Deuterostomes

The deuterostomes include three phyla: chordates (includes vertebrates), echinoderms (starfish and sea urchins), and hemichordates (strange little animals). The chordate phylum includes three existing subphyla: urochordates (tunicates or sea squirts), cephalochordates, and vertebrates. This section lays the groundwork for the analysis of the origin of the vertebrates in Chapter 9.

Figure 8‑34. Rhabdopleura normani, Rhabdopleurida, Pterobranchia, Hemichordata. Credit: Sedgwick, A, Lister, JJ, Shipley, AE (1898) A student's textbook of zoology.

The hemichordates are one of the three deuterostome phyla. They include two subphyla, the pterobranches and the enteropneusts (acorn worms). Some scientists think that the hemichordate phylum is at or near the base of deuterostome evolution. The pterobranches (Figure 8-34) secrete a substance that forms the tube in which they live. They are filter feeders and extend branches from their tubes into the water to catch particles in the water. Their body includes a proboscis, collar, and trunk. The proboscis secretes the organic material that forms the tube.

The other subphyla of the hemichordates are the enteropneusts, some of which are like pterobranches and are filter feeders while other are detritivores (acorn worms, Figure 8-35), which means they burrow through the soil and pass mud through their bodies, filtering out the food in the mud. An enteropneust fossil was found in the Burgess Shale (517 Ma), which indicates that they were early deuterostomes.

Acorn worms have no eyes, brains, internal skeleton, or sense organs; however, acorn worms have pharyngeal gill slits, which are like vertebrate gills. Thus, some scientists connect them to the evolution of the vertebrates.

Figure 8-35. Enteropneust acorn worms. Credit: Johann Wilhelm Spengel, 1893.

The chordates include cephalochordates, urochordates, and vertebrates. They are united by four characteristics in at least some portion of the life cycle. The first is a notochord, which is a semiflexible rod and which is present for at least part of the life cycle. Second is a nerve cord in a hollow sheath. Third is a pharyngeal pouch, which develops into gills in fish and structures such as the middle ear cavity in vertebrates. Fourth, they have a muscular post-anal tail. They also have a mouth and anus like other coelomates. The following video describes cephalochordates and urochordates.

Figure 8‑36. Lancelet (cephalochordate). Credit: Hans Hillewaert. Used here per CC BY-SA 4.0.

After burying most of their bodies in the sand, cephalochordates extend a filter into the water column and feed on passing nutrients. Cephalochordates (Figure 8-36) have no sense of sight, smell, or hearing, no brain, and no vertebrae; however, they have some physical and DNA similarities with vertebrates.

Figure 8‑37. Pikaia gracilens. Smithsonian Miscellaneous Collections. Cambrian Geology and Paleontology, Vol II. By Charles Walcott. 1914. p. 139.

Pikaia fossils in the Burgess Shale (Figure 8‑37) and Cathaymyrus fossils in the Chengjiang Lagerstatten (Figure 9-4) might have been cephalochordates or were like cephalochordates; however, their identity is uncertain,

"For the Cambrian Period, there are two famous sites where an exceptional preservation of the soft tissues has occurred. One is the Burgess Shales, in Canada, and the other is Chengjiang. Fossils resembling chordates (the large animal group that includes the vertebrates) or vertebrates have been recorded from both, such as Pikaia from the Burgess, and Yunnanozoan and Cathaymyrus from Chengjiang. Although probably close relatives of the vertebrates, none of these fossils looks entirely familiar to the vertebrate specialist." [1]

