So it’s not especially surprising that sponges were already common and highly diverse in the Cambrian, with representatives of the major modern groups all present – demosponges, glass sponges, and calcareous sponges.
During the Cambrian explosion, a time full of incredibly weird-looking evolutionary experiments, Opabinia regalis was one of the weirdest of all – so ridiculous, in fact, that when its anatomy was first revealed at a presentation the audience laughed.
Known from the mid-CambrianBurgess Shale fossil deposits in Canada, this bizarre creature lived around 508 million years ago and had a body measuring just 4-7cm long (~1.5-2.75″).
It had five stalked eyes on its head, and a long flexible proboscis that resembled a vacuum cleaner hose ending in a pincer-like grasping structure. Its mouth was located on the bottom of its head, behind the base of its proboscis, and the opening pointed backwards forming a U-bend in its digestive tract.
The rest of its segmented body had overlapping swimming lobes and a tail fan, and small triangular structures that may have been legs on its underside.
It was probably a bottom-feeding predator or a detritvore, swimming along above the seafloor using its proboscis to snatch up small soft prey or organic material and passing it up to its mouth.
It also seems to have been a fairly rare member of the Burgess Shale ecosystem, with less than 50 specimens known from the thousands of fossils found there.
For a while Opabinia was thought to represent a completely new phylum, but after further discoveries of similar animals like Anomalocaris it’s now considered to be a “stem-arthropod”, a close evolutionary cousin to modern insects, arachnids, myriapods, and crustaceans. Its exact relationships with other stem-arthropods are still being debated, however, and some studies suggest its closest living relatives may actually be tardigrades.
Capinatator praetermissus, an arrow worm from the Mid-Cambrian of Canada (~508 mya). Discovered in the famous Burgess Shale fossil deposits, it was one of the earliest known arrow worms and also much larger than most modern forms, measuring around 10cm in length (4″).
Its mouth was surrounded by 50 hooked spines, which could be extended out to grasp onto its prey – probably feeding on whatever smaller animals it could catch – but when not in use these spines would have been kept folded up inside a fleshy “hood” around its head.
It may have been a transitional form between early large-predator arrow worms and the smaller plankton-feeders that the group later became.
Orthrozanclus elongata, from the mid-Cambrian of China. Only about 2cm long (~0.8″), this tiny creature was covered in both long spines and extensive armor – with tile-like scales on its back, overlapping dagger-shaped plates around its sides, and a small shell on its head.
It’s the second species of Orthrozanclus to be discovered, extending the genus’ range about 10 million years older than the Canadian O. reburrus.
Although it would have been a member of the Lophotrochozoa (the group that contains modern molluscs, annelid worms, and brachiopods), its exact evolutionary relationships are still uncertain. It might have been a transitional form between Wiwaxia and the halkieriids, or it could be closer related to the brachiopods.
Dinomischus isolatus, an enigmatic animal from the mid-Cambrian Burgess Shale Formation in British Columbia, Canada (~505 mya). Only about 2cm (0.8″) in total length, it had a soft cup-shaped body topped with a whorl of about 20 solid plate-like “petals”, and lived attached to the seafloor by a thin stalk.
Impressions of its internal anatomy show the presence of a U-shaped gut, with its mouth and anus positioned next to each other in the center of the “petals”. It probably fed in a similar manner to crinoids, filtering small particles of food from the surrounding sea water.
But what type of creature it actually was is still unknown. Although comparisons have been made with several different groups – particularly the tiny entoprocts – Dinomischus doesn’t seem to quite fit in anywhere.
Despite this ongoing mystery, a few other similar fossils have been found that seem to be its relatives. Specimens of another species of Dinomischus from slightly older deposits in China show different “petal” shapes, and have been named as D. venustum. Another Burgess Shale animal called Siphusauctum gregarium may also be closely related.
Synophalos xynos, a shrimp-like arthropod from the Early Cambrian of China (~515 mya). Thought to be closely related to stem-crustaceans like Waptia, it was about 2cm long (0.75″) and had a bivalved carapace with a segmented body ending in a forked tail.
Unlike any other known arthropods, however, it formed long “conga line” chains of up to twenty individuals, with the tail of each animal locking securely into the shell of the next. The function of the these chains is unknown, although suggestions include some sort of mating behavior, migration, or defense against predators.
Only one specimen was found completely on its own, and its slightly longer carapace suggests it may represent a different solitary life stage of these strange little creatures.
Amiskwia was a tiny soft-bodied creature from the Middle Cambrian, known from a fairly small number of fossils – about 18 specimens from the Burgess Shale in Canada (505 mya) and an additional one from the Maotianshan Shales in China (515 mya).
Despite only measuring about 2.5cm long (1”), it was one of the larger animals alive at the time. Its body features a head with two tentacles and a small mouth, a pair of stubby fins, and a flattened paddle-shaped tail, suggesting it was an active swimmer. Its internal anatomy has been well-preserved in some specimens, revealing a brain, gut, and traces of what may be blood vessels and a nerve cord.
But we don’t know what type of animal it is. At all.
It was initially thought to be an early arrow worm. However, fossils of Cambrian representatives of that group have since been found, and Amiskwia lacks their characteristic spines and teeth. A relationship to ribbon worms or molluscs has also been suggested, but these hypotheses have the same problems with missing key features.
So, for now, Amiskwia remains one of the “weird wonders” of the Cambrian Explosion with no obvious affinities to any other known group.
Vetulicolians were a group of small marine animals best described as “problematic”, known from the Early Cambrian (~518-507 mya) of China, Greenland, Canada, and Australia. They had bulbous but streamlined bodies with a mouth opening at the front, no eyes, a thick exoskeleton-like cuticle, and a segmented swimming tail. Most also had five pairs of openings which may have been gill slits.
The image above depicts Vetulicola rectangulata, a 7cm long (2.75″) vetulicolian with a fairly “typical” body plan for the group, and the more unusual 14cm long (5.5″) Skeemella clavula.
Their evolutionary affinities have been uncertain for a long time, and at different points they’ve been classified as arthropods, chordates, kinorhynchs, basal deuterostomes, or even given their own unique phylum. A genus named in 2014, Nesonektris, has been interpreted as having a possible notochord – making vetulicolians chordates, and potentially placing them close to the tunicates – but their exact relationships are still unresolved.
(Skeemella also complicates matters, having some features considered more arthropod-like than other vetulicolians. But since it’s only known from a single specimen, more fossil material is needed to figure out what’s going on with it.)
