Ptyctodontid placoderms get a face lift

Thursday, January 25, 2024 14:15

% of readers think this story is Fact. Add your two cents.

Ptyctodontid placoderms
(Figs 1–4) are narrower than other placoderms.

Previously
ptyctodontids were reconstructed with parrot-like, down-turned, toothless beaks (Figs 1–4). That’s a problem because that is the opposite of their LRT ancestors and relatives, like Shenacanthus and Roumundina (Fig 1). These all have a convex upper jaw margin with an upturned tip.

Unfortunately,
due to their less-fully ossified and more loosely sutured skulls, many ptyctodontids are preserved scattered. So they require assembly (Fig 4).

Today tested taxa will undergo that reassembly
in order to more closely match less jumbled ancestors and relatives in the LRT. This is called ‘working from a Bauplan’. If the Bauplan or method is wrong, let me know.

Figure 1. Early Silurian Shenacanthus is the earliest narrow-skull taxon in the lineage of Romundina and the ptyctodontids. Note the elevation of the thoracic armor.

Turns out,
after reassembly – none – of the ptyctodontids had a sharp, pointed rostrum. Instead a convex upper jaw margin can be assembled from the scattered, broken and jumbled parts of these biological puzzles. The taxa were reassembled following the blueprint of an ancestral taxon, Romundina (Fig 1).

Figure 2. Campbellodus revised here based on the Bauplan of Romundina. Scattered bones here are reassembled here. Very risky.

This is risky business,
reassembling published diagrams and broken bones, but the new diagrams and photo comps more closely match ancestors and relatives, like Romundina (Fig 1). That’s the way evolution works. Small changes. Many similarities.

Figure 3. The ptyctodontid placoderm Austroptyctodus as originally reconstructed and revised here in frame 2 of 2. Now all the parts match ancestral and descendant taxa (see figure 2) and fit together with fewer questions.

The possible exception,
Materpiscis (Fig 4), aka: the pregnant ptyctodontid, was given a straight upper jaw
margin in the diagram provided by Trinajstic et al 2012. Fortunately the authors also published photos of the jumbled and broken mouth bones. Here (Fig 4) the Materpiscis bones are reassembled. Here they form a convex upper jaw margin, matching a concave lower jaw margin. All other bones appear to follow the pattern of Romundina and the other ptyctodontid reconstructions provided in this blogpost. So, so far, there are no tested exceptions.

Figure 4. This Materpiscis diagram from Trinajstic et al 2012 is the best data I’ve been able to find. The authors applied a tall bone to the rostrum not replicated in other taxa. Their labels are occasionally different from mine, perhaps because they did not consider Romundina ancestral to this clade.

This experiment started with
Early Silurian Shenacanthus (Fig 1), which had a narrow skull, distinct from other flatter Silurian taxa. So more study was required. The results (Figs 1–4) indicate narrow-skull placoderms were related to one another, sharing more traits with each other than with other placoderms. That means ptycodontids go back to the Early Silurian. Note the loss of palatine teeth (blue) in derived ptyctodontids.

Austroptyctodus gardineri
(right, originally Ctenurella (Miles and Young 1977; Long 1997; Late Devonian, Fig 3) was toothless, like other ptyctodontids, but did not have a parrot-like beak.

Figure 5. Cheirodus (Chirodus, Amphicentrum) as traditionally reconstructed alongside Platysomus a relative with a convex upper jaw margin. See figure 5 for a fossil Chirodus. These taxa are not considered to be placoderms anywhere else but the LRT.

So, where did the traditional parrot beak paradigm come from?
Maybe Cheirodus (= Chirodus = Amphistrum, fig 5) was the source. It was illustrated over a hundred years ago (Fig 5) with a pointed beak (Fig 2). If this is the original fossil of Cheirodus (Fig 6), it appears to be damaged throughout, but especially so at the snout. Given this situation, maybe we need a fish paleontologist to revisit this genus to provide a new µCT scan.

All tested ptyctodontids,
like Romundina, Platysomus and Eurynotus have a convex upper jaw margin, created by the preoperculum homolog (light yellow), not the maxilla, which is a tooth-bearing bone lacking in placoderms.

Figure 6. Unnumbered specimen attributed to Chirodus/Cheirodus/Amphicentrum. Note the damage to the taphonomic rostrum and face. If you have a better fossil image of Chirodus with an undamaged rostrum, please send it over.

As you can imagine
wading through traditional data can sometimes be fascinating and sometimes lead to frustration. Trust is necessary, but sometimes your cladogram will tell you where traditional mistakes have been made. Better data = better scores = more resolution in cladograms.

Evolution is a seamless spectrum of morphologies,
punctuated only by the absence of fossils in long stretches.

References
Long JA 1997. Ptyctodontid fishes from the Late Devonian Gogo Formation, Western Australia, with a revision of the German genus Ctenurella Orvig 1960. Geodiversitas 19: 515-555.
Miles RS and Young GC 1977. Placoderm interrelationships reconsidered in the light of new ptyctodontids from Gogo Western Australia. Linn. Soc. Symp. Series 4: 123-198.
Trinajstic K, Long JA, Johanson Z, Young G and Senden T 2012. New morphological information on the ptyctodontid fished (Placodermi, Ptyctodontida) from Western Australia. Journal of Vertebrate Paleontology 32(4):757–780.

wiki/Austroptyctodus
wiki/Ptyctodontida

Source: https://pterosaurheresies.wordpress.com/2024/01/26/ptyctodontid-placoderms-gets-a-face-lift/

Story Views
Now:
Last hour:
Last 24 hours:
Total:

Online:
Visits:	1,601,229,742
Stories:	8,143,760

Comments