Cambrian Explosion Month #17: Phylum(?) Vetulicolia & Other Early Deuterostome Weirdos

Vetulicolians were a group of odd Cambrian animals known from between about 520 and 505 million years ago. The front half of their bodies were large and streamlined, with a prominent mouth, no eyes, and five pairs of openings that seem to have been gills, with some species having a rigid exoskeleton-like carapace. Their back half was slender, segmented, and flexible, and functioned as a tail for swimming, giving them an overall appearance like alien tadpoles.

Their evolutionary affinities have been problematic for a long time, but evidence of a notochord in some specimens suggest they were probably related to the chordates in some way. Sometimes they’re considered to represent their own phylum, but they might also be stem-chordates or stem-tunicates.

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Cambrian Explosion Month #11: Phylum Echinodermata – Increasing Asymmetry

During their early evolution, echinoderms started developing unusual asymmetric body plans – and some of them were so strange-looking that for a while it wasn’t clear if they even were echinoderms.

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Cambrian Explosion Month #10: Phylum Echinodermata – Bilateral Origins

Modern echinoderms typically have five-way radial symmetry as adults, and don’t at all resemble other deuterostomes – but their larvae give away their ancestry, still retaining bilateral traits and only developing radial symmetry when they mature and metamorphose.

The earliest definite echinoderms are known from the early Cambrian, about 525 million years ago, which seems to be around the point when early members of the group first developed biomineralized skeletons and became much more likely to fossilize. However, they must have an evolutionary history going back further than that, and molecular clock estimates suggest their last common ancestry with their closest relatives the hemichordates was in the Ediacaran about 570 million years ago.

For a long time the transition from bilateral to radial symmetry was a mystery, but various fossil discoveries are starting to reveal how this unique group of animals evolved.

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