Cambrian Explosion Month #16: Phylum Chordata – Vertebrata

Vertebrates are by far the most numerous and diverse group of chordates today, with over 65,000 known species including fish, amphibians, reptiles, birds, and mammals. Genetic studies show that they’re closely related to the weird bag-like tunicates, and their shared common ancestor was probably something lancelet-like.

And the earliest true vertebrates would have looked something like Haikouichthys ercaicunensis.

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Cambrian Explosion Month #15: Phylum Chordata – Early Forms & Tunicates

Chordates are one of the most diverse animal phyla, ranging from tiny lancelets to sac-like tunicates to all fish and tetrapods. They share a common deuterostome ancestor with echinoderms and hemichordates, probably diverging from them sometime in the Ediacaran Period, and are characterized by having specific anatomical features at some point during their life cycle – a notochord, a dorsal nerve cord, pharyngeal slits, a post-anal tail, and an endostyle.

The earliest chordates were all small soft-bodied animals with no mineralized tissues, so their fossil record is poor aside from rare locations with exceptional preservation. But one of the best known examples is Pikaia gracilens from the Canadian Burgess Shale fossil deposits (~508 million years ago).

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Myllokunmingia

Myllokunmingia, from the Early Cambrian of China (~530 mya).

Just under 3cm long (or just over 1″), this tiny animal seems to have been a very close relative of the true vertebrates – almost a vertebrate itself but not quite there yet. A single known fossil specimen shows evidence of a cartilage skull and skeletal elements, five or six gill pouches, a large sail-like dorsal fin, and paired finfolds on its underside.