Sciency Thoughts: Rhegmaspis xiphoidea: A streamlined Galeaspid Fish from the Early Devonian of Yunnan Province.

Sunday 20 September 2015

Rhegmaspis xiphoidea: A streamlined Galeaspid Fish from the Early Devonian of Yunnan Province.

Galeaspid Fish were jawless Bony Fish distinguished by their heavily armoured head-shields with a median opening which apparently served as a water intake and nostril, as was as complex patterns of sensory canals. In most Galeaspid Fish this head-shield was flattened, with the median opening and eye orbits on the top, suggesting that the living Fish had a lifestyle similar to that of Skate or Flatfish, remaining on the bottom in coastal or other shallow waters, where the flattened shape would help to prevent it being shifted by currants.

In a paper published in the journal Vertebrata PalAsiatica on 20 April 2015, Gai Zhi-kun, Zhu Min, Jia Lian-Tao and Zhao Wen-Jin of the Key Laboratory of Vertebrate Evolution and Human Origins and Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences describe a new species of Galeaspid Fish from the Early Devonian Posongchong Formation of Yunnan Province, China.

The new species is named Rhegmaspis xiphoidea, where ‘Rhegmaspis’ means ‘Broken-shield’ and ‘xiphoidea’ means ‘swordlike’. It is unusual for a Galeaspid Fish, in that its body is torpedo-shaped, rather than flattened, with eyes on the side of the head and an extended rostrum (snout) with a sword-like or paddle-like shape, suggesting that it lived above the seafloor, and led an active swimming lifestyle.

Restoration of Rhegmaspis xiphoidea. (A) Dorsal view; (B) ventral view; (C) lateral view. Abbreviation: br.f, branchial fossa; m, mouth; md.o, median dorsal opening; obr.f, oralobrachial fenestra; orb, orbital opening; pi, pineal fossa; ro, rostral process; soc2, posterior supraorbital canal. Gai et al. (2015).

Rhegmaspis xiphoidea is described from a complete head-shield, an almost complete head-shield, a partial head-shield and a cast of a brain-case. The complete head-shield is 35.9 mm in length and 9.5 mm in width. The almost complete head-shield is 27.6 mm in length and 12.1 mm in width. The orbits (eye openings in the skull) of all specimens are small compared to other Galeaspid Fish. The skull endocast shows a number of swellings on the brain, which are thought to correspond to divisions of the brain.

Photograph (A), line drawing (B), and restoration (C) of the endocranium of Rhegmaspis xiphoidea. (A), (B) Lateral view; (C) dorsal view. Abbreviations: aa, anterior ampulla; asc, anterior semicircular canal; br.a, branchial arch; com, commissural division of two vertical semicircular canals; mar.v/a, marginal vein or artery; mes, mesencephalic division; met, metencephalic division; mye, myelencephalic division; nc, neural canal; pa, posterior ampulla; psc, posterior semicircular canals; vcl, lateral head vein or dorsal jugular vein; other abbreviations as in top figure. Gai et al. (2015).

Rhegmaspis xiphoidea is thought to have been closely related to two other Early Devonian Galeaspid Fish, Gantarostrataspis gengi and Wumengshanaspis cuntianensis, the three species together being referred to as the Gantarostrataspidae. Neither of the two previously described species is known from complete head-shields, and both had previously been interpreted as having typical flattened forms. However the torpedo-shape of Rhegmaspis xiphoidea suggests that these two other forms may also have lacked the projections which extend the head-shield into a flattened shape, an interpretation which fits well with our current knowledge of these Fish.