Since Ernst Haeckel first proposed the idea in the 1860s, the scientific consensus has generally been that the vertebrates descended from a cephalochordate. Up until a paper published in March 2021 in the journal Nature,[2] most scientists thought that the first vertebrates in the fossil record were lamprey fish. They thought this because the larval phase of the lamprey looks like a cephalochordate. For over a century, scientists have thought that vertebrates descended from cephalochordates because of this similarity; however, the larval phase of the lamprey apparently evolved long after the Cambrian, according to this new study in Nature. It now looks like the larval phase of the lamprey and the similarity with cephalochordates is a case of convergent evolution in which species evolve to have the same form due to similar environmental conditions and needs. Because there was no larval phase in the early fish in the Chengjiang Lagerstatten, the lamprey is no closer to the origin of vertebrates than any other fish. In fact, the number of Hox gene clusters in the lamprey (7) probably means that the lamprey is further from the origin of fish than other fish. On the other end of the connection, there are many reasons (listed below) to doubt that cephalochordates are ancestral to vertebrates. This lack of physiological connection between early vertebrates and invertebrates probably means that the gap between invertebrates and vertebrates is growing larger.

Although there are many similarities between vertebrates and cephalochordates, the placement of cephalochordates at the base of vertebrate evolution is called into question by the discredited larval lamprey at the base of vertebrates and by a phylogenomic study of cephalochordates conducted in 2018.[3] At the time of Zhang's study, nobody had conducted a phylogenomic analysis of cephalochordates.[4] The phylogenomic study by Zhang et al. indicates a divergence of cephalochordates beginning approximately 100 million years ago (Figure 8-38). The following list indicates that modern cephalochordates evolved from lampreys in the last few hundred million years.

Lamprey larvae evolved 200 million years after the Cambrian
Cephalochordates are extremely similar to lamprey larvae (ammocoetes)
Phylogenomic analysis does not indicate the presence of modern cephalochordates prior to 100 million years ago.
Cephalochordates could be a branch of lampreys in which the lamprey larvae that never mature to become mature lampreys.
The interpretation of Cathaymyrus and Pikaia as Cambrian cephalochordates is uncertain.
Scientists say that the molecular clock of cephalochordates is "extremely slow." Assuming that the basis of this statement is the lack of difference between cephalochordate and vertebrate molecular clock genes, the lack of difference would make sense if they diverged in the last few hundred million years rather than 500 million years ago.

The concept that modern cephalochordates are degenerate vertebrates is not new. In 1910, Gaskell proposed that amphioxus (modern cephalochordate) is a degenerate vertebrate that had lost its skeleton and cranial regions. Although modern cephalochordates might have evolved from vertebrates, Pikaia and Cathaymyrus from the Cambrian might have been similar in morphology to the modern cephalochordates and might have been ancestral to vertebrates; however, making genetic comparisons between modern cephalochordates and vertebrates in order to understand the relationship between Cambrian invertebrates and vertebrates is not valid if modern cephalochordates evolved from vertebrates in the last few hundred million years. For example, modern cephalochordates have essentially the same brain pattern as vertebrates. This does not mean that the brain patterns of Pikaia and Cathaymyrus in the Cambrian were also the same. Because of the lack of continuity, it might make sense to classify Pikaia and Cathaymyrus as pseudocephalochordates. This discussion will be resumed in section 9-2.

Figure 8‑38. "Estimates of the divergence time frame of cephalochordate evolution using MCMCTree. The lower-case letters above the lilac bar represent internal node labels (nodes a–r). Each lilac bar represents the 95% CI of the corresponding estimate. Cephalochordates include Branchiostoma and Asymmetron. Credit: Zhang et al. [5]. Used here per CC-BY.

Figure 8‑39. Sea squirt (tunicate) internal anatomy. Credit: John Houseman. Used here per CC BY-SA 3.0.

Sea squirts (Figure 8-39, 8-40 and 8-42) are urochordates. They are also called tunicates, and ascidians. Tunicate is the subphylum and refers to the fact that it grows a tunic around itself. Ascidian in the class of tunicates that anchor themselves to the seafloor (benthic). The tunicate lifestyle is apparently successful since there are 3,000 tunicate species.

