Palaeobranchiostoma hamatotergum is an extinct chordate from the Early Permian-aged Whitehill Formation in South Africa. It is known from a single, 11 mm long specimen found in black shale. The body shows the presence of a notochord-like structrure running along the length of the body, gill slits, and several unpaired fins, including a tail (caudal) fin with rays that is round and small, a pronounced pelvic fin and a humped dorsal fin covered in spikes. In its original description it was considered to be a cephalochordate (also known as a lancelet), though some other authors have considered its placement in the group as not definitive, due to a number of aspects of its anatomy like the fins being strongly divergent from living lancelets.
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👉 Palaeobranchiostoma in the context of Lancelets
The lancelets (/ˈlænslɪt, ˈlɑːn-/ LA(H)N-slit), also known as amphioxi (sg.: amphioxus /ˌæmfiˈɒksəs/ AM-fee-OK-səs), consist of 32 described species of somewhat fish-like benthic filter-feeding chordates in the subphylum Cephalochordata, class Leptocardii, and family Branchiostomatidae.
Lancelets diverged from other chordates during or prior to the Cambrian period. A number of fossil chordates have been suggested to be closely related to lancelets, including Pikaia and Cathaymyrus from the Cambrian and Palaeobranchiostoma from the Permian, but their close relationship to lancelets has been doubted by other authors. Molecular clock analysis suggests that modern lancelets probably diversified much more recently, during the Cretaceous or Cenozoic.