Acanthothoraci

From Palaeos.org

Gnathostomata
`--o PLACODERMI
   | ?--Stensioella
   | ?--Pseudopetalichthyida
   `==Acanthothoraci (paraphyletic)
      |--Rhenanida
      |--Antiarchi
      `--+--Ptyctodontida
         |--Petalichthyida
         `--Arthrodira
            `--Phyllolepida

Two acanthothoracid placoderms, Murrindalaspis wallacei and Brindabellaspis stensioi of Taemas-Wee Jasper, from the Emsian (Early Devonian) of South-East Gondwana. Graphic © ‎Stanton F. Fink

[edit] Introduction

The Acanthothoraci ("spiny chests") were a group of chimaera-like placoderms who were closely related to the rhenanid placoderms. Superficially, the acanthoracids resembled scaly chimaeras, lightly armored arthrodires, or (relatively) heavily armored ptyctodonts. They were distinguished from chimaeras by the presence of large scales and plates, tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms due to differences in the anatomy of their skulls, and due to patterns on the skull plates and thoracic plates that are unique to each order.

o Placodermi
|?--Brindabellaspis stensioi (Brindabellaspida)
`--o Acanthothoraci
   |--o Palaeacanthaspidae
   |  |--o--o--Romundina stellina
   |  |  |  |--Palaeacanthaspis vasta
   |  |  |  `--Antiarchi
   |  |  `--o--Radotina kosorensis
   |  |     |--Radotina tesselata
   |  |     |--Kossoraspis peckai
   |  |     |--Kimaspis tienshanica
   |  |     `--Rhenanida
   |  `--o--"Radotina" prima
   |     |--o--Ptyctodontida
   |     |  `--Petalichthyida
   |     `--Arthrodira
   `--o Weejasperaspididae
      |--Murrindalaspis wallacei
      |--Murrindalaspis bairdi
      '--Weejasperaspis gavini

[edit] Phylogeny

Even more than with any other placoderm groups, phylogenetic relationships are highly controversial.

One theory is that the acanthothoracids were closely related to the rhenanids because of similar arrangements of the plate armor, and that the patterns on the former's plates resemble the patterns of the latter's tubercle mosaics. Some authorities unite the two groups, though, they differ in which order to place the other in. According to Young 1980, 1986, the acanthothoracids can be considered primitive rhenanids on the basis of the dorsal (upper) position of their nostrils.

A very different hypothesis, by Goujet 1984, and followed by Janvier 1996 pp.170-2, holds that the Acanthothoraci, and not the Ptyctodontida, were the basal placoderms, and that the Acanthothoraci (as we currently know them) were paraphyletic. In this hypothesis, the acanthothoracids as a basal assemblage from which the other Placoderm lineages (the Ptyctodontid-Petalichthyid-Arthrodire clade, the Antiarchi, and the Rhenanida) emerged.

These two theories are actually not incompatible, since if the Acanthothoraci are a paraphytletic grade, there would still be those that evolve into - and hence represent primitive representatives and sister clades of - the Rhenanida.

The cladogram used here is based on that given by Janvier, following Goujet 1984, except that Brindabellaspis is here presented as a more basal Placoderm. This cladogram should not be taken as anything other than one possible interpretation among many.

[edit] Ecology and Lifestyle

From what can be inferred from the mouthplates of fossil specimens, the acanthothoracids were ecologically similar to modern-day chimaeras, being a clique of shellfish hunters. Competition with their relatives, the ptyctodont placoderms, may have been one of the main reasons for the acanthothoracids' extinction prior to the Mid Devonian extinction event.

[edit] Stratigraphic Range

?Silurian, Early to Mid Devonian

[edit] Taxonomy

• Order Acanthothoraci Stensiö, 1944 [Acanthothoraciformes]
  • Palaeacanthaspidae (author?) (year?)
    • Dobrowlania
    • Kimaspis
    • Kolymaspis
    • Kosoraspis
    • Palaeacanthaspis
    • Radotina [Holopetalichthys]
    • Romundina
  • Family Weejasperaspididae White, 1944
    • Murrindalaspis
    • Weejasperaspis

[edit] External Links

• Mikko's Phylogeny Archive [1]

[edit] References

• Goujet, D. (1984). Placoderm interrelationships: a new interpretation, with a short review of placoderm classification. Proceedings of the Linnean Society of New South Wales. 107, 211-241.
• Janvier, Philippe. Early Vertebrates Oxford, New York: Oxford University Press, 1996. ISBN 0-19-854047-7
• Long, John A. The Rise of Fishes: 500 Million Years of Evolution Baltimore: The John Hopkins University Press, 1996. ISBN 0-8018-5438-5
• Young, G. C. (1986). The relationships of placoderm fishes. Zoological Journal of the Linnean Society, 88, 1-57.

[edit] Credits

Stanton 060924, modified MAK061002