A missing premaxilla in the placoderm Holonema?
According to Wikipedia,
Holonema (Fig 1) is an extinct genus of relatively large, barrel-shaped arthrodire placoderms that were found in oceans throughout the world from the Mid to Late Devonian. Although previously thought to have fed in the bottom seafloor mud, more recent morphological studies of the jaw and toothplates suggest a more planktivorous lifestyle. This species reached around 60.6 cm (23.9 in) long.
Note the comb-like (but more tightly packed, like baleen) dentary teeth of Holonema (Figs 1, 2). When biting, these would have likely contacted (or slid past like scissors) the palatine teeth of similar striated design. No upper marginal teeth (e.g. maxillary teeth) had yet evolved.
When Roger Miles 1997 illustrated the lateral view,
(Fig 1) he gave Holonema a rostrum overhanging the palatine teeth (dark blue) as in related Dunkleosteus and kin. However, when Miles illustrated the palatal view (Fig 2) he put the striated palatines together at the mid-line and moved them anteriorly to and beyond the rostrum margin.
Let’s correct that discrepancy today, if it works to do so.
To make the Holonema palate more like those of other similar placoderms,
here (Fig 2, frame 2) the palatines are separated by missing vomers, together with the addition of a little missing premaxilla copied from the ANU V 244 specimen (Fig 3), which has a tentative premaxilla with dermal pustules for teeth (= the tooth carpet of catfish).
This is a falsifiable experiment.
Let’s compare the traditional and the newly revised Holonema palate to those of other placoderm palates (Fig 3) , which are rarely described. If there is a Dunkleosteus palate somewhere in the literature. I will add it here when found and processed.
How advanced or primitive was Holonema?
To which clade did it most closely relate to? How easy was it for placoderms to lose those early vomers and palatines? There’s a lot to learn. All esoteric.
That suture separating the anterior palate
(= pterygoid) from the posterior palate (= basipterygoid, Figs 2, 3) is a clue that Holonema was more derived and therefore more like ANU V 244.
References
Miles RS 1971. The Holonematidae (placoderm fishes): a review based on new specimens of Holonema from the Upper Devonian of Western Australia. Philosophical Transactions of the Royal Society of London 263 B: 101-234.
Trinajstic K 1999. New anatomical information on Holonema (Placodermi) based on material from the Frasnian Gogo Formation and the Givetian-Frasnian Gneudna Formation, Western Australia. Geodiversitas. 21 (1): 69–84.
Source: https://pterosaurheresies.wordpress.com/2023/12/23/a-missing-premaxilla-in-the-placoderm-holonema/
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