Odd shell-like structures that resemble angular ribbed cones with four-way symmetry appear in the fossil record starting around the mid-to-late Cambrian (with a possible Ediacaran record).
Known as conulariids, these fossils are so distinctive and different from anything else that for a long time their evolutionary affinities were unknown, and they were considered to be a “problematic” group. But in recent years they’ve been identified as being cnidarians, generally thought to be close relatives of modern stalked jellyfish.
None of these species have been given their own names, and they’re all tiny, only around 1cm in diameter (0.4″). But their anatomy is still preserved in enough detail to tentatively classify them into known lineages, including the box jelly, narcomedusan, and semaeostomean shown here.
Much larger Cambrian jellyfish have been also found in Death Valley, California, and in Wisconscin, representing preserved mass stranding events on ancient shorelines. Some of these jellies were up to about 50cm in diameter (20″), indicating that large soft-bodied animals were much more common in Cambrian seas than previously thought.
The anthozoan lineage of cnidarians (corals, anemones, and sea pens) spend their adult lives as polyps attached to the seafloor, either solitary or colonial, and since many lineages have hard calcium carbonate skeletons their fossil record is generally much better than that of the soft-bodied medusozoan jellyfish.
While corals are major contributors to reef ecosystems in modern times, back during the Cambrian they were actually rather rare. The weird little archaeocyathan sponges were the main reef-builders in the early-to-mid Cambrian, and after their decline reefs were mainly formed by algae and other types of sponges.
But, sometimes, growing among these reefs were also some tiny Cambrian corals.
Much like the sponges, the ctenophores (commonly known as “comb jellies”), are one of the oldest animal lineages, but their exact position in the evolutionary family tree is a little uncertain. Traditionally they’re placed between sponges and all other animals, as the earliest branch of the eumetazoans, but some studies have suggested that they might be much more ancient, possibly branching off before even the sponges did.
And while their fossil record is poor due to their soft gelatinous bodies, some of what we do have is starting to hint that their ancestry was very different from their modern jellyfish-like representatives – and they might even have links to some weird Precambrian creatures.
Sponges were major reef builders during the Cambrian Explosion, and for the first half of the Cambrian Period the dominant reef-forming group were the bizarre archaeocyathans.
Although their reign was geologically short, lasting only about 15 million years, these tiny calcified sponges were incredibly numerous and diverse during that time, with hundreds of different species known from warm shallow marine waters all around the world. They came in a huge range of shapes, including cups, cones, funnels, towers, and irregular blobs, and were so weird that they weren’t even properly recognized as being sponges until the 1990s.
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.
Unlike modern baleen whales it was small, about the size of a modern porpoise at around 2m long (6’6″), and the wear on its multi-cusped teeth suggest it was a predator taking slicing bites of fish – possibly using suction-assisted feeding like its close relatives the aetiocetids.
Its fossilized remains are also a rare example of an ancient whale fall, with characteristic bore holes in its bones from Osedax worms.
Living in Scotland during the mid-Carboniferous period, about 326 million years ago, this 1.5m long (~5′) stem-tetrapod had an incredibly unusual head compared to its relatives – wide and flat, almost square in shape, with its jaws lined with hundreds of tiny chisel-like teeth.
Most other stem-tetrapods had deep skulls with large teeth, adapted for fish-eating, so clearly Spathicephalus was specialized for a very different diet. Some comparisons have been made to flat-headed ambush predator plagiosaurid temnospondyls like Gerrothorax, but a better ecological comparison might actually be filter-feeders like “pancake crocs“.
Along with another recently-discovered species, Skybalonyx skapter, and the weird burly arms of Drepanosaurus, this suggests that instead of tree-climbing some drepanosaurs were instead much more specialized for digging. They may have been Triassic equivalents to modern anteaters or pangolins, using their enlarged claws to excavate burrows and rip their way into insect nests.