This page covers the stem group of the cyclostomes (Clade Cyclostomata, subphylum Vertebrata), a group of extant primitive fishes that represents the jawless vertebrates. It contains two orders, the lampreys and the hagfishes.
As indicated in Figure 1 of the Vertebrates page, the Cyclostomata are monophyletic. This has long been the consistent result of molecular analysis (Janvier, 2015) although a recent paper (Theofanopoulou et al, 2021) suggests on the basis of molecular data that the cyclostomes might actually be paraphyletic. This is also the conclusion of some studies using morphological data (e.g. Larouche et al, 2017; Chevrinais et al, 2018; Clements et al, 2019) while other morphological analyses support monophyly (e.g. Keating and Donoghue, 2016; Hirasawa et al, 2016; Terrill et al, 2018; Miyashita et al, 2019). This issue remains unresolved, but we will follow, at least for now, the monophyletic interpretation given the strong molecular evidence for that view.
On the basis of phylogenetic analysis of morphological characteristics, several stem-group cyclostomes have been identified by Dearden et al (2023), as illustrated in the following phylogenetic time tree, constructed using the R package "strap" (Bell and Lloyd, 2015) and selecting the "equal" option that requires specification of the age of the root of the tree:
As indicated in Figure 1 of the Vertebrates page, the Cyclostomata are monophyletic. This has long been the consistent result of molecular analysis (Janvier, 2015) although a recent paper (Theofanopoulou et al, 2021) suggests on the basis of molecular data that the cyclostomes might actually be paraphyletic. This is also the conclusion of some studies using morphological data (e.g. Larouche et al, 2017; Chevrinais et al, 2018; Clements et al, 2019) while other morphological analyses support monophyly (e.g. Keating and Donoghue, 2016; Hirasawa et al, 2016; Terrill et al, 2018; Miyashita et al, 2019). This issue remains unresolved, but we will follow, at least for now, the monophyletic interpretation given the strong molecular evidence for that view.
On the basis of phylogenetic analysis of morphological characteristics, several stem-group cyclostomes have been identified by Dearden et al (2023), as illustrated in the following phylogenetic time tree, constructed using the R package "strap" (Bell and Lloyd, 2015) and selecting the "equal" option that requires specification of the age of the root of the tree:
Figure 1. Time tree of the stem-Cyclostomata. Root age specified as 529 million years. Note that in this tree stem-Vertebrata occur in a polytomy with cyclostomes and gnathostomes.
The above tree (Figure 1) is one of several possible alternatives presented by Dearden et al (2023) and by some other authors (Miyashita et al, 2021 and Reeves et al, 2023). All are agreed that the Anaspida (a group of scaly jawless fish) belong to the cyclostome stem group. However, depending on the analytical approach followed in deriving the phylogenetic tree, another group of animals, the Euconodonta, can appear either in the cyclostome stem group or in the crown group. Euconodonts are an infraclass of condonts, which are a group of animals known mainly by scattered elements of their feeding apparatus (Aldridge et al, 1993); not all of the latter are known with certainty to represent vertebrates, but the euconodonts, or "true" conodonts, have been classified as vertebrates since they were found as fossils in which their soft-tissue anatomy could be seen (Donoghue and Keating, 2014). The above tree, which is the one placed in the main text of Dearden et al (2023) places the euconodonts in the stem-Cyclostomata. This interpretation seems more reasonable than assignment to the crown cyclostomes because that would imply that the anaspids are younger than the crown node.
The oldest known fossil representative of the stem-Cyclostomata is Cambropustula kinnekullensis, the oldest known Euconodont, described from the Late Cambrian Alum Shale at the Gum quarry in Västergötland, Sweden (Müller and Hinz, 1991; Müller and Hinz-Schallreuter, 1998). No image of this species is available in the public domain, but the younger euconodont Promissum pulchrum is illustrated below with other members of the cyclostome stem group for which images are available in the public domain (click on image for larger version):
The oldest known fossil representative of the stem-Cyclostomata is Cambropustula kinnekullensis, the oldest known Euconodont, described from the Late Cambrian Alum Shale at the Gum quarry in Västergötland, Sweden (Müller and Hinz, 1991; Müller and Hinz-Schallreuter, 1998). No image of this species is available in the public domain, but the younger euconodont Promissum pulchrum is illustrated below with other members of the cyclostome stem group for which images are available in the public domain (click on image for larger version):
Figure 2. Images of stem-group cyclostomes
Two of the genera shown in the above tree, Birkenia and Rhyncholepis, are characterized by mineralized scales. They occupy a basal position relative to Ciderius, Cornovichthys and the euconodonts, which all lack scales. Reeves et al (2023) interpret suggest that the lack of mineralization seen in modern cyclostomes is a result of secondary loss in the cyclostome stem group; they interpret Lasanius problematicus, which has weakly-mineralized scales, as a transitional form between basal stem cyclostomes and the non-mineralized crown cyclostomes. However, such a transition is less clear from the above time tree (Figure 1) than it is from the phylogenetic tree presented by Reeves et al (2023), in that Euphanerops, Jamoytius and Achanarella, which have weakly- or non-mineralized scales, are not resolved as being crownward of Birkenia and Rhyncholepis.
Some idea of the nature of the transition from the stem group to the crown group of the cyclostomes can be derived from a comparison of the above images with the examples of early crown cyclostomes shown below:
Some idea of the nature of the transition from the stem group to the crown group of the cyclostomes can be derived from a comparison of the above images with the examples of early crown cyclostomes shown below:
Figure 3. Examples of early crown-Cyclostomata
Page last updated July 6th, 2026.
References
Aldridge, R. J., Briggs, D. E. G., Smith, M. P., Clarkson, E. N. K., & Clark, N. D. L. (1993). The anatomy of conodonts. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 340(1294), 405-421.
Bell, M. A., & Lloyd, G. T. (2015). strap: an R package for plotting phylogenies against stratigraphy and assessing their stratigraphic congruence. Palaeontology, Vol. 58, No. 2, pp. 379-389.
Chevrinais, M., Johanson, Z., Trinajstic, K., Long, J., Morel, C., Renaud, C. B., & Cloutier, R. (2018). Evolution of vertebrate postcranial complexity: axial skeleton regionalization and paired appendages in a Devonian jawless fish. Palaeontology, 61(6), 949-961.
Clements, T., Purnell, M., & Gabbott, S. (2019). The Mazon Creek Lagerstätte: a diverse late Paleozoic ecosystem entombed within siderite concretions. Journal of the Geological Society, 176(1), 1-11.
Dearden, R. P., Lanzetti, A., Giles, S., Johanson, Z., Jones, A. S., Lautenschlager, S., ... & Sansom, I. J. (2023). The oldest three-dimensionally preserved vertebrate neurocranium. Nature, 621(7980), 782-787.
Donoghue, P. C., & Keating, J. N. (2014). Early vertebrate evolution. Palaeontology, 57(5), 879-893.
Hirasawa, T., Oisi, Y., & Kuratani, S. (2016). Palaeospondylus as a primitive hagfish. Zoological letters, 2(1), 1-9.
Janvier, P. (2015). Facts and fancies about early fossil chordates and vertebrates. Nature, 520(7548), 483-489.
Keating, J. N., & Donoghue, P. C. (2016). Histology and affinity of anaspids, and the early evolution of the vertebrate dermal skeleton. Proceedings of the Royal Society B: Biological Sciences, 283(1826), 20152917.
Larouche, O., Zelditch, M. L., & Cloutier, R. (2017). Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates. BMC biology, 15(1), 1-26.
Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Currie, P. J. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146-2151.
Miyashita, T., Gess, R. W., Tietjen, K., & Coates, M. I. (2021). Non-ammocoete larvae of Palaeozoic stem lampreys. Nature, 591(7850), 408-412.
Reeves, J. C., Wogelius, R. A., Keating, J. N., & Sansom, R. S. (2023). Lasanius, an exceptionally preserved Silurian jawless fish from Scotland. Palaeontology, 66(2), e12643.
Terrill, D. F., Henderson, C. M., & Anderson, J. S. (2018). New applications of spectroscopy techniques reveal phylogenetically significant soft tissue residue in Paleozoic conodonts. Journal of Analytical Atomic Spectrometry, 33(6), 992-1002.
Theofanopoulou, C., Gedman, G., Cahill, J. A., Boeckx, C., & Jarvis, E. D. (2021). Universal nomenclature for oxytocin–vasotocin ligand and receptor families. Nature, 592(7856), 747-755.
Žigaitė, Ž., & Goujet, D. (2012). New observations on the squamation patterns of articulated specimens of Loganellia scotica (Traquair, 1898) (Vertebrata: Thelodonti) from the Lower Silurian of Scotland. Geodiversitas, 34(2), 253-270.
Bell, M. A., & Lloyd, G. T. (2015). strap: an R package for plotting phylogenies against stratigraphy and assessing their stratigraphic congruence. Palaeontology, Vol. 58, No. 2, pp. 379-389.
Chevrinais, M., Johanson, Z., Trinajstic, K., Long, J., Morel, C., Renaud, C. B., & Cloutier, R. (2018). Evolution of vertebrate postcranial complexity: axial skeleton regionalization and paired appendages in a Devonian jawless fish. Palaeontology, 61(6), 949-961.
Clements, T., Purnell, M., & Gabbott, S. (2019). The Mazon Creek Lagerstätte: a diverse late Paleozoic ecosystem entombed within siderite concretions. Journal of the Geological Society, 176(1), 1-11.
Dearden, R. P., Lanzetti, A., Giles, S., Johanson, Z., Jones, A. S., Lautenschlager, S., ... & Sansom, I. J. (2023). The oldest three-dimensionally preserved vertebrate neurocranium. Nature, 621(7980), 782-787.
Donoghue, P. C., & Keating, J. N. (2014). Early vertebrate evolution. Palaeontology, 57(5), 879-893.
Hirasawa, T., Oisi, Y., & Kuratani, S. (2016). Palaeospondylus as a primitive hagfish. Zoological letters, 2(1), 1-9.
Janvier, P. (2015). Facts and fancies about early fossil chordates and vertebrates. Nature, 520(7548), 483-489.
Keating, J. N., & Donoghue, P. C. (2016). Histology and affinity of anaspids, and the early evolution of the vertebrate dermal skeleton. Proceedings of the Royal Society B: Biological Sciences, 283(1826), 20152917.
Larouche, O., Zelditch, M. L., & Cloutier, R. (2017). Fin modules: an evolutionary perspective on appendage disparity in basal vertebrates. BMC biology, 15(1), 1-26.
Miyashita, T., Coates, M. I., Farrar, R., Larson, P., Manning, P. L., Wogelius, R. A., ... & Currie, P. J. (2019). Hagfish from the Cretaceous Tethys Sea and a reconciliation of the morphological–molecular conflict in early vertebrate phylogeny. Proceedings of the National Academy of Sciences, 116(6), 2146-2151.
Miyashita, T., Gess, R. W., Tietjen, K., & Coates, M. I. (2021). Non-ammocoete larvae of Palaeozoic stem lampreys. Nature, 591(7850), 408-412.
Reeves, J. C., Wogelius, R. A., Keating, J. N., & Sansom, R. S. (2023). Lasanius, an exceptionally preserved Silurian jawless fish from Scotland. Palaeontology, 66(2), e12643.
Terrill, D. F., Henderson, C. M., & Anderson, J. S. (2018). New applications of spectroscopy techniques reveal phylogenetically significant soft tissue residue in Paleozoic conodonts. Journal of Analytical Atomic Spectrometry, 33(6), 992-1002.
Theofanopoulou, C., Gedman, G., Cahill, J. A., Boeckx, C., & Jarvis, E. D. (2021). Universal nomenclature for oxytocin–vasotocin ligand and receptor families. Nature, 592(7856), 747-755.
Žigaitė, Ž., & Goujet, D. (2012). New observations on the squamation patterns of articulated specimens of Loganellia scotica (Traquair, 1898) (Vertebrata: Thelodonti) from the Lower Silurian of Scotland. Geodiversitas, 34(2), 253-270.
Image credits - Stem-Cyclostomes
- Figure 2 (Birkenia elegans): Ghedoghedo, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Birkenia elegans, life restoration): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Rhyncholepis parvulus): Ghedoghedo, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
- Figure 2 (Rhyncholepis parvulus, life restoration): Apokryltaros, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons
- Figure 2 (Lasanius problematicus): Open Access article Reeves, J. C., Wogelius, R. A., Keating, J. N., & Sansom, R. S. (2023). Lasanius, an exceptionally preserved Silurian jawless fish from Scotland. Palaeontology, 66(2), e12643.
- Figure 2 (Lasanius problematicus, life restoration): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Jamoytius kerwoodi): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Euphanerops longaevus): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- igure 2 (Ciderius cooperi): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Cornovichthys blaauweni): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 2 (Achanarella trewini): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Gilpichthys greenei): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Myxinikela siroka): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Priscomyzon riniensis): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license
- Figure 3 (Mayomyzon pieckoensis): Nobu Tamura under Creative Commons Attribution- ShareAlike (CC BY-SA) license