DESCRIPTION OF THE SUPERCLASS PETROMYZONTOMORPHI (NELSON 2006)
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SUPERCLASS PETROMYZONTOMORPHI LINKS
| Petromyzontomorphi (pet-ro-mi-ZON-to-MORF-i) is a combination of three Greek roots that mean rock [petra (πέτρα)] sucking [myzo (μυζώ)] forms [morphi (μορφή)]. The name refers to the way that lampreys attach to stones. The name was adopted by Nelson (2006), which was modified from Petromyzontiformes (Berg 1940, cited in Donoghue et al. 2000). |
| INTRODUCTION TO THE PETROMYZONTOMORPHI Like hagfish, lampreys are jawless, armorless fish without paired appendages; however, their mouth is rigid, conical, and lined with keratinized “teeth” (See Figure 1). Many lampreys are “parasitic” (really predatory), and use their mouths to attach to their prey, usually fish, after which they use their tooth-covered rasping tongue to dig into the bodies of their prey. They draw water into a common respiratory tube which exits through numerous gill openings. This mechanism allows them to continue to draw water across their gills while they are feeding (at which time their mouth is completely occluded). Lampreys breed in freshwater, where they may spend all or some of their lives. The ammocetes larva (Figure 2) is a small, amphioxus-like stage that is free-living in streams. Usually, they return to the sea where they become sexually mature. After a maturation period in the marine environment, lampreys return to freshwater streams where they reproduce. Lampreys, are similar to hagfish (Myxinomorphi) but differ from them in several important ways, chief among them is the absence of vertebrae in mature hagfish. They also differ in the ways they draw water into the pharynx and across the gills. Furthermore, lampreys have two semicircular canals in the middle ear and control the heart by the nervous system. Hagfish have a single semicircular canal and control heart rate by the endocrine system. Similarities between hagfish and lampreys were recognized early by Dumeril (1806), who called them collectively the Cyclostomata (round-mouths). Because of major anatomical differences between the groups, hagfish were considered to be basal craniates, but they are not vertebrates. The paraphyletic (text with tooltip) A paraphyletic group is a collection of taxa that is not natural or monophyletic. That is the taxa do not share apomorphic characters but are joined by shared primitive (synpleisiomorphic) characters. relationship between the lampreys and the hagfishes [see Benton (2005) and Janvier (2008a)] seemed to be the norm and was manifest in the Craniate Hypothesis (all animals with crania are not vertebrates). The view of hagfish-lamprey paraphyly was is not universally accepted, though. Molecular work tended to support the monophyly of the cytostomes (lampreys+hagfish) in opposition to the growing support for their separation based on morphology. For example, Stock and Whitt (1992, using rRNA), Kuraku et al. (1999, using nuclear DNA-coded genes), Delarbre et al. (2002, using mitochondrial DNA nucleotide sequence analyses, nuclear gene nucleotide sequence analyses, and amino acid sequence analyses), Furlong and Holland (2002, using 18S rDNA and mitochondrial genes and protein-coding nuclear DNA), Mallatt and Sullivan (2002, using 28S and 18S rDNA), and Heimberg et al. (2010, using microRNAs) all supported a robust monophyletic cyclostome clade that was sister to the gnathostomes. Since then, molecular work has consistently supported a monophyletic cyclostome clade. Resolution to the conflict between the anatomists and geneticists regarding this question finally came by careful studies of hagfish development. First reported by Ota et al. (2007), hagfish development appeared to be similar in major ways to other vertebrate animals. Ota et al. (2011) identified structures in the developing hagfish that were incipient vertebrae; thus, the fundamental anatomical separation of the hagfish from the other vertebrates vanished. Subsequent studies, especially Oisi et al. (2013) showed that the details of the embryology of hagfish and lampreys, especially of the pituitary and nasal organs, are very similar even though the adult forms are quite different. The arguments from evo-devo in this question caused Janvier (2013), a longtime advocate of the Craniate hypothesis, to declare, “the anatomical features that make present-day hagfish seem more primitive than lampreys are the result of a subsequent loss of features in hagfish during evolution.” |
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| FIGURE 1. An adult lamprey attached to the glass of an aquarium. Note the circular mouth with its keratinized “teeth”. Image from: http://people.cornellcollege.edu/b-hess/geo105/Misc.htm | FIGURE 2. a. The ammocetes larva of a lamprey. b. Branchiostoma, a lancet (bottom). Image from: a. http://biology.nebrwesleyan.edu/courses/Labs/Biology_of_Animals/Image %20Web%20Pages/Chordata/Ammocetes.html b. http://record.wustl.edu/2002/04-26-02/worm.html |
FIGURE 3. A cladogram that illustrates the position of the two classes of the Petromyzontomorphi in the jawless fishes is based primarily on Benton (2005). However, Petromyzontida (the lampreys) are presented as sisters to the Euphaneropsids as proposed by Janvier (2008a). Together with the Myxinomorphi they make up the Cyclostomata, which is sister to the other vertebrates. Euphaneropsida is in red because it has no living members.
