Your Bony Jaw Origins Revealed by Early, Armored Fish

We take our teeth and jaws for granted. Although our back-boned body plan originated over 540 million years ago (Shu et al., 2003), it was not until at least 450 million years ago that our common ancestors began to sport jaws. Data from careful anatomical comparisons, observations of embryonic development, and study of developmental genes strongly indicate that our jaws evolved from the cranial (those towards the head) gill supports called branchial arches (Cohn, 2002; Cerny et al., 2004; Rücklin et al., 2012; Compagnucci et al., 2013). Once jaws arrived, the world was never the same: vertebrates could now grow larger and support this growth by consuming larger prey.

In all jawed vertebrates (including you, dear readers), our upper and lower jaws first appear during our embryonic development as cartilaginous elements. The upper element is called the palatoquadrate and will become the base of support for the upper jaw, whereas the lower element is called the Meckel’s cartilage, and this will undergird the lower jaw (Liem et al., 2001). In cartilaginous fishes, such as sharks, these two elements are the jaws. This is because, unlike most jawed vertebrates, sharks and their kin never develop a bony skeleton. We belong to the clade of jawed vertebrates named the Osteichthyes and all bony fish to tetrapods encase or replace our cartilaginous skeleton with bone. During our time as embryos, the palatoquadrate becomes encased in the tooth-bearing bones of the