| Placodermi | ||
| The Vertebrates | Acanthothoraci |
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Abbreviated Dendrogram
Gnathostomata
│ └─Acanthodii etc (if traditional placement)
│
└─Placodermi
├─○Palaeacanthaspidae (paraphyletic Acanthothoraci, in part)
│ ├─○
│ │ ├─○
│ │ │ ├─Romundina stellina
│ │ │ ├─Palaeacanthaspis vasta
│ │ │ └─Antiarchi
│ │ └─○
│ │ ├─Radotina kosorensis (paraphyletic Acanthothoraci, in part)
│ │ ├─Radotina tesselata
│ │ ├─Kossoraspis peckai
│ │ ├─Kimaspis tienshanica
│ │ └─Rhenanida
│ └─○
│ ├─"Radotina" prima
│ ├?─Brindabellaspis stensioi (Brazeau 2009)
│ ├─○
│ │ ├─Ptyctodontida
│ │ └─Petalichthyida
│ └─┬─Arthrodira
│ └?─Acanthodii etc ( Brazeau 2009)
└─○Weejasperaspididae (paraphyletic Acanthothoraci, in part)
├─Murrindalaspis wallacei
├─Murrindalaspis bairdi
└─Weejasperaspis gavini
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Contents
Overview |
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.
Even more than with any other placoderm groups, phylogenetic relationships of the Acanthothoraci 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 paraphyletic 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.
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. Stanton 060924, MAK061002
Brindabellaspis stensioi Young, 1980
Emsian (Early Devonian) of Taemas-Wee Jasper, New South Wales (south-east Gondwana).
basal Placodermi? or Placodermi ::: (petalichthyids + ptyctodontids) + arthrodires) + * .
Comment: As of 1993, there has been a growing consensus to remove the Taemas-Wee Jasper placoderm Brindabellaspis stensioi from the Acanthothoraci, and place it in its own order, "Brindabellaspida." According to Janvier 1996 pp.170-1, the braincase of Brindabellaspis is remarkably like that of the galeaspida and osteostraca, which may indicate that this is actually a transitional group. Alternatively it may simply belong in the Romundina-Antiarch clade; basal, but not that basal. More recently Brazeau 2009 provides a cxladogram which places Brindabellaspis near (or just basal to) petalichthyids, ptyctodontids, and arthrodires. MAK061019 111026
References: Janvier 1996.
Emsian (Early Devonian) of Taemas-Wee Jasper, New South Wales (south-east Gondwana).
basal Placodermi? .
Comments: As with other acanthothoracids, the weejasperaspids were heavily armored, with a pair of spines that emanated from their chests, and had a large spine emanating from the top of their backs. They had blunt snouts, and are suspected of being shellfish eaters. The main reasons why the weejasperaspids are not considered to be closely related to other non-acanthothoracid placoderms, as opposed to the palaeacanthaspids, are that their skull anatomies and plate histologies are generalized, and do not bear any similarities to any specific non-acanthothoracid group, and that the patterns of ornamentation on their dermal plates are unique to this family. Stanton 070406 Wikipedia
References: Long1996.
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