Chen et al. 2019 bring us a new look
at the first teeth and jaws to evolve in the clade Gnathostomata.
A word of caution:
Watch out for, and be able to differentiate between ‘the origin of jaws and teeth’ vs ‘the reduction of jaws and teeth’ here. The two can be confused with one another if you don’t have a good outgroup taxon.
Therefore it is helpful to start with a jawless outgroup
for polarity. In the large reptile tree, (LRT, 1593 taxa; subset Fig. 6) the thelodont, Thelodus (Fig. 1), is that outgroup taxon. In other words, Thelodus is the last common ancestor of all vertebrates with jaws in the LRT. That means some undiscovered taxon or Thelodus itself, documents the genesis of jaws. Since all Thelodus specimens are crushed flat body fossils with no internal details shown or known, we’ll have to wait to find out what is inside some to-be-discovered 3D Thelodus. Perhaps the transition occurs within that genus. Meanwhile…
Figure 1. Manta compared to Thelodus and Rhincodon. All three have a terminal mouth essentially transverse between the lateral eyes set far forward on the skull. Based on Thelodus, the outgroup, this is the primitive condition.
From the abstract:
“Osteichthyan dentitions are characterized by cyclic tooth replacement and linear tooth rows.”
Not all of them. Whale sharks (Fig. 1; Rhincodon), manta rays (Fig. 1; Manta ) have tooth carpets and no marginal teeth on the jaws (Fig. 1).
Figure 2. Whale shark (Rhincodon) tooth pads, not that much different from catfish tooth pads.
Chen et al. continue:
“The acquisition of these characters can be explained by an intimate relationship between the growth of jawbone and the initiation of teeth, supported by substantial evidence from synchrotron microtomography that reveals the 3D pattern of successor teeth, vascular canals, growth arrested and resorption surfaces in various Silurian-Devonian gnathostomes.
Jawbones (premaxilla, maxilla and dentary) are actually the last bones to get teeth. So, according to the LRT, the process of getting marginal teeth was a little more complicated than Chen et al. proposed. Perhaps it happened twice by convergence. Catfish (Fig. 3) show an early version of this in bony fish. Sharks evolved marginal teeth by convergence.
Figure 3. Catfish teeth from Usman et al. 2013, colors added. The labeled maxillary teeth are actually premaxillary teeth. Compare these carpets of short teeth with those in figure 2.
Chen et al. continue:
The growing bone provides space for new teeth to attach and the succession of larger teeth maintains the number of teeth as the animal grows.”
OK. Gong with the flow, let’s just focus on jawbones now.
“In non shedding dentitions, whether the spiral addition of acanthodian tooth whorls, the anterior addition of ischnacanthid dentigerous jawbones, or the radial addition of arthrodire gnathal plates, the sequential addition of teeth is synchronized with the appositional growth of bone.”
Figure 4. Onychodus and Ischnacanthus share enough traits to make them sisters, apart from Brachyacanthus + Pteronisculus. Circles show the acanthodian tooth whorls mentioned above.
Chen et al. continue:
“The most primitive teeth of the most basal stem gnathostome Radotina and Kosoraspis already display the lingual addition of tooth rows shared by the gnathostome crown group.”
The LRT recovers more primitive taxa with teeth on the jaws, like Squatina, the extant angel shark (Fig. 5). It is basically a large Thelodus. You know it has primitive jaws because surrounding the jaws on either side are gill bars. These slowly disappear in all more derived taxa. Radotina and Kosoraspis are both Early Devonian placoderms, a derived gnathostome clade (Fig. 6). Neither preserve lateral gill bars.
On a side note, and regarding placoderms, Vaskaninova and Ahlberg 2017 wrote, “A key development in the understanding of this stem group has been the recognition that the placoderms (armoured jawed fishes of Silurian to Devonian age), which until recently were regarded as a clade branching off the gnathostome stem group, probably form a paraphyletic segment of that stem group. Some groups of placoderms appear to be very primitive and close to jawless vertebrates whereas others possess what were previously regarded as osteichthyan autapomorphies (notably a maxilla, premaxilla and dentary) and are probably close to the gnathostome crown-group node.”
The LRT does not support these placoderm hypotheses. All tested placoderms nest together in the LRT at the base of the lobefin-rayfin split. Some bottom dwelling placoderms revert to a near jawless condition. So do certain catfish, like the kind that cling to aquarium glass.
Figure 5. Squatina skull. Note the gill bars framing the mouth and the marginal teeth, the most primitive examples in the shark lineage. Heterodontus has the most primitive teeth in the bony fish line, despite the lack of bones in Heterodontus.
Chen et al. continue:
When in situ resorption evolved in osteichthyans, the first-generation teeth of the stem osteichthyan Lophosteus are shed semi-basally forming deeply overlapping tooth rows. As the bone growth slows down at later developmental stages, the succeeding teeth overlap the preceding ones entirely, causing the preceding teeth to be shed basally and replaced in situ.
“Therefore, tooth replacement may have emerged via a tooth initiation rate higher than the bone growth rate. When a lingual shelf is formed on the marginal jawbones of crown osteichthyans, the lingual growth of bone is restricted, and new tooth rows cannot be added lingually to the previous rows, only apically.”
In the LRT, other than sharks, the most primitive instance of marginal teeth occurs in chimaeras like Falcatus (Fig. 7).
Figure 7. The skull of Falcatus with DGS tracing above. Note the primitive marginal teeth.
Chen et al. continue:
“The replacement of the marginal linear tooth row of the basal actinopterygian Moythomasia is actually a vertical piling of alternate tooth rows by semi-basal resorption. Thus a single linear tooth row may have transformed from a lingual-labial compressed version of transverse tooth files.”
Moythomasia was added to the LRT, but it nests off a rather derived node, far from the most primitive taxa in the LRT with jaws. Like Falcatus (Fig. 7), Moythomasia has deep jaws that extend to the back of its skull. By contrast the whale shark and other basal taxa have much shallower, more transverse jaws, more like those of Thelodus (Fig. 1).
References
Chen et al. 2019. Development relationships between teeth and jawbones in stem gnathostomes and stem osteichthyans revealed by 3D histology: insights into the evolution of tooth replacement and tooth organization.
Vaskaninova V and Ahlberg PE 2017. Unique diversity of acanthothoracid placoderms (basal jawed vertebrates) in the Early Devonian of the Prague Basin, Czech Republic: A new look at Radotina and Holopetalichthys. PLoS ONE 12(4):eo174794.
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