“Oldest known echinoderm with a bilateral body plan” omits chordate and nematode outgroup taxa
According to phy.org
Atlascystis (Fig 1) “bridges the evolutionary gap between the closest living relatives of chinoderms, all of which have bilateral symmetry, and the familiar pentaradial (five-armed) forms alive today.”
From the publicity department,
“Echinoderms eventually landed on five ambulacra, giving them their unique pentaradial form. However, researchers still aren’t sure why starfish and their relatives developed this form as opposed to any other form.
Figure 1. Atlascystis in several views, DGS colors were published. This taxon bears some resemblance to a sea cucumber. Note the relatively large oral cavity surrounded by buccal cirri now calcified.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/atlascystis588.jpg?w=284″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/atlascystis588.jpg?w=584″ class=”size-full wp-image-93424″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/atlascystis588.jpg” alt=”Figure 1. Atlascystis in several views, DGS colors were published. ” width=”584″ height=”617″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/atlascystis588.jpg?w=584&h=617 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/atlascystis588.jpg?w=142&h=150 142w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/atlascystis588.jpg?w=284&h=300 284w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2025/06/atlascystis588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 1. Atlascystis in several views, DGS colors were published. This taxon bears some resemblance to a sea cucumber. Note the relatively large oral cavity surrounded by buccal cirri now calcified.
This is a tiny Cambrian fossil.
Woodgate et al 2025 reported, “The theca is bilaterally symmetrical, dorsoventrally flattened, and sub-triangular in outline. It ranges in size from approximately 3.3 to 9.9 mm in length and 2.9 to 8.1 mm in width. The authors’ reported, “The bilaterally symmetrical fossil Ctenoimbricata spinosa (Zamora et al 2012) was recovered as sister to all other echinoderms.”
In the present hypothesis, the authors have their cladogram upside down, with derived taxa at the base – which is lacking a simple bilateral, worm-like taxon with a radiating oral cavity, like the nematode, Enoplus (Fig 2). The oral cavity is where the origin of the radial symmetry has its genesis.
Figure 4. The nematode ancestors of hagfish have no eyes.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/06/nematode_mouth588.jpg?w=254″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/06/nematode_mouth588.jpg?w=584″ class=”size-full wp-image-56600″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/06/nematode_mouth588.jpg” alt=”Figure 4. The nematode ancestors of hagfish have no eyes.” width=”584″ height=”689″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/06/nematode_mouth588.jpg?w=584&h=689 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/06/nematode_mouth588.jpg?w=127&h=150 127w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/06/nematode_mouth588.jpg?w=254&h=300 254w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/06/nematode_mouth588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 2 The nematode ancestors of hagfish have no eyes. Note the odd number of buccal cirri here, sometimes reduced to 13 in basal echinoderms, then down to 5 in starfish. Not sure why the freehand diagram has symmetrical 6 tentacles = cirri.
13 years ago, co-author Zamora et al 2012 wrote:
“Here we report new fossils from the early middle Cambrian of southern Europe that are the first echinoderms with a fully bilaterian body plan as adults. Morphologically they are intermediate between two of the most basal classes, the Ctenocystoidea and Cincta.”
According to Wikipedia – Ctenocystoidea
“had bilateral symmetry, or were only very slightly asymmetrical. They are believed to be one of the earliest-diverging branches of echinoderms,”
According to Wikipedia – Cincta
“Cinctans were sessile, asymmetrical animals with a skeleton made of stereom plates and a racquet-shaped body composed of a theca and stele. They may have had a lifestyle similar to modern tunicates, filter-feeding by pumping water through gill slits in their pharynx.”
Figure 2. A hypothetical transitional series from lancelets to sea cucumbers = echinoderms.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_to_sea_cucumber588.jpg?w=93″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_to_sea_cucumber588.jpg?w=316″ class=”size-full wp-image-57639″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_to_sea_cucumber588.jpg” alt=”Figure 2. A hypothetical transitional series from lancelets to sea cucumbers = echinoderms.” width=”584″ height=”1892″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_to_sea_cucumber588.jpg?w=584&h=1892 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_to_sea_cucumber588.jpg?w=46&h=150 46w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_to_sea_cucumber588.jpg?w=93&h=300 93w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_to_sea_cucumber588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 3. A hypothetical transitional series from lancelets to sea cucumbers = echinoderms. Note the oral cavity is radial.
