Here’s a YouTube video featuring
Dr. Karma Nanglu (Smithsonian National Museum of Natural History) showing and discussing Cambrian taxa from the Burgess Shale ancestral to living hemichordates, pterobranchs (= graptolites and kin) and enteropneusts = acorn worms).
“This talk will guide you through a series of recent studies using Burgess Shale fossils that shine a light on hemichordate origins, one of the most mysterious parts of the animal tree of life. These exceptional fossils reveal unanticipated combinations of morphological and ecological characteristics early in the history of this animal group, including surprising combinations of those found in their modern relatives.”
Unfortunately, when it came time to present last common ancestors at an hour into the presentation, Nanglu left much of the work undone. His graphic showed question marks at the ancestral nodes (Fig. 1).
By contrast and thirty years ago
Peters 1991 found hemicordates arose from basal chordates (Fig. 3) like the lancelet, Branchiostoma (Fig. 2), itself derived from nearly featureless roundworms (Fig. 3). You might recall that adult lancelets are sessile feeders, anchoring themselves tail first into sandy and muddy substrates, distinct from their free-swimming tiny hatchlings that more greatly resemble tiny fish in their activity. All this occurred during the Cambrian.
Distinct from chordates,
sessile (= essentially immobile) pterobranchs emphasize and enlarge the suspension feeding cirri made sticky with mucous strands (Fig. 3).
Distinct from chordates,
worm-like enteropneusts emphasize the rostrum (= proboscis, Fig. 3).
gave up the chevron-shaped swimming muscles and internal gill basket found in lancelets and fish. However, enteropneust hatchlings present a vestigial post-anal tail that is resorbed or transformed in adults.
Nanglu 2021 confirms this 30-year-old hypothesis of interrelationships
(Fig. 3) as he nests chordates basal to hemichordates and echinoderms.
Nanglu also presents
a tube-building, vermiform last common ancestor between pterobranchs and enteropneusts, with post-anal attachment and possible tube building. In Peters 1991 pterobranchs are basal to crinoids, blastoids and other echinoderms, taxa that further emphasize and enlarge the gracile cirri that encircles the mouth of lancelets until the cirri comprise the entire anatomy of the starfish. So starfish are walking on their greatly enlarged and elaborate mouth parts, having given up or absorbed the rest of the ancestral lancelet anatomy.
We looked at chordate origins
in more detail earlier here (summarized in Fig. 3).
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Romer AS 1971. The Vertebrate Body – Shorter Version 4th ed. WB Saunders.