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.
The earliest branches of the dinocaridids were the “gilled lobopodians”, which had lobopodian-like legs on their undersides and gills on the upper surfaces of their body lobes. The flap-like structures may have initially evolved just to provide a larger surface area for respiration, but they were quickly co-opted for swimming purposes and opened up a whole new range of ecological opportunities to the ancestral dinocaridids.
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.