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The traditional coelacanth, Miguashaia, joins the Middle Devonian placoderm ‘flounder’ clade in the LRT

Friday, January 5, 2024 18:31

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Sometimes it takes a similar Bauplan,
in other words: a familiar face, to recognize interrelationships.

This one escaped notice until today – and for good reason.

Figure 1. Miguashaia juvenile and adult. These are traditional lobe-fin porolepiformes. Compare these to the fossil in situ in figure 2. These may represent some other specimens.

Here little Middle Devonian
Miguashaia (Figs 1–3), a traditional basalmst coelacanth (Fig 1), joins the star-gazing, big mouth, bottom feeding placoderms of the Middle Devonian: Yinosteus, Homosteus and Heterosteus (Fig 3). Miguashaia has large coelacanth-like scales, not seen in other placoderms. These three putative relatives (Fig 2) lack post-crania and therefore squamation, if present, remains unknown.

That’s a phylogenetic problem resolved here by the LRT, at least for the present, Convergence is rampant. Scoring is always biased. Let’s see how this one falls out when others test with a similar taxon list and a reconstruction that matches the fossil.

Figure 2. Miguashaia (MHNM 06-494) in situ and reconstructed. Colors added here. Note the lack of lateral fins preserved in this specimen. Note the oversized mandible, as in related taxa seen in figure 3. Note the 90º axial rotation necessary to align the middle of the skull with the top portion of the tail fin.

These taxa are nicknamed
placoderm ‘flounders’ because Miguashaia provides the only post-crania for this clade, and it looks like the wide flat skull is axially rotated 90º to the flattened body, presently lacking any substantial lateral fins, contra the traditional reconstruction (Fig 1).

Figure 3. Phylogenetically the top three nest together close to the Gemuendina clade of ray-like placoderms. The largest specimen preserves too few traits to score at this time. See figure 4.

Miguashaia bureaui
(Schultze 1973, Cloutier 1996; Middle Devonian; up to 45cm) was traditionally considered the sister group (outgroup) of the Actinista (coelocanths). Notably Miguashaia retains a heterocercal tail and it is usually reconstructed with lobe fins (see Fig 1). The maxilla was considered taphonomically missing, but the premaxilla is wide. The dentary in the reconstruction is a standard coelacanth design, not what we see in this fossil in situ.

Here this specimen is reinterpreted as a Devonian ‘flounder’ with a wide skull, a tremendous underbite and an torso axially rotate 90º to lie flat of the sea floor, like a flounder. Lateral fins are missing, perhaps due to taphonomy. Phylogenetically it matches larger Middle Devonian taxa lacking post-crania. If a placoderm thoracic carapace was once present, as in related taxa, it is gone now, ripped off during taphonomy (Fig 2).

Figure 4. This is the bottom-dwelling placoderm, Gemuendina. Based on the scale bar, this a surprisingly small specimen, full scale on a typical computer monitor.

Mondéjar-Fernández J, et al 2023 reported,
“Despite their rich fossil record, comprising more than 60 genera and 130 species, few surveys of the dermal skeleton of coelacanths have been undertaken. To our knowledge, few detailed accounts on the palaeohistology of the scales and dermal bones of extinct coelacanths have been published or properly illustrated.”

The authors nested Miguashaia as the basalmost coelacanth with no outgroup taxon indicated. That gives it a ‘wildcard’ status. not nesting between two coelacanths.

The authors concluded,
“Overall, the combination of traits described in Miguashaia do not represent a drastic departure from the histological structure of the scales of other coelacanths.”

The LRT does not test microscopic histological traits, but it does nest taxa with previously overlooked and untested taxa.

The nesting of Miguashaia
close to another bottom-dwelling placoderm, Gemuendina (Fig 4) should not be surprising. Perhaps Miguashaia can provide some insight into the genesis of the ray-like placoderms.

This appears to be a novel hypothesis of interrelationships.
If not please provide a citation so I can promote it here.

References
Cloutier R 1996. The primitive actinistian Miguashaia bureaui Schultze (Sarcopterygii). Pp. 227–247 in: Schultze H-P and Cloutier R editors. Devonian fishes and plants of Miguasha, Quebec, Canada. Dr. Friedrich Pfiel; München: 1996.
Mondéjar-Fernández J, et al 2023. A microanatomical and histological study of the scales of the Devonian sarcopterygian Miguashaia bureaui and the evolution of the squamation in coelacanths.
Schultze H-P 1973. Crossopterygier mi heterozerker Schwanzfloss aus dem Oberdevon Kanadas, nebst einer Beschreibung von Onychodontida-Resten aus dem Middledevon Spaniens und aus dem Karbon der USA. Palaeontograhica A 143:188–208.

wiki/Miguashaia

Source: https://pterosaurheresies.wordpress.com/2024/01/06/the-traditional-coelacanth-miguashaia-joins-the-middle-devonian-placoderm-flounder-clade-in-the-lrt/

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