Biology:Acanthothoraci

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Short description: Extinct order of fishes

Acanthothoraci
Temporal range: Early to Middle Devonian
Murrindalaspis wallacei.jpg
Weejasperaspis gavini, Murrindalaspis wallacei & Brindabellaspis stensioi
Scientific classification e
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: †Placodermi
Order: †Acanthothoraci
Stensiö, 1944
Families

Weejasperaspididae †
Hagiangellidae[1] †
Palaeacanthaspidae †

Synonyms
  • Acanthothoraciformes

Acanthothoraci (spine chests) is an extinct group of chimaera-like placoderms closely related to the rhenanid placoderms. Superficially, the acanthoracids resembled scaly chimaeras and (relatively) heavily armored ptyctodonts. They were distinguished from chimaeras by their large scales and plates, a pair of large spines that emanate from their chests (thus, the order's name), tooth-like beak plates, and the typical bone-enhanced placoderm eyeball. They were distinguished from other placoderms by differences in skull anatomy and by patterns on the skull plates and thoracic plates that are unique to this order.

Fossil record

Fossils of the Acanthothoracids are found in various deposits from the Lower Devonian throughout the world. Fossils of the Palaeacanthaspids are found in Eurasia and Canada, while the Weejasperaspids have only been found in the Taemas Wee Jasper reef, in Southeastern Australia .

Ecology

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.

Families

Three families have been recognized:

  • Palaeacanthaspidae is the most widespread of the three families. Fossils of Palaeacanthaspids are found in Lower Devonian strata of Europe, Canada, Saudi Arabia, and East Asia. Palaeacanthaspids have short rostrums and large nostrils situated dorsally on the forehead, almost directly between the eyes. They also had stout spines emanating from the back of the median dorsal plates of their shoulder-girdles and a spine emanating on plates directly in front of the pectoral fins. Dermal plates were decorated with either tubercules or scales with stellate patterns.
  • Weejasperaspididae is restricted to the Emsian Taemas-Wee Jasper "reef" in what is now New South Wales, Australia . Compared to the corresponding spines of Palaeacanthaspids, the spines of Weejasperaspids are long and markedly stout. The armor of Weejasperaspids were decorated with small tubercules.
  • Hagiangellidae is a monogeneric family, currently represented by Hagiangella goujeti, which is restricted to Lochkovian strata of the Khao Loc formation in Tung Vai, Ha Giang Province of Northern Vietnam. From what can be discerned from the various scrappy fossils, H. goujeti had a relatively high-domed head, especially when compared to the bun-shaped heads of other acanthothoracids, and had serrations on the spines in front of its pectoral fins.

Relation to other placoderms

Most placoderm experts have reached a consensus that Acanthothoracida is the sister group of the rest of Placodermi, save for, perhaps, Stensioella and Pseudopetalichthyida. This is the result of a careful reexamination of the various members of the Acanthothoracid family Palaeacanthaspidae in that particular species within that family share various anatomical similarities with other placoderm orders, particularly the anatomies of their braincase, dermal plate arrangement and bone histology. In 2011 the genus Hagianella, of the monotypic family Hagianellidae, was reappraised as possibly being the sister-group of Ptyctodontida in light of similarities of skull anatomies.[1] Accordingly, Palaeacanthaspidae and Hagianellidae are now considered paraphyletic in view of similarities of their members to primitive members of other placoderm orders.

The family Weejasperaspididae, on the other hand, is considered monophyletic. Because of the Weejasperaspids' generalized anatomy and strong similarities with the Palaeacanthaspids, but no overt similarities with any other order save Brindabellaspida, they are regarded as either basal placoderms or very close to the basal placoderm.

Timeline of genera

<timeline> ImageSize = width:1000px height:auto barincrement:15px PlotArea = left:10px bottom:50px top:10px right:10px

Period = from:-416 till:-359.2 TimeAxis = orientation:horizontal ScaleMajor = unit:year increment:5 start:-416 ScaleMinor = unit:year increment:1 start:-416 TimeAxis = orientation:hor AlignBars = justify

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 id:paleozoic     value:rgb(0.6,0.75,0.55)
 id:cambrian   value:rgb(0.49,0.63,0.33)
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 id:latedevonian   value:rgb(0.96,0.89,0.71)
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bar:periodtop
from: -416    till: -411.2   color:earlydevonian    text:Lochkovian
from: -411.2    till: -407   color:earlydevonian    text:Pragian
from: -407    till: -397.5   color:earlydevonian    text:Emsian
from: -397.5    till: -391.8   color:middledevonian    text:Eifelian
from: -391.8    till: -385.3   color:middledevonian    text:Givetian
from: -385.3    till: -374.5   color:latedevonian    text:Frasnian
from: -374.5    till: -359.2   color:latedevonian    text:Famennian

bar:eratop
from: -416    till: -359.2   color:devonian    text:Devonian

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align:left fontsize:M mark:(line,white) width:5 anchor:till align:left

color:earlydevonian	bar:NAM1	from:	-416	till:	-414.4	text:	Kimaspis
color:earlydevonian	bar:NAM2	from:	-416	till:	-409.8	text:	Kosoraspis
color:earlydevonian	bar:NAM3	from:	-416	till:	-407	text:	Radotina
color:earlydevonian	bar:NAM4	from:	-412.8	till:	-411.2	text:	Dobrowlania
color:earlydevonian	bar:NAM5	from:	-412.8	till:	-411.2	text:	Palaeacanthaspis
color:earlydevonian	bar:NAM6	from:	-412.8	till:	-411.2	text:	Romundina
color:earlydevonian	bar:NAM7	from:	-411.2	till:	-407	text:	Breizosteus
color:earlydevonian	bar:NAM8	from:	-407	till:	-397.5	text:	Kolymaspis
color:earlydevonian	bar:NAM9	from:	-407	till:	-397.5	text:	Murrindalaspis
color:earlydevonian	bar:NAM10	from:	-407	till:	-397.5	text:	Weejasperaspis

PlotData= 

align:center textcolor:black fontsize:M mark:(line,black) width:25 
 
bar:period
from: -416    till: -411.2   color:earlydevonian    text:Lochkovian
from: -411.2    till: -407   color:earlydevonian    text:Pragian
from: -407    till: -397.5   color:earlydevonian    text:Emsian
from: -397.5    till: -391.8   color:middledevonian    text:Eifelian
from: -391.8    till: -385.3   color:middledevonian    text:Givetian
from: -385.3    till: -374.5   color:latedevonian    text:Frasnian
from: -374.5    till: -359.2   color:latedevonian    text:Famennian

bar:era
from: -416    till: -359.2   color:devonian    text:Devonian

</timeline>

References

  1. ↑ 1.0 1.1 Dupret, V. et al, Journal of Vertebrate Paleontology 31(3):531–538, May 2011 ©2011 by the Society of Vertebrate Paleontology "THE SKULL OF HAGIANGELLA GOUJETI JANVIER, 2005, A HIGH-CRESTED ACANTHOTHORACID (VERTEBRATA, PLACODERMI) FROM THE LOWER DEVONIAN OF NORTHERN VIETNAM"

Further reading

External links

Wikidata ☰ Q3603904 entry



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