Palæos:		Unit 050: Thelodonti
The Vertebrates		750: Cornuata (2)

Thelodonti: Cornuata (2)

CRANIATA
|--CONODONTA
`--+--PTERASPIDOMORPHI
   `--Thelodonti
      |--+--Furcacaudiformes
      |  `--Thelodontida 
      `--+--+--Katoporida  
         |  `--Cephalaspidomorphi 
         |     |--Galeaspida
         |     |  |--Eugaleaspidiformes
         |     |  `--Polybranchiaspidida
         |     `--+--Pituriaspida
         |        `--Osteostraci
         |           |--Ateleaspis
         |           `--Cornuata
         |              |--Kiaeraspidida
         |              `--Thyestiida
         `--+--Loganiidae            
            `--Gnathostomata
               |--PLACODERMI
               `--+--CHONDRICHTHYES
                  `--TELEOSTOMI

1. Kiaeraspidida X

2. Thyestiida X

Kiaeraspidida: Acrotomaspis, Axinaspis, Gustavaspis, Kiaeraspis, Nectaspis, Norselaspis  

Range: Early Devonian of Europe (Spitzbergen).

Phylogeny: Cornuata:::: Thyestiida + *. 

Characters: Very small; some with mouths opening anterodorsally; supraoral sensory field present and large; many with lateral sensory fields reduced; cornual processes reduced; headshield extends posteriorly into long abdominal division; anterior dorsal scute absent; first canal to lateral sensory fields bifurcates near orbit; posterior ventral myodome reduced or absent. 

Notes: Lochkovian to Emsian.  These are tiny forms, barely larger than a fingernail.  The cornual "horns" are reduced and there is a tendency also towards reduction or subdivision of the "fields".  They are restricted to the early Devonian of Spitsbergen (northern Euramerica). MAK001023.

References: Janvier (1996).

Thyestiida: Dartmuthia, Procephalaspis, Sclerodus, Thyestes, Tremataspis, Witaaspis  

Range: Middle Silurian to Early Devonian of Europe.

Phylogeny: Cornuata:::: Kiaeraspidida + *.

Characters: infraorbital sensory line passes medially to lateral fields; anterior dorsal scute absent; first canal to lateral sensory fields bifurcates near orbit; posterior ventral myodome reduced or absent; loss of. paired fins common; may be burrowers.

Image: from North Eastern Geologic

Notes: Wenlock to Pridoli.  These very early osteostracans can variously be considered the ancestral lineage of cornuates (the evolutionary-systematic paradigm) or as the most specialized crown group (the cladistic paradigm).  Thyestiida became diversified very early in the Baltic area (central eastern Euramerica) and may have later reached Great Britain, Timan and the Urals but seem to have disappeared by the Early Devonian. Their most primitive members (Procephalaspis, Auchenaspis) had the typical cornuate shield shape whereas the most advanced or derived (Tremataspididae) had lost paired and unpaired fins, possibly as a consequence of an adaptation to burrowing habits.  They were all quite small.  Tremataspis, an advanced form, was about 8 or 10 cm in length.  There is a tendancy in many of these forms towards a reduction of the "fields". MAK001023.

Links: 122.htm; Thyestes verrucosus; 123.htm; geologic; Ovning4.pdf (Swedish).

References: Janvier (1996).

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