility that this sort of well-developed “median eye” may have been more widespread in early arthropods than previously thought.
Along with the third eye, some of the Stanleycaris specimens preserve fine internal details of its nervous system and show that its brain was made up of two segments instead of the three seen in modern arthropods. It also had gills positioned on its underside, unlike most other radiodonts which had them on their backs.
Euthycarcinoids were a group of arthropods that lived between the mid-Cambrian and the mid-Triassic – but despite existing for over 250 million years their fossil record is incredibly sparse, and it’s only within the last decade that they’ve been recognized as being close relatives of modern centipedes and millipedes.
The earliest members of this group were marine, living in shallow tidal waters, but they quickly specialized into brackish and freshwater habitats and were even some of the very first animals to walk on land. Fossil trackways show they were amphibious, venturing out onto mudflats to feed on microbial mats, avoid aquatic predators, and possibly lay their eggs in a similar manner to modern horseshoe crabs.
Most euthycarcinoid species are known from tropical and subtropical climates, but Antarcticarcinus pagoda here hints that these arthropods were much more widespread and diverse than previously thought. Discovered in fossil deposits in the Central Transantarctic Mountains of Antarctica, it lived in freshwater lakes during the Early Permian (~299-293 million years ago), at a time when the region was in similar polar latitudes to today with a cold icy subarctic climate.
About 8.5cm long (3.3″), it would have had a similar three-part body plan to other euthycarcinoids – with a head, a limb-bearing thorax, and a limbless abdomen ending in a tail spine – but its most distinctive feature was a pair of large wing-shaped projections on the sides of its carapace. These may have helped to stabilize its body when resting on soft muddy surfaces, spreading out its weight, or they might even have functioned as a hydrofoil generating lift while swimming.
Ever since the bizarre anatomy of Opabinia was first recognized in the 1970s, it’s been a persistently unique “weird wonder” of the Cambrian period. Over the decades we’ve figured out that it was an early type of arthropod in an evolutionary position between lobopodians and radiodonts, but this whole time it’s still been sitting there alone as the only known representative of a weird stem-lineage with no other known close relatives.
Only about 3cm long (1.2″), Utaurora had 15 pairs of swimming flaps along the sides of its body, and a tail region with a 7-part fan and a pair of serrated spines. Hair-like gill blades covered both its back and the bases of its swimming flaps, and although its head region was poorly preserved it probably had an arrangement of 5 eyes and a long flexible claw-tipped proboscis similar to that of Opabinia.
Its discovery extends both the geographical and temporal known range of opabiniids, and suggests that their continued scarcity in other Cambrian fossil sites compared to other soft-bodied arthropods may simply be because they were just incredibly rare animals in those habitats at the time.
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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