Tunicates suck water into a pharyngeal basket, which filters out food and transfers the food to the stomach. The internal organs are below the basket Figure 8-39). The tunic is constructed from polysaccharides, one of which is a form of cellulose, which is the material that give structural strength to plants. Scientists think that tunicates gained the ability to produce cellulose by gene transfer from a bacterium.

Figure 8‑40. Sea squirt (ascidian) and frog larval stage. Credit: British Museum (1901).

Many animals have two life stages, an early larval stage and an adult stage. The larval sea squirt is free swimming (8-40). It looks like a frog (vertebrate) tadpole (larval stage of the frog). It then buries its nose in the ocean mud as an adult and grows a cellulose tunic around itself. Because sea squirts begin as juvenile fish, later develop a tunic around themselves, some of which bury their noses in the sand and become nonmoving animals, most scientists have thought that the sea squirts evolved from a free-swimming fish, rather than vice-versa. Likewise, the frog tadpole and the frog are known to have evolved from a free-swimming fish.

Figure 8-41. "Reconstruction of the Cambrian tunicate, Shankouclava anningense, of the Chengjiang Fauna of Middle Cambrian China." Credit: Apokryltaros. Used here per CC BY-SA 3.0.

The tunicates share several common features with vertebrates: “All share segmentation of the muscles of the body wall, separation of upper and lower nerve and blood vessel branches, and many newly evolved hormone and enzyme systems.” [6] Thus, there is a close evolutionary relationship, but the question is in what direction did evolution proceed, from vertebrate to tunicate or from tunicate to vertebrate?

Phylogenetics is the study of the evolutionary history of a group. Scientists have interpreted the Cambrian fossil, Shankouclava, as an early ascidian tunicate, the class of tunicates that anchor themselves to the sea floor; however, recent phylogenetics papers indicate that the tunicates evolved long after the vertebrates, which would imply that they are degenerate vertebrates rather than vertebrate ancestors.[7] In addition, the phylogenetic studies indicate that the first tunicates were free swimming and then gradually proceeded toward sessile organisms (stationary ascidians) that anchored to the sea floor. This is significant because Shankouclava (Figure 8-41) is the only supposed tunicate in the fossil record of the Cambrian Period and is interpreted as a tunicate that anchors itself to the sea floor (Ascidian). This interpretation is based on the extension, which could be an anchor or holdfast:[8] however, the extension could also be the tail of an arthropod. After Shankouclava, there are no tunicates in the fossil record for hundreds of millions of years. Molecular estimates of DNA variation between species indicates that the tunicates first evolved 447 Ma. [9] The first of the Ascidian tunicates, the type that anchor to the sea floor, was the Stolidobranchiata [10], which, according to DNA molecular clock studies, appeared less than 400 Ma. [11] Thus, Giribet places the origin of the tunicates after the origin of the vertebrates. [12]

Figure 8‑42. A colony of sea squirts. Credit: Samuel Chow. Used here per CC BY 2.0.

The sea squirt life cycle begins with a notochord, a segmented v-shaped muscle structure, and a coelem, which is a fluid filled gap between the internal organs and the muscles, as are found in chordates. However, in the sea squirt, the notochord and muscle structure vanish, and the unnecessary coelom stops developing in the nonmoving tunic. The organs are safely set in permanent position within the tunic. If the notochord, muscle structure, and coelom vanish in tunicates, and this might indicate that the ancestor of sea squirts was free swimming.

Linda Holland evaluated the relationships between tunicates and cephalochordates/vertebrates (Figure 8-43). She stated, “Tunicates have clearly lost much of what their ancestor had, in terms of both structures and genes, making it quite difficult, if not impossible, to reconstruct this long extinct ancestor.” [13] Why devote so much of this section to the evolutionary relationship between tunicates and vertebrates? It is important because there are some unique genes in tunicates and vertebrates that are not in cephalochordates and other invertebrates. Whether they evolved from vertebrates or are ancestral to vertebrates is an important question.