The authors’ text does not include
the terms ‘nematode’, ‘lancelet’ or ‘sea cucumber’, taxa nesting at the base of the echinoderms and all with bilateral symmetry – except around the oral cavity where buccal cirri evolve to become echinoderm arms = ambulacra. See figure 3–5.
Figure 1. Starfish evolution from 2021.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_basal_to_starfish588.jpg?w=88″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_basal_to_starfish588.jpg?w=301″ class=”size-full wp-image-57636″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_basal_to_starfish588.jpg” alt=”Figure 1. Starfish evolution from 2021.” width=”584″ height=”1984″ srcset=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_basal_to_starfish588.jpg?w=584&h=1984 584w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_basal_to_starfish588.jpg?w=44&h=150 44w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_basal_to_starfish588.jpg?w=88&h=300 88w, https://pterosaurheresies.wordpress.com/wp-content/uploads/2021/07/lancelets_basal_to_starfish588.jpg 588w” sizes=”(max-width: 584px) 100vw, 584px” />
Figure 4. Starfish evolution from 2021.
So starfish are all mouth and surrounding tissues.
They have lost their ancestral head, torso and tail.
Figure 5. The origin of the asymmetric echinoderms from bilateral ancestors.
Woodgate et al wrote,
“In contrast to living echinoderms, however, the bilateral symmetry exhibited by Atlascystis is more similar to the body plans of hemichordates and chordates.”
Yes, but look how many taxa were omitted to come to that conclusion (Figs 3–5). This is the only time the term ‘chordate’ is used in the text.
Side note: At one time and for one English professor, Richard Jefferies, asymmetric echinoderms were chordate ancestors. See the calcichordate hypothesis.
Crinoids and their sessile relatives
turn the torso + tail into a stem to elevate the feeding structure off the sea floor.
Asymmetric echinoderms and their relatives
apparently were low-energy echinoderms that laid one side down on the seafloor without sensory organs or mobility organs. They had shapeless and variable armored outlines. So, they were lucky if food drifted in or walked into their oral cavity.
Atlascystis (Fig 1) appears have been basal to these increasingly asymmetrical sea floor dwelling echinoderms (Fig 5) with reduced ambulacra lengths.
References
Woodgate SC et al (5 co-authors including Zamora S) 2025. A new Cambrian stem-group echinoderm reveals the evolution of the anteroposterior axis, Current Biology. DOI: 10.1016/j.cub.2025.05.065
Zamora S, Rahman IA and Smith AB 2012. Plated Cambrian Bilaterians Reveal the Earliest Stages of Echinoderm Evolution. PLoS ONE 7(6): e38296. doi:10.1371/journal.pone.0038296
wiki/Ctenocystoidea
wiki/Cincta
wiki/Mitrate
Publicity
From DiscoverMagazine.com
“Echinoderm evolution took quite the circuitous route from the bilateral symmetry of Atlascystis to the five or more arms of modern starfish. The researchers followed this path by tracking the changes in ambulacra throughout their evolutionary history, which didn’t exactly follow a straight line.
“So originally the echinoderms had two ambulacra,” said Rahman. “There is then a reduction to one, before what we think is a duplication event of that one remaining arm-like structure to give rise to three and then later to five ambulacra.”
phys.org/news/2025-06-million-year-fossil-reveals-starfish.html
discovermagazine.com/planet-earth/discover-how-starfish-gained-five-arms-after-evolving-from-500-million-year
Source: https://pterosaurheresies.wordpress.com/2025/06/29/oldest-known-echinoderm-with-a-bilateral-body-plan-omits-chordate-and-nematode-outgroup-taxa/
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