Sea cucumbers have a central role in echinoderm origins

Earlier we looked at the origin of chordates
hemichordates and molluscs from nematodes. Earlier we also looked at the transition from the bilateral body to the radial body (with an oral axis) due to gradual loss of the post-mouth morphology in starfish. This left only the buccal cirri (= tentacles surrounding the oral cavity) that enlarge and become organs of locomotion. This happens by convergence in the starfish and octopus, but both start with the same buccal cirri surrounding the oral cavity.

Today
let’s look at the origin of various echinoderm clades. The present hypothesis of interrelationships is distinct from traditional studies that, according to Wikipedia, posit an origin from crinoids leading to sea cucumbers + sea urchins on one branch, then starfish and brittle stars on the other branch. This is based on gene studies, which have a bad reputation here.

A new phylogenetic order based on a new outgroup taxon
Here (Figs. 1–6) the traditional order of primitive and derived echinoderms is flipped on its head. This is based on a new hypothesis that posits the origin of echinoderms from primitive burrowing sea cucumbers. These, in turn, arise from burrowing lancelets with tiny buccal ciirri that evolve to become branching tentacles. Sea cucumbers lose the mobile post-anal tail, so they have a terminal anus. Today’s hypothesis takes a closer look at echinodoerm transitional taxa only hinted at earlier here.

There is little text to support this hypothesis.
Why get long-winded when the figures (2-6) tell the story of gradual accumulation (or loss) of character traits more parsimoniously than other hypotheses.

The derivation of echinoderms from sea cucumber and lancelet ancestors
appears to be a novel hypothesis of interrelationships. If not, please provide a citation so I can promote it here.

Smith 2005
discussed the “pre-radial history of echinoderms” and claimed to provide “critical evidence for how the latest common ancestor of hemichordates and echinoderms was transformed into a pentaradiate crown-group echinoderm.”

The Smith 2005 hypothesis is not confirmed here (Figs. 1–6). Hemichordates (Fig. 2) do not evolve to become echinoderms. Pentaradiates (five-arm echinoderms) are not primitive, but derived. Thirteen arms, as in sun stars, is the primitive number for basal asterozoans based on the same thirteen ‘arms’ arising from transitional sea cucumber outgroup taxa, like Euapta (Fig. 4).

Rahman and Zamora 2009 reported they found
“the oldest cinctan carpoid (stem-group Echinodermata)” and were able to discuss “the evolution of the water vascular system” that supports and locomotes echinoderms.

The Rahman and Zamora hypothesis is not confirmed here (Figs. 1–6). Cinctan carpoid echinoderms are highly derived (Fig. 5), not primitive. They have lost their ‘arms’ and have become asymmetric as they became smaller. Cinctan carpoids appear in the Middle Cambrian so that indicates a prior wide radiation of more primtive echinoderms.

According to Rahman and Zamora,
“Cinctans (Fig. 7) have a large atrial opening in addition to mouth and anus, and are interpreted as pharyngeal basket feeders. The paired grooves associated with the mouth indicate the presence of a hydrovascular system, but not necessarily one built along the echinoderm plan (that is, derived from just the left hydrocoel).”

Just the opposite. Here cinctans have lost many echinoderm traits. They are not echinoderm precursors, but tiny descendants leaving no ancestors.

Rahman and Zamora continue,
“Stylophorans (= Calcichordata, Fig. 5) have a bilateral body plan that is externally masked by torsion and possess gills, either unpaired and external, or paired and internal, opening into an atrial cavity. Their bilateral appendage is a locomotory organ, not an ambulacrum, and there is no evidence that stylophorans ever possessed a well-developed hydrovascular system homologous to the water vascular system of echinoderms and the tentacles of pterobranch hemichordates. Solutes are the most crownward, having a true echinodermal ambulacral system with a single hydropore and no pharyngeal gill openings.”

Just the opposite. I found no bilateral body plan, no gills, only an atrium. Lefebvre et al. 2019 found a hydrovascular system in the anterior tentacle = arm. The tail is absent in Cothurnocystis (Fig. 5). In its place two elongate immobile processes appear at the rear corners of the atrium. These could have anchored Cothurnocystis in the substrate in lieu of an anchoring tail.

Hunter and Ortega-Hernanez 2017
considered Early Ordovician Cantabrigiaster “the most primitive of all the Asterozoa, and most likely evolved from ancient animals called crinoids that lived 250 million years before dinosaurs.”

Perhaps not. Here (Fig. 4) the five arms of pentaradiate starfish are reduced from a primitive number of 13, derived from a sea cucumber ancestor with 13 arms = tentacles.

Ruta 2003 created a supertree of stylophorans
(Fig. 7), but employed an all-zero outgroup to root his cladogram. Results may be helpful, but perhaps were less than optimal without a solid outgroup taxon. Unfortunately supertrees have a less than stellar history of success.

Dr. Richard Jefferies considered stylophoran echinoderms basal to chordates.
He presented his hypothesis in the early 1990s shortly after the publication of “From the Beginning – The Story of Human Evolution.” The present hypothesis does not support Dr. Jefferies’ ‘chalcichordate’ hypothesis.

Sea cucumbers on their own
(Fig. 3) evolved a wide variety of crawling, swimmng, burrowing and walking forms that are still recognized as derived sea cucumbers (Holothuroidae). Therefore we should not be surprised to extend this list of adaptations to include the sessile, stalked, radial, asymmetric, armored, and spiny sea cucumber descendants (Figs. 4–6) that are currently not recognized for what they are. Lack of an appropriate and valid outgroup is the problem in echinoderm systematics. Here (and earlier, in Peters 1992), a candidate outgroup series is provided awaiting confirmation or refutation.

References
Hunter A and Ortega-Hernández J 2017. “A primitive starfish ancestor from the Early Ordovician of Morocco reveals the origin of crown group Echinodermata”. bioRxiv.
Jefferies RPS 1987. The ancestry of the vertebrates. Cambridge University Press. AbeBooks.
Lefebvre B et al. (9 co-authors) 2019. Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes. Geobios 52:27–36.
Peters D 1992. From the Beginning: The Story of Human Evolution. Wm Morrow. 128 pp.
Rahman IA, Zamora S 2009. The oldest cinctan carpoid (stem-group Echinodermata), and the evolution of the water vascular system. Zoological Journal of the Linnean Society, Volume 157, Issue 2, October 2009, Pages 420–432. doi.org/10.1111/j.1096-3642.2008.00517.x
Ruta M 2003. A species-level supertree for stylophoran echinoderms. Acta Palaentolgica Polonica 48(4):559–568.
Smith AB 2005. The pre-radial history of echinoderms. First published: 20 June 2005 https://doi.org/10.1002/gj.1018

popularmechanics.com/science/animals/a26469816/stylophoran-weird-ancient-animal/

theconversation.com/starfish-rare-fossil-helps-answer-the-mystery-of-how-they-evolved-arms-153481

wiki/Echinoderm
wiki/Stylophora

livescience.com/64832-ancient-starfish-relative-mystery-solved.html

echinoblog.blogspot.com/2016/04/the-diversity-of-echinoderm-anuses.html