Figure 8-43. A. Cephalochordate, B. Larval tunicate, C. Adult tunicate. 1. Notochord, 2. Nerve chord, 3. Buccal cirri, 4. Pharynx, 5. Gill slit, 6. Gonad, 7. Gut, 8. V-shaped muscles, 9. Anus, 11. Inhalant syphon, 12. Exhalant syphon, 13. Heart, 14. Stomach, 15. Esophagus, 16. Intestines, 17. Tail, 18. Atrium, 19. Tunic. Credit: Basketball1713 (Wikipedia). Used here per CC BY-SA 4.0.

Figure 8‑44. Reconstruction of various members of the vetulicola phylum and the yunnanozoan in the Maotianshan shale biota (Chengjiang biota), of the Early Cambrian. From top, Yuyuanozoon magnificissimi, Heteromorphus longicaudatus, Vetulicola cuneata, Xidazoon stephanus, and Yunnanozoon lividum. Credit: Apokryltaros. Used here per CC BY 2.5.

The Chengjiang Lagerstatten (517 Ma) contains several fossils that are classified within the vetulicolan phlylum (Figure 8‑44). They were first interpreted as arthropods, but lack of segmentation and the fact that they seem to have pharyngeal slits (the row of holes on the side in Figure 8‑44) has caused many paleontologists to classify them as early deuterostomes.[14] They have an enlarged anterior forebody with five gill slits on the sides. Segmented tails with an anus at the end of the tail extend in various ways from the forebody (Figure 8‑44).

Like the early jawless vertebrates, vetulicolians had a cylindrical mouth with spines in concentric rings. The large anterior ends of the fossils were filled with sediment, which might indicate that they were partially fluid filled. Recently, scientists observed that some of the anterior ends were filled with possibly small symbiotic parasite worms. The fossils consist of casts, but nothing remains of the insides. On some, the anterior ends appear to include seven large plates while other vetulicolians are classified as soft. The posterior end is generally a tail, which has a notochord, The vetulicolians were possibly filter feeders or possibly planktivores.

Figure 8‑45. Sea urchins (echinoderms). Credit: Nick Hobgood. Used here per CC BY-SA 3.0.

The deuterostome phylum, Echinodermata, includes starfish and sea urchins (Figure 8-45). Molecular (DNA) evidence groups the echinoderms, chordates, and hemichordates as close evolutionary cousins.[15] The echinoderms have a larval stage like hemichordates, [16] but the Echinodermata develop five arms. Valerie Morris showed that the pentaradiality of the sea urchin originates in the gills of hemichordate-like echinoderm larvae: the legs grow out through the gills. [17] This indicates that the pentaradiality of the echinoderms originated in a hemichordate ancestor. [18] The first echinoderms (sea urchins) appear early in the fossil record at approximately at 520 Ma (Cambrian stage 3). Other echinoderms, such as starfish and sea cucumbers, appeared much later in the fossil record and were further evolutionary developments within the echinoderms. Sometimes, there are fossils of a certain phylum in in one paleoecological environment but not in others. There are no fossil enchinoderms in the Chengjiang fauna or other Burgess Shale siliclastic environments; however, there are early echinoderm fossils (Figure 8‑46) in carbonate environments from the Cambrian. [19] Early echinoderms did not have spines, but spines evolved in sea urchins to protect the animals from predators. Although sea urchins look stationary, they move on their tube feet that are powered by water pressure. Starfish also have tube feet.

Figure 8-46. Early echinoderms. Credit: Nobu Tomura. Used here per CC BY-SA 4.0

References

[1] Janvier, Phillippe. Rise of the Dragon: Readings from Nature on the Chinese Fossil Record (2001): 53.

[2] Miyashita, Tetsuto, Robert W. Gess, Kristen Tietjen, and Michael I. Coates. "Non-ammocoete larvae of Palaeozoic stem lampreys." Nature 591, no. 7850 (2021): 408-412.

[3] Zhang, Qi-Lin, Guan-Ling Zhang, Ming-Long Yuan, Zhi-Xiang Dong, Hong-Wei Li, Jun Guo, Feng Wang, Xian-Yu Deng, Jun-Yuan Chen, and Lian-Bing Lin. "A phylogenomic framework and divergence history of Cephalochordata amphioxus." Frontiers in physiology 9 (2018): 1833.

[4] Zhang, Phylogenomic.

[5] Zhang, Phylogenomic.

[6] Fastovsky, David E., and David B. Weishampel. Dinosaurs: a concise natural history. Cambridge University Press, 2016.

[7] Giribet, Gonzalo. "Phylogenomics resolves the evolutionary chronicle of our squirting closest relatives." BMC biology 16, no. 1 (2018): 49.

[8] Chen, Jun-Yuan, Di-Ying Huang, Qing-Qing Peng, Hui-Mei Chi, Xiu-Qiang Wang, and Man Feng. "The first tunicate from the Early Cambrian of South China." Proceedings of the National Academy of Sciences 100, no. 14 (2003): 8314-8318.

[9] Delsuc, Frédéric, Hervé Philippe, Georgia Tsagkogeorga, Paul Simion, Marie-Ka Tilak, Xavier Turon, Susanna López-Legentil, Jacques Piette, Patrick Lemaire, and Emmanuel JP Douzery. "A phylogenomic framework and timescale for comparative studies of tunicates." BMC biology 16, no. 1 (2018): 39.

[10] Giribet, Chronicle.

[11] Delsuc, Tunicates.

[12] Giribet, Chronicle.

[13] Holland, L. Z. "Invertebrate origins of vertebrate nervous systems." In Evolutionary Neuroscience, pp. 51-73. Academic Press, 2020.

[14] Gee, Henry. Across the Bridge: Understanding the Origin of the Vertebrates. University of Chicago Press, 2018.

[15] A Smith, K Peterson, G. Wray, D. Littlewood, From bilateral symmetry to pentaradiality. In Assembling the Tree of life, J Cracraft and M. Donoghue (NY: Oxford University Press, 2004), pp. 365-383.

[16] Smith, Bilateral.

[17] Valerie Morris, Origins of radial symmetry identified in an echinoderm during adult development and the inferred axes of ancestral bilateral symmetry, Proceedings of the Royal Society (2007) 274(1617): 1511-1516.

[18] Smith, Bilateral.

[19] Clausen, Sébastien, Xian-Guang Hou, Jan Bergström, and Christina Franzén. "The absence of echinoderms from the Lower Cambrian Chengjiang fauna of China: Palaeoecological and palaeogeographical implications." Palaeogeography, Palaeoclimatology, Palaeoecology 294, no. 3-4 (2010): 133-141.

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"Diversity of Deuterostomia. Mediterranean red sea star (Echinaster sepositus), wild boar (Sus scrofa), Oscar (Astronotus ocellatus), Bennett's feather star (Oxycomanthus bennetti), Panther chameleon (Furcifer pardalis), Ox heart ascidian (Polycarpa aurata), Green sea urchin (Strongylocentrotus droebachiensis) and (Tockus nasutus epirhinus). Credit: Charles James Sharp - File:Zeester.JPG, File:A young wild boar in his environment.jpg, File:Astronotus ocellatus.jpg, (File:Sea Squirts (Polycarpa aurata) (8474256286).jpg), File:Comasteridae - Oxycomanthus bennetti-001.jpg, File:Panther chameleon (Furcifer pardalis) female Montagne d’Ambre (2).jpg, Sea Squirts (Polycarpa aurata), File:Urchin.jpg, Sharp Photography, sharpphotography.co.uk (File:African Grey Hornbill (Lophoceros nasutus epirhinus) female.jpg). Public domain.

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