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Current status:
all 2019 content uploaded
–older works in progress

Cambrian Explosion Month #07: Phylum Cnidaria – The Weird Ones

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.

Continue reading “Cambrian Explosion Month #07: Phylum Cnidaria – The Weird Ones”

Cambrian Explosion Month #06: Phylum Cnidaria – Medusozoa

 The medusozoans are a group of cnidarians that includes modern true jellyfish, box jellyfish, stalked jellyfish, hydrozoans, and the weird fish egg parasite Polypodium.

Due to their soft gelatinous bodies their fossil record is very sparse. While vague fossilized blobs tend get interpreted as jellyfish fairly often, many of them turn out to be trace fossils or inorganic structures, and definite preserved medusae are only found in a few sites of exceptional preservation.


Among those rare examples of fossil jellies there are some amazingly well-preserved specimens known from the mid-Cambrian, discovered in the Marjum Formation in Utah, USA (~505 million years ago).

Cambrian Narcomedusae, Cubozoa, and Semaeostomeae

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.

Cambrian Explosion Month #05: Phylum Cnidaria – Anthozoa

Cnidarians are a diverse group that includes modern corals, sea anemones, sea pens, jellyfish, hydra, and even some parasitic forms. They’re the closest relatives of bilaterians in the animal evolutionary tree, and their ancestry goes back at least 560 million years into the Ediacaran Period, with the polyp-like Haootia being one of the earliest definite cnidarian fossils – and molecular clock estimates suggest the group might have actually originated much much earlier than that, possibly as much as 740 million years ago.

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.

Continue reading “Cambrian Explosion Month #05: Phylum Cnidaria – Anthozoa”

Cambrian Explosion Month #04: Phylum Ctenophora (And Petalonamae?)

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.

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Cambrian Explosion Month #03: Phylum …Porifera?

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.

Continue reading “Cambrian Explosion Month #03: Phylum …Porifera?”

Cambrian Explosion Month #02: Phylum Porifera

Sponges are one of the very oldest branches of the animal family tree, originating sometime in the Proterozoic Eon. Fossils are known from at least 600 million years ago, and their ancestry probably goes back even further back than that into the Cryogenian Period or late Tonian Period, at least 750 million years ago.

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.

Continue reading “Cambrian Explosion Month #02: Phylum Porifera”

Borealodon

Modern mysticete whales all have baleen plates in their mouths, but before the evolution of these specialized filter-feeding structures the early members of their lineage still had toothy jaws.

Borealodon osedax here was one of those “toothed mysticetes”, living about 30-28 million years ago during the mid-Oligocene off the coast of Washington state, USA.

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.

Spathicephalus

Spathicephalus mirus here was part of a group of amphibian-like animals called the baphetoids, a lineage that weren’t quite true tetrapods themselves but were still very closely related to them.

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“.

Ancistronychus

Drepanosaurs were already some extremely weird animals, even among all the other weirdos of the Triassic period.

These strange little tree-climbing reptiles had chameleon-like bodies, humped backs, long necks, and oddly bird-like skulls with toothless beaks – and then some of them also had bizarre forelimb anatomy with a single enormous claw on the second finger of each hand, along with a claw on the tip of their prehensile tail.

But new discoveries are showing that some members of this bizarre group were doing something different.

Ancistronychus paradoxus here lived during the late Triassic, about 227 million years ago, in what is now the southwestern United States. Measuring around 50cm long (1’8″), its enormous hand claws were unusual compared to its close relatives, with a distinctly wide and hooked shovel-like shape.

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.