Now we get to the bilaterians, all the animals with bilateral symmetry as an ancestral feature.
The hypothetical common ancestor of all bilaterians (the “urbilateria“) was probably a tiny worm-like species, and likely originated sometime in the early Ediacaran Period. The earliest definite body fossils of bilaterians come from about 558 million years ago, and possible burrow traces are a little older, from about 585 million years ago – but it was during the Cambrian Explosion that this group rapidly diverged into a wide variety of forms and ecological niches.
The deuterostomes are one of the two major evolutionary branches of the bilaterians, made up of modern hemichordates, echinoderms, and chordates (including the vertebrates). They split from their last common ancestor with the protostomes sometime during the Ediacaran, but the earliest probable deuterostome fossils are weird tiny potato-like blobs from the very start of the Cambrian.
(…We’ll talk about those later in the month.)
Hemichordates are the closest living relatives to echinoderms, and are a small phylum with only around 130 known modern species (most of which are acorn worms). But in the distant past they were much more numerous, with graptolites being major components of Paleozoic planktonic ecosystems.
All living members of this lineage have a three-part body plan, but they’re otherwise very different in both appearance and ecology. Acorn worms are solitary worm-like animals living on in the sediment of the sea bed, and are mostly detritivores, while pterobranchs are tiny colonial filter-feeders that build protective tubular structures.
Continue reading “Cambrian Explosion Month #08: Phylum Hemichordata – Pterobranchia”