Complete

Phylogeny of Stegocephalians

Michel Laurin

The phylogeny of stegocephalians is controversial. Comparisons between various phylogenies are complicated by partly overlapping sets of terminal taxa. However, recent studies by Carroll (1995), Laurin and Reisz (1997, 1999), Laurin (1998a-c), and Vallin and Laurin (2004) agree in most respects. The minor differences between them reflect primarily the choice of included taxa. Here is a strict consensus of the four most parsimonious trees found by Carroll (1995):

     ========================================== Eusthenopteron
|
| ======================================= Panderichthys
| |
| | ==================================== Acanthostega
| | |
| | | ================================= Ichthyostega
| | | |
| | | | ============================== Crassigyrinus
| | | | |
| | | | | =========================== Loxommatids
| | | | | |
| | | | | | ===================== Colosteids
| | | | | |=====|
| | | | | | ===================== Temnospondyls
| | | | | |
| | | | | | ===================== Anthracosaurs
=====| | | | | | |
===| | | | | | ================== Seymouriamorphs
===| | | | | |
===| | | | | ============ Westlothiania
===| | | | |
===| | | ===| ========= Diadectomorphs
| | | | ===|
| | | | ========= Amniotes
======| | | __
===| | ============ Nectrideans |
===| | |
| |=========== Microsaurs |
| | |
===| === Adelogyrinids |Lepospondyls
| ===| |
======| === Aïstopods |
| |
====== Lysorophids __|

The phylogenies of Carroll (1995), Laurin and Reisz (1997), and Laurin (1998a-c) differ from previous ones in placing lepospondyls (an extinct group of small amphibians that superficially resembled salamanders and snakes) closer to amniotes than to seymouriamorphs. Indeed, few previous studies dealt in any detail with the systematic position of lepospondyls and even the monophyly of this group has been repeatedly questioned (Carroll, 1988 and 1995; Panchen and Smithson, 1988).

Laurin and Reisz (1997), Laurin (1998a-c), and Vallin and Laurin (2004) agreed with Carroll (1995) in most respects, but they also suggested that lissamphibians are derived from lepospondyls:

==================================================== Osteolepiformes
|
| ================================================== Panderichthyidae
| |
| | ============================================== Acanthostega
| | ==|
| | | ============================================== Ichthyostega
| | |
| | | ============================================== Crassigyrinus
| | | |
| | | | ============================================ Loxommatidae
| | | | |
| | | | | =========================== Colosteidae
| | | | | |
| | | | | | ========================= Dendrerpeton
| | | | | | |
| | | | | ==Temnospondyli| | ======================= Ecolsonia
| | | | | | ==| |
| | | | | | ==| ===================== Tersomius
| | | | | | ==|
| | | | | | ===================== Amphibamus
| | | | | |
| | | | | | ============================ Proterogyrinus
| | | | | | ==Embolomeri|
| | | | | | | ============================ Archeria
| | | | | | |
| | | | | | | ====================================== Gephyrostegidae
| | | | | | | |
| | | | | | | | =================== Kotlassia
| | | | | | | | |
| | | | | | | | ==Seymouriamorpha| ================= Seymouria
| | | | | | | | | ==|
| | | | | | | | | ================= Ariekanerpeton
| | | | | | | | |
| | | | | | | | | ============================== Aistopoda
| | | | | | | | | ==|
| | | | | | | | | | ============================== Adelogyrinidae
====| | | | | | | | | |
==| | | | | | | | | ============================== Nectridea
==| | | | | | | | |
| | | | | | | | | ============================ Pantylus
==| | | | | | | | |
==| | | | | | | | ========================== Rhynchonkos
==| | | | | | | |
| | | | | | | | ======================== Brachystelechidae
| | | | | | | | |
| | | | | | | | | ====================== Lysorophia
| | | | | | | | | |
==| | | | | | | | | ==== Triadobatrachus
| | | ==| | | | | | |
==| | | | | | | | | ====| == Discoglossidae
==| | ==| | | | | | ==|
| | ==| | | | | == Pipidae
| | ==| | | |
==| ==| | |======= Hynobiidae
| ==| |
| ==Lissamphibia|======= Sirenidae
| |
| |======= Proteidae
| |
| | ==== Eocaecilia
| | |
| ====| == Rhinatrematidae
| ==|
| == Ichthyophiidae
|
| ================ Limnoscelis
| ==Diadectomorpha|
| | ================ Diadectes
| |
==| ======================= Synapsida
| |
==Amniota| ===================== Captorhinidae
==|
===================== Procolophonidae

