Pikaia, the Middle Cambrian lancelet, poorly revisited in 2024

Lacalli 2024 looked at the origin of chordate somites
(= torso muscles also found in fish that enable wriggling for propulsion) by reexamining Middle Cambrian Pikaia (Figs 1–3).

Caption in Lacalli 2024 describing figure 1.
“The anterior end of Pikaia, oriented with the mouth facing down. The anterior appendages (ap) are generally interpreted as gills or gill-related structures, and repeat in register with the anterior segments (pink).

Those are not “gills or gill-related structures.” Those appendages are cirri as in related lancelets and nautili. They are feeding structures. Gills are internal.

“The pharynx, dorsal organ and ventral blood vessel are shown in outline. The point of the diagram is to show that there is no evidence for appendages positioned forward of the putative somite series, so if the latter indicates the beginning of the trunk, the region constituting what is effectively the head of Pikaia is exceedingly small.”

The preoral hood present in Pikaia and related taxa is overlooked by the author

Lacalli 2024 concluded,
“If Pikaia is a chordate, then even without a notochord it would represent a comparatively advanced member of that lineage based on the way its body is organized and the mode of swimming that organization implies.”

Phylogenetic analysis recovers last common ancestors preceding every node. That alone would determine if Pikaia was a chorate, even if it did not have a notochord, though that would be well neigh impossile.

“Pikaia is nevertheless difficult to place in relation to living chordates.

By contrast, Pikaia is easy to place in the LRT.

“It has, on the one hand, structures with no obvious chordate counterpart (e.g., the dorsal organ and the apparent head shield, or “anterior unit”, while on the other, the evidence for structures it should have, like the notochord and nerve cord, is equivocal.”

This is not true. One can demonstrate (Figs 1–3) all the elements are homologous with structures in  lancelets and related taxa.

“There are nevertheless questions concerning chordate origins that one can address using Pikaia as a point of reference, with that of somite origins being the most obvious. The problem, in essence, is how to convert a large coelom spanning the trunk to a repeating series of smaller coelomic compartments.”

Segmentation occurs in invertebrates and vertebrates.

“Ruppert’s ideas on the hemichordate collar cord provide a useful starting point by highlighting the role of structures of this type in innervating muscles distant from the body surface.”

The colloar cord is not useful. It is a derived trait.

“The extension of an initial, anterior invagination, to form an extended neural tube, would follow as the targets of innervation, the myotomes, were themselves replicated as an extended anteroposterior series.

Marine nematodes, like Enoplus meridionalis,  provide good ancestral taxa.

“In this scenario, the neural tube can be seen as an innovation at the anatomical level that solves a histological problem, of the inherent limitations of intraepithelial innervation”

That’s how the embryo develops, even in humans.

“Any chordate with somites would then be expected to have a neural invagination comparable to the collar cord that extends as far along the anteroposterior axis as the somites themselves, or a neural tube of comparable length.”

Wrong Bauplan.

“The argument for homology between the collar cord and the chordate nerve cord remains open, but the case in favor is supported by evidence at the cellular and molecular level of how similar the nervous systems of hemichordates and protochordates are, with vertebrates being, in some respects, an outlier.”

The author has it backwards. Hemichordates are derived, not primitive.

“The position of the neural tube in ancestral chordates would have depended on whether a notochord was present or not, which then begs the question of how the body was supported in the absence of a notochord.”

Add taxa. Looks at Enoplus, the nematode (Fig 4). Don’t speculate. Observe.

“Since improvements to the locomotory abilities and efficiency of ancestral chordates can be assumed to have been progressive, it is by no means clear that a structural support as substantial as a notochord would have been required in the first instance to act as a compression strut.”

Worms can’t swim. Lancelets can swim. That’s what a notochord gives to a taxon.

“Whether Pikaia has a notochord or supports its body by other means remains an unresolved problem. If the dorsal organ, whatever its tissue of origin, is in some way concerned with body support, then Pikaia is potentially representative of a transitional step in the sequence by which chordates evolved body support systems of increasing efficiency.”

Leaving the sea floor is what turns a nematode into a lancelet.

“And, if the dorsal organ is not simply an overlarge notochord, one would have to conclude that a notochord of typical chordate type evolved considerably later than the somite series.”

Probably in lock-step with one another.

“This would place the dorsal organ at center stage as an early but temporary solution to the problem of body support.”

Make that a permanent solution in a structure similar to or identical to cartilage. So body support once wriggled left, snaps back right in a continuous sine wave of open water locomotion.

“An alternative, that Pikaia represents an independent, now extinct chordate lineage with somites but no notochord, is also possible, or it could once have had a notochord that has been lost as its functions were taken over by the later evolution of the dorsal organ. There remain, in sum, a number of unresolved questions concerning this remarkable fossil and how it is to be interpreted.”

The terms ‘preoral hood; ‘cirri’, ‘lancelet’, ‘myxiia’, ‘lamprey, ‘hagfish’ and ‘nematode’ are not mentioned in the text,

Localli 2024 followed the outdated enteropneust Bauplan model.
Enteropneusts are derived lancelets with fewer, weirder parts. Better to use taxa related to Pikaia in the LRT. These are recovered by adding taxa.

References
Bardack D and Richardson ES Jr 1977. New aganathous fishes from the Pennsylvanian of Illinois. Fieldiana Geology 33(26):489–510.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Lacalli T 2024. The Cambrian fossil Pikaia, and the origin of chordate somites. BMC EvoDEvo 15:1.
Mallatt J and Holland ND 2013. Pikaia gracilens Walcott: stem chordate or already specialized in the Cambrian? Journal of Experimental Zoology, Part B, Molecular and Developmental Evolution 320B: 247–271.
Walcott CD 1911. Cambrian geology and paleontology. Middle Cambrian annelids. Smithsonian Miscellaneous Collections 57(5):109–144.

wiki/Myxine
wiki/Pikaia

Source: https://pterosaurheresies.wordpress.com/2024/06/12/pikaia-the-middle-cambrian-lancelet-poorly-revisited-in-2024/