| Lampreys were once thought to be living descendants of armored jawless fishes; however, their developmental history and fossil history (Mississippian to the present) suggest that they are primitive rather than being secondarily reduced. Benton (2005) proposes that lampreys are sisters to the Conodontomorphi (see the cladogram of the craniate classes based on Benton 2005), and the most basal of all the vertebrate groups (Figure 3). Janvier (2008), and Janvier and Arsenault (2007) consider the lampreys to be sisters to Euphanerops (Figure 4; see the cladogram of “agnathans” based on Janvier 2008a). Indeed, the naked, or nearly scaleless, Euphanerops has many of the same general characters common to lampreys: the same kind of tail and in-line fins, branchial basket, and gill openings. That they share these characters suggest that Euphanerops and other naked agnathans may be in the lamprey lineage; however, the characters may be primitive, in which case the true phylogenetic relationships remain unclear. |
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| FIGURE 4. A reconstruction of Euphanerops, a likely relative of the lamprey line. Note the many gill pouch openings and the strongly hypocercal tail. Image from: http://www.sepaq.com/pq/mig/miguasha/mig-en/euphanerops.html |
| LITERATURE CITED Benton, M. J. 2005. Vertebrate Paleontology. Third Edition. Blackwell Publishing, Malden, MA. Berg, L. S. 1940. Classification of Fishes, both Recent and Fossil. Travaux de l’Institute Zoologique de l’Academie des Sciences de l’URSS 5: 1-517. [in Russian with English translation 1947]. Delarbre, C., C. Gallut, V. Barriel, P. Janvier, and G. Gachelin. 2002. Complete mitochondrial DNA of the hagfish, Eptatretus burgeri: the comparative analysis of mitochondrial DNA sequences strongly supports the cyclostome monophyly. Molecular Phylogenetics and Evolution. 22: 184–192. Donoghue, P. C. J., P. L. Forey, and R. J. Aldridge. 2000. Conodont affinity and chordate phylogeny. Biol. Rev. 75: 191-251. Dumeril, A. M. C. 1806. Zoologie analytique methode naturelle de classification des animaux, rendue plus facile a l’aide de tableaux synoptiques. Allais, Paris. pp. i-xxxiii, 1-334. Furlong, R. F. and P. W. H. Holland. 2002. Bayesian phylogenetic analysis supports monophyly of Ambulacraria and of cyclostomes. Zoological Science. 19: 593-599. Janvier, P. 2008a. Early jawless vertebrates and cyclostome origins. Zoological Science. 25: 1045-1056. Janvier, P. and M. Arsenault. 2007. The anatomy of Euphanerops longaevus Woodward, 1900, an anaspid-like jawless vertebrate from the Upper Devonian of Miguasha, Quebec, Canada. Geodiversitas. 29(1): 143-216. Janvier, P. 2013. led by the nose. Nature. 493. 169-170. Kuraku, S., D. Hoshiyama, K. Katoh, H. Suga, T. Miyata. 1999. Monophyly of lampreys and hagfishes supported by nuclear DNA-coded genes. Journal of Molecular Evolution. 49: 729-735. Mallatt, J. and J. Sullivan. 1998. 28S and 18S rDNA sequences support the monophyly of lampreys and hagfishes. Molecular Biology and Evolution. 15(12): 1706-1718. Nelson, J. S. 2006. Fishes of the World. 4th edition. John Wiley and Sons, Inc. New York. Oisi, Y., K. G. Ota, S. Kuraku, S. Fugimoto, and S. Kuratani. 2013. Craniofacial development of hagfishes and the evolution of vertebrates. Nature, 493: 175-180. Ota, K. G., S. Fugimoto, Y. Oisi, and S. Kuratani. 2011. Identification of vertebra-like elements and their possible differentiation from sclerotomes in the hagfish. Nature Communications. 2:373 doi:10.1038/ncomms1355 Ota, K. G., S. Kuraku, and S. Kuratani. 2007. Hagfish embryology with reference to the evolution of the neural crest. Nature. 446: 672-675. Stock, D. W. and G. S. Whitt. 1992. Evidence from 18S ribosomal RNA sequences that lampreys and hagfishes form a natural group. Science. 257: 787-789. |
| By Jack R. Holt and Carlos A. Iudica. Last revised: 02/03/2018 |
