Hyomandibular evolution + Introducing the postsquamosal

Revised July 19, 2020
with new bone identities given to several lobefin fish, correcting the mistakes of Thomson 1966.

The hyomandbular is the largest bone
in the entire body of the basal ray-finned fish Trachinocephalus (Fig. 1). Over time and phylogeny it evolves to become the smallest bone in the human body, the stapes (Fig. 3), one of the ultra tiny sound-conducting bones of the middle ear.

Along the way,
the large reptile tree (LRT, 1656+ taxa) presents a new (and heretical) lineage of tetrapod ancestry, distinct from the traditional one that includes Ichthyostega and Acanthostega. Today we go below the surface and formally introduce ‘the lineup.’

Figure 1. Hyomandibular evolution from the first dichotomy of bony fish to Gephyrostegus. The hyomandibular evolves to become the stapes. Note the hyomandibula contact with the intertemporal, quadrate and pterygoid, sometimes fused to these bones. The hyomandibular is poorly ossified in Onychodus, so it restored here. Note how the maxilla splits to produce the quadratojugal.

Figure 1. Hyomandibular evolution from the first dichotomy of bony fish to Gephyrostegus. The hyomandibular evolves to become the stapes. Note the hyomandibula contact with the intertemporal, quadrate and pterygoid, sometimes fused to these bones. The hyomandibular is poorly ossified in Onychodus, so it restored here. Note how the maxilla splits to produce the quadratojugal.

Some bones are relabeled
from the diagram found in Thomson 1966 (modified in Fig. 2), who presented several layers of skull bones (cranial, palatal and dermal) in the the Permian megalichthyid rhipidistian fish, Ectosteorhachis, a late-survivor of an earlier (Mid-Devonian) radiation that ultimately produced tetrapods and humans. Thomson mislabeled the dentary as a maxilla (mx) in his diagram, but all other labels are traditional.

In most fish 
the hyomandibular is roofed over by the otherwise unremarkable intertemporal, which anchors it dorsally.

That brings up a problem in Thomson’s diagram
(Fig. 2). To remedy that problem, here the dorsal rim of the traditional palatoquadrate is relabeled as the hyomandibular fused to the pterygoid and other palatal elements. Second, the labeled hyomandibular (h) is now the preopercular. Third, the traditional preopercular (pop) requires a new name: the postsquamosal. It is not homologous with the preopercular of teleost fish.

The disappearance of the traditional preopercular
Trachinocephalus (Fig. 1) retains a traditional preopercular. Pteronisculus (Fig. 1) has a postsquamosal and lacks a traditional preopercular on the surface. Cheirolepis (Fig. 1) lacks both. The squamosal and postsquamosal appear to be fused or else the tiny postsquamosal is overwhelmed by the advancing squamosal. The traditional rhipidistians have a postsquamosal. The tetrapods (Fig. 1) lack a postsquamosal.

The more derived traditional transitional tetrapods,
Acanthostega and Ichthyostega, have a postsquamosal, but this appears as a reversal, a neotonous trait. These two are secondarily more aquatic than their ancestor, Ossinodus.

Figure 2. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view.

Figure 2. Ectosteorhachis skull from Thomson 1966 with layers to show the brain case and palatoquadrate. Some bones are relabeled in the revised view.

The hyomandibular decreases in size
in most later tetrapods (Fig. 3), where it continues to shrink into the auditory channel where it is then known as the stapes.

At the same time, the intertemporal
disappears or fuses to nearby skull bones in several derived basal tetrapods and basal reptiles, all by convergence.

Figure 4. Evolution of the tetrapod mandible and ear bones leading to humans.

Figure 4. Evolution of the tetrapod mandible and ear bones leading to humans in lateral and medial views, first printed in From The Beginning, the Story of Human Evolution (Peters 1991), colors added here.

The quadratojugal first appears in the tetrapod lineage
in Gogonasus (Fig. 1) after the elongate maxilla of Onychodus splits in two.

Finally,
sharks also have a palatoquadrate, but it is composed of a fused lacrimal, jugal and squamosal with a tooth-bearing premaxilla and maxilla fused to the ventral rim. The pseudo- or plesio-palatoquadrate illustrated by Thomson 1966 in Ectosteorhachis and other rhipidistians,is not homologous and is comprised of different bones. 


References
Thomson KS 1966. The evolution of the tetrapod middle ear in the rhipidistian-amphibian transition. American Zoologist 6:379–397.

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