However, most scientists have argued that lissamphibians are the descendants of temnospondyls (Bolt, 1969; Milner, 1988; Panchen and Smithson, 1988; Trueb and Cloutier, 1991; Ruta et al. 2003a, b). Comparing the support for these theories is difficult because simply listing synapomorphies of a clade may give only part of the relevant data, but these two theories may be compared by listing synapomorphies present in lissamphibians and their putative closest known relatives (lysorophians, a group of lepospondyls, and the temnospondyl Apateon). In both cases, these synapomorphies appear at various levels, some being unique to lysorophians and lissamphibians (and to Apateon and lissamphibians), some being shared with other lepospondyls (and dissorophoid temnospondyls). Since the lists of characters below are given to explain the relative support of both theories, only synapomorphies unique to lissamphibians and one of the two putative ancestral group (lepospondyls or temnospondyls) have been retained in the lists below; derived characters present in at least some temnospondyls and some lepospondyls have not been retained.

Synapomorphies present in lissamphibians and lysorophians include (Laurin and Reisz, 1997; Laurin 1998a, b):

Synapomorphies of lissamphibians and some temnospondyls include (Trueb and Cloutier, 1991):

The computer-assisted phylogenetic analyses of Carroll (1995), Laurin and Reisz (1997), and Laurin (1998a-c) considered most of the major groups of terrestrial vertebrates. Earlier studies based on computer-assisted phylogenetic analysis of a data matrix (Gauthier et al., 1988; Trueb and Cloutier, 1991) considered a subset of these taxa. Therefore, the results of these study are difficult to compare with those of Carroll (1995), Laurin and Reisz (1997), and Laurin (1998a-c). However, some studies not based on a data matrix considered all or most of the relevant taxa (Milner, 1988 and 1993; Panchen and Smithson, 1988).

All the phylogenies discussed below suggest that living amphibians are derived from temnospondyls.

Panchen and Smithson (1988) suggested that lepospondyls were a paraphyletic group of amphibians. They proposed two phylogenies because they did not agree about the affinities of Anthracosauroideae (embolomeres). Smithson suggested the following:

                 === Temnospondyli + Lissamphibia
===|
===| === Microsauria
===| |
===| | ====== Colosteidae
| | |
| | ========= Nectridea
| |
| ============ Ichthyostegidae
|
=====| ============ Loxommatoidea
| |
| | ========= Crassigyrinus
| | |
| | | === Anthracosauroideae
===| | ===|
===| | === Seymouriamorpha
===|
| === Diadectomorpha
===|
=== Amniota

Panchen preferred this tree:

                 === Temnospondyli + Lissamphibia
===|
===| === Microsauria
===| |
===| | ====== Colosteidae
| | |
| | ========= Nectridea
| |
| ============ Ichthyostegidae
|
=====| ============ Loxommatoidea
| |
| | ====== Crassigyrinus
| | ===|
| | | ====== Anthracosauroideae
===| |
===| ====== Seymouriamorpha
| |
===| === Diadectomorpha
===|
=== Amniota

Trueb and Cloutier (1991) provided the most thorough account of the theory that living amphibians are derived from temnospondyls and based their argument on a computer- assisted analysis of a large data matrix. They suggested that branchiosaurs, a group of small and possibly immature temnospondyls, were the closest relatives of lissamphibians. Indeed, Branchiosauridae was a paraphyletic group in their phylogeny:

     ==================================== Edopoidea
|
| ============================== Trimerorhachoidea
| ===|
| | ============================== Dendrerpeton
| |
| | ============================== Eryopoidea
| | |
| | | ======================== Actiobates
| | | ===|
| | | | ======================== Trematops
| | | |
| | | | ======================== Broiliellus
| | | | |
| | | | |======================= Dissorophus
| | | | |
=====| | | | | =============== Tersomius
===| | | | |
| | | | ===| ============ Amphibamus
===| | | | ===|
===| | | ============ Doleserpeton
| | |
===| | =============== Micromelerpeton
| | | ____
======| | ============ Leptorophus |
| | | |
| | | ========= Schoenfelderpeton |
===| | | | Branchiosaurs
===| |======== Branchiosaurus |
===| |
| === Apateon |
======| ____|
=== Lissamphibia

Trueb and Cloutier (1991) considered more temnospondyl taxa than any other published phylogenetic analysis. However, their matrix and their phylogeny include only temnospondyls and lissamphibians. Therefore, the results of their study were constrained by the assumption that lissamphibians were derived from temnospondyls.

Most scientists now agree that the three main groups of modern amphibians (anurans, urodeles, and gymnophiones) are closely related to each other, but a few dissenting opinions have been expressed (Carroll and Currie, 1975; Carroll and Holmes, 1980). However, all computer-assisted phylogenetic analyses based on morphological characters have suggested that extant amphibians form a closely related group that excludes all known Paleozoic taxa.

The phylogeny of Carroll (1995) suggest that only diadectomorphs and Westlothiana are closely related to amniotes. Laurin (1998a) further suggested that even Westlothiana is not more closely related to amniotes than to lissamphibians. Most previous studies have suggested that at least embolomeres, seymouriamorphs, Gephyrostegus, and Solenodonsaurus were more closely related to amniotes than to lissamphibians (Carroll, 1970; Gauthier et al., 1988; Panchen and Smithson, 1988). However, only Gauthier et al. (1988) based their results on a computerized analysis of a data matrix, and their results were constrained by their choice of taxa, as can be seen in their phylogeny:

     =============== Amphibia (temnospondyls and lissamphibians)
|
| ============ Anthracosauroideae (embolomeres)
| |
| | ========= Seymouriamorpha
=====| | |
===| | ====== Solenodonsaurus
===| |
===| === Diadectomorpha
===|
=== Amniota (mammals, reptiles, and birds)

Amphibia was one of the outgroups in Gauthier et al. (1988), and lepospondyls were not included in their analysis. Therefore, it is difficult to compare the results of Gauthier et al. (1988) with the more recent analyses (Carroll, 1995; Laurin and Reisz, in press).

Lombard and Sumida (1992) reviewed recent work on stegocephalians and proposed the following phylogeny by combining the results of several analyses. Their phylogeny represents the accepted consensus in the early 1990s:

     ================== Ichthyostegalia
|
| ========= Temnospondyli *
| ===|
| | ========= Microsauria
| ===|
| | | ========= Nectridea
| | ===|
| | ========= Aistopoda
=====| |
===| ============ Loxommatoidea
| |
| | ========= Anthracosauroidea
| | |
===| | ====== Seymouriamorpha
===| |
===| === Diadectomorpha
===|
=== Amniota

* Even though Lombard and Sumida (1992) did not explicitly state this, their Temnospondyli probably included Lissamphibia.

All the phylogenies mentioned above suggest that extant amphibians form a monophyletic group that is closely related to amniotes. Therefore, the controversy lies entirely on the identity of the extinct relatives of extant amphibians and amniotes.

References

Bolt, J. R. 1969. Lissamphibian Origins: Possible Protolissamphibian from the Lower Permian of Oklahoma. Science 166: 888-891.

Carroll, R. L. 1964. Early evolution of the dissorophid amphibians. Bulletin of the Museum of Comparative Zoology 131: 161-250.

Carroll, R. L. 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society B257: 267-308.

Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. New York: W. H. Freeman and Company.

Carroll, R. L. 1995. Problems of the phylogenetic analysis of Paleozoic choanates. Bulletin du Muséum national d'Histoire naturelle de Paris 4ème série 17: 389-445.

Carroll, R. L. & P. J. Currie. 1975. Microsaurs as possible apodan ancestors. Zoological Journal of the Linnean Society 57: 229-247.

Carroll, R. L. & R. Holmes. 1980. The skull and jaw musculature as guides to the ancestry of salamanders. Zoological Journal of the Linnean Society 68: 1-40.

Fracasso, M. A. 1983. Cranial osteology, functional morphology, systematics and paleoenvironment of Limnoscelis paludis Williston. PhD Thesis, Yale, 624 pp.

Gauthier, J., A. G. Kluge, & T. Rowe. 1988. The early evolution of the Amniota. In Michael J. Benton (ed.) The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds: 103-155. Oxford: Clarendon Press.

Laurin M. 1998a. The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I-systematics, middle ear evolution, and jaw suspension. Annales des Sciences Naturelles, Zoologie, Paris, 13e Série 19: 1-42.

Laurin M. 1998b. The importance of global parsimony and historical bias in understanding tetrapod evolution. Part II-vertebral centrum, costal ventilation, and paedomorphosis. Annales des Sciences Naturelles, Zoologie, Paris, 13e Série 19: 99-114.

Laurin M. 1998c. A reevaluation of the origin of pentadactyly. Evolution 52: 1476-1482.

Laurin, M. & R. R. Reisz. 1997. A new perspective on tetrapod phylogeny. In S. Sumida and K. Martin (eds.) Amniotes Origins: Completing the Transition to Land: 9-59. London: Academic Press.

Laurin M. & R. R. Reisz. 1999. A new study of Solenodonsaurus janenschi, and a reconsideration of amniote origins and stegocephalian evolution. Canadian Journal of Earth Sciences 36: 1239-1255.

Lombard, R. E. & S. S. Sumida. 1992. Recent progress in understanding early tetrapods. The American Zoologist 32: 609-622.

Milner, A. R. 1988. The relationships and origin of living amphibians. In M. J. Benton (ed.) The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds: 59-102. Oxford: Clarendon Press.

Milner, A. R. 1993. The Paleozoic relatives of lissamphibians. Herpetological Monographs 7: 8-27.

Nussbaum, R. A. 1977. Rhinatrematidae: A new family of caecilians (Amphibia: Gymnophiona). Occasional papers of the Museum of Zoology, University of Michigan 682: 1-30.

Panchen, A. L. & T. R. Smithson. 1988. The relationships of the earliest tetrapods. In M. J. Benton (ed.) The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds: 1-32. Oxford: Clarendon Press.

Ruta M., M. I. Coates, & D. D. L. Quicke. 2003. Early tetrapod relationships revisited. Biological Reviews of the Cambridge Philosophical Society 78: 251-345.

Ruta M., J. E. Jeffery, & M. I. Coates. 2003. A supertree of early tetrapods. Proceedings of the Royal Society of London, Series B 270: 2507-2516.

Trueb, L. & R. Cloutier. 1991. A phylogenetic investigation of the inter- and intrarelationships of the Lissamphibia (Amphibia: Temnospondyli). In Hans-Peter Schultze and Linda Trueb (eds.) Origins of the higher groups of tetrapods-Controversy and Consensus: 223-313. Ithaca: Comstock Publishing Associates.

Vallin G. & M. Laurin. 2004. Cranial morphology and affinities of Microbrachis, and a reappraisal of the phylogeny and lifestyle of the first amphibians. Journal of Vertebrate Paleontology 24: 56-72.

Wellstead, C. F. 1991. Taxonomic revision of the Lysorophia, Permo-Carboniferous lepospondyl amphibians. Bulletin of the American Museum of Natural History 209: 1-90.

About This Page

Michel Laurin
Muséum National d'Histoire Naturelle, Paris, France

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