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There are many homologous characteristics between vertebrates and modern cephalochordates; however, the problem is that modern cephalochordates might have evolved 200 million years after the origin of the vertebrates along with lamprey larvae (Section 8-8). If this is the case, then many of their homologous characteristics with vertebrates result from the descent of cephalochordates from vertebrates. Likewise, phylogenomic studies also indicate that the urochordates are homologous with the vertebrates because they evolved from the vertebrates (Section 8-8).
Even though the fossil record and phylogenomic studies call into question the descent of vertebrates from modern cephalochordates or urochordates, there are other viable candidates for the vertebrate ancestor. Various researchers have proposed that vertebrates originated within the hemichordates, vetulicolians, annelid worms, or the cephalochordates (Cathaymyrus and Pikaia).
The vertebrate embryogenesis pattern with radial embryo cleavage is like that of other deuterostomes (hemichordates, cephalochordates, and vetulicolians), but it is different from annelid worms, which have spiral embryo cleavage; in addition, all deuterostomes have a mouth that forms in a second opening, but annelid worms and other protostomes have a mouth that forms in the blastopore. Proponents of annelid worms as the ancestors of vertebrates have argued that the annelid worm pattern is not opposite but is to the side and could shift to form the vertebrate mouth in a second opening.
The vertebrate body orientation is like cephalochordates but opposite from that of annelids. The dorsal (stomach and front) -ventral (back) orientation of vertebrates and cephalochordates are aligned while the vertebrates and annelid worms have the opposite orientation (Figure 9-2). Annelids and other protostomes have the nerve cord on the ventral side (stomach) while chordates have the nerve cord on the dorsal (back) side. The notochord is also common between vertebrates and cephalochordates. Because they have a stiff notochord, vertebrates and cephalochordates swim with a side-to-side motion. Instead of a notochord, annelid worms have an axochord filled with muscles and swim with up and down undulations. It appears that at least some of the vetulicolians also swam with a side to side motion. Unfortunately, there are no living vetulicolians so the much of the morphology and all of the DNA of the vetulicolians is unknown.
Figure 9‑2. Dorsal (back)-ventral (stomach) orientation of arthropods and other protostomes on the left and vertebrates and other chordates on the right. Credit L'ontogenese. Used here per CC BY-SA 3.0.
Figure 9‑3. Embryonic vertebrate brain. Credit: Nrets. Used here per CC BY-SA 3.0.
The modern cephalochordate nervous system and brain have the same pattern (Figure 9‑3) “consisting of a diencephalic forebrain, small midbrain, hindbrain, and spinal cord.” [1] The cerebral vesicle in cephalochordates is homologous to the diencephalic forebrain and midbrain in vertebrates, and the vertebrate spinal cord and hindbrain are homologous with the rest of the cephalochordate nerve cord (spinal cord).[2] The brain patterning genes in cephalochordates and vertebrates are also homologous. The following video describes the structural and genetic similarity of vertebrate and cephalochordate brains. Again, it should not be a surprise that modern cephalochordates are homologous with vertebrates if they descended from vertebrates and are thus degenerate vertebrates. In the following video, the assumption is that modern cephalochordates predated vertebrates although this assumption is questionable.
Figure 9‑4. Cathaymyrus from Chengjiang Lagerstatten (517 Ma), an early cephalochordate. Credit: Apokryltaros. Used here per CC BY-SA 4.0.
Cathaymyrus (Figure 9-4) in the Chengjiang Lagerstatten (517 Ma) and Pikaia (Figure 9-5) in the Burgess Shale (505 Ma) may not have been modern cephalochordates, but they might have been similar to cephalochordates and are generally classed within the chordates.
Morris and Caron evaluated over 100 fossils of Pikaia. It had a notochord, nerve cord, longitudinal blood vessel, and possible pharyngeal features, which also had appendages. It resembled the chordates although it is uncertain whether it was a chordate. Morris and Caron showed the swimming behavior with an up and down motion. [3] although Figure 9-5 shows a side to side motion. It had no eyes, and there was no mention of hearing, sense of smell, or brain. Thurston Lacalli described the muscles and swimming behavior of Pikaia. Muscles were not the same as cephalochordates and vertebrates. Lacalli stated that they were slow twitch muscles and thus the organism would not have been a fast swimmer. [4]
McMenamin described Cathaymyrus as follows:
"Assigned to the Cephalochordata, Cathaymyrus includes two, possibly synonymous, species, C. diadexus and C. haikoensis. Some researchers consider the genus to be ”a chordate of uncertain affinity”, with D. Shu calling it “an amphioxus-like creature”. The animal is remarkable for its elongate, narrowly tapered body divided by S-shaped myomeres. A notochord and a discontinuous gut trace may be present. The animal may possess a notochord but lacks a well-defined cranial region. Cathaymyrus is possibly synonymous with Zhongxiniscus." [5]
Figure 9‑5. Pikaia gracilens from Burgess Shale. A likely cephalochordate (505 Ma). Credit: Nobu Tomura. Used here per CC BY-SA 4.0.
In summary, there were several species in Chengjiang on the vertebrate side of chordates and others on the invertebrate side of chordates. Table 9-1 shows the timing of vertebrates and other early chordate fossils in Cambrian stages 3 and 4. Vertebrates and cephalochordates both appear in the fossil record during Cambrian Stage 3. The diversity of cephalochordates, vertebrates, and vetulicolians in Cambrian Stage 3 indicates that they began to evolve prior to Cambrian Stage 3.
Table 9-1. Fossil record of chordates and related phyla. [6] Haikouichthys common name changed to Ostracoderm.
Figure 9-6. Ostracoderms from class Osteostraci ("bony shields"). Credit: Philippe Janvier. Used here per CC BY 3.0.
Now that lampreys are no longer at the base of vertebrate evolution, the earliest fish might be classified as ostracoderms (jawless fish). Henry Gee thinks that vetulicolians are the most likely ancestors of the vertebrates because of possible microscopic early vetulicolians at the base of the Cambrian.[7] Ostracoderms have a hard armored shell on the anterior end of the body (Figure 9-6), which is similar to the vetulicolians; however, the first vertebrates in the fossil record, Haikouichthys, Myllokunmingia, and Metaspriggina, did not have a hardened anterior. It probably means that the anterior armor of the ostracoderms was derived after the origin of the vertebrates and that this is not evidence of descent from vetulicolians.
References
References
[1] Holland, L.Z. “Invertebrate origins of vertebrate nervous systems.” In Evolutionary Neuroscience, pp. 51-73. Academic Press, 2020. (Figure 4.2).
[2] Holland, Nervous systems.
[3] Morris, Simon Conway, and Jean‐Bernard Caron. "Pikaia gracilens Walcott, a stem‐group chordate from the Middle Cambrian of British Columbia." Biological Reviews 87, no. 2 (2012): 480-512.
[4] Lacalli, Thurston. "The Middle Cambrian fossil Pikaia and the evolution of chordate swimming." EvoDevo 3, no. 1 (2012): 1-6
[5] McMenamin, Mark AS. "Cambrian chordates and vetulicolians." Geosciences 9, no. 8 (2019): 354.
[6] McMenamin, Cambrian Chordates.
[7] Gee, Henry, Across the bridge: understanding the origin of the vertebrates. University of Chicago Press, 2018.
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Figure 8-43. Larval stage of the vertebrate lamprey. Credit Tracyanne. Used here per CC BY-SA 3.0
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