Nanglu et al 2023 wrote:
“Tunicates are an evolutionarily significant subphylum of marine chordates, with their phylogenetic position as the sister-group to Vertebrata making them key to unraveling our own deep time origin.
False. Don’t cite this myth. Test it. Paleo authors cite myths all the time with Ornithodira, Parareptilia, Cetacea, Afrotheria, etc. It’s lazy, trusting and therefore unscientific.
“Tunicates greatly vary with regards to morphology, ecology, and life cycle, but little is known about the early evolution of the group, e.g. whether their last common ancestor lived freely in the water column or attached to the seafloor.”
Adding taxa resolves this problem. Peters 1991 did it in a children’s book.
“Here we describe Megasiphon thylakos nov., a 500-million-year-old tunicate from the Marjum Formation of Utah, which features a barrel-shaped body with two long siphons and prominent longitudinal muscles.
Megasiphon is shown in figure 1 (bottom). The atrium exit near the atrium entrance indicates this taxon was more derived in the Middle Cambrian than several extant sea squirts.
Nanglu et al 2023 continue:
“The ascidiacean-like body of this new species suggests two alternative hypotheses for early tunicate evolution. The most likely scenario posits M. thylakos belongs to stem-group Tunicata, suggesting that a biphasic life cycle, with a planktonic larva and a sessile epibenthic adult, is ancestral for this entire subphylum”.
“Most likely scenario” = tradition, myth, untested assumption, personally preferred.
“Alternatively, a position within the crown-group indicates that the divergence between appendicularians and all other tunicates occurred 50 million years earlier than currently estimated based on molecular clocks.
Don’t trust molecules (= DNA). Genomic testing has a history of failing too often.
Ultimately, M. thylakos demonstrates that fundamental components of the modern tunicate body plan were already established shortly after the Cambrian Explosion.”
This is true. However, this is contra the “astonishing” headline in Science Daily, who wrote, “reveals astonishing secrets of tunicate origins.”
Since Nanglu et al cited Delsuc 2006,
let’s see what they wrote, “Tunicates or urochordates (appendicularians, salps and sea squirts), cephalochordates (lancelets) and vertebrates (including lamprey and hagfish) constitute the three extant groups of chordate animals”.
Now we know cephalopods are also chordates. The siphuncle is the notochord.
In the world of monophyly, vertebrates are cephalocordates. So are tunicates.
“Traditionally, cephalochordates are considered as the closest living relatives of vertebrates, with tunicates representing the earliest chordate lineage1,2.
Tradition doesn’t cut it any more. Testing reveals tunicates are derived from lancelets, which are derived from enoplid nematodes (Fig 1), not the other way around.
“This view is mainly justified by overall morphological similarities and an apparently increased complexity in cephalochordates and vertebrates relative to tunicates”
Increased complexity doesn’t cut it anymore. Sessile organisms lose body parts they no longer need. Active organisms don’t arise from sessile ones.
“Despite their critical importance for understanding the origins of vertebrates3, phylogenetic studies of chordate relationships have provided equivocal results. Taking advantage of the genome sequencing of the appendicularian Oikopleura dioica, we assembled a phylogenomic data set of 146 nuclear genes (33,800 unambiguously aligned amino acids) from 14 deuterostomes and 24 other slowly evolving species as an outgroup.
We’ve learned not to trust genomic deep time studies. Enoplid nematodes (Fig 1) were not tested in the 2006 or 2023 studies. Don’t omit pertinent outgroups.
Here we show that phylogenetic analyses of this data set provide compelling evidence that tunicates, and not cephalochordates, represent the closest living relatives of vertebrates.”
Trait analysis with more taxa does not confirm that “compelling evidence”.
References
Delsuc F, Brinkmann H, Chourrout D and Philippe H 2006. Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature 439, 965–968.
Nanglu N, Lerosey-Aubril R, Weaver JC and Ortega-Hernández J 2023. A mid-Cambrian tunicate and the deep origin of the ascidiacean body plan. Nature Communications 14 (1) DOI: 10.1038/s41467-023-39012-4
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Publicity
Science daily wrote this headline:
“Discovery of 500-million-year-old fossil reveals astonishing secrets of tunicate origins.”
Then backtracked in this summary:
“Researchers describe a 500 million-year-old tunicate fossil species. The study suggests that the modern tunicate body plan was already established soon after the Cambrian Explosion.”
In reality: Megasiphon was already highly derived in the Middle Cambrian, so nothing to do with tunicate origins or ancestry.
BTW: Primitive fish were already present in the Early Cambrian. So nematode, lancelet, tunicate, echinoderm origins further back into the Ediacaran.
On the author’s prompting, Science Daily wrote,
“Tunicate’s close relationship to vertebrates makes studying them critical for understanding our own evolutionary origins.”
This is false. Tunicates and chordates were already well on their own ways when Megasiphon (Fig 1) appeared in the Middle Cambrian.
sciencedaily.com/releases/2023/07/230706124616.htm
The Pterosaur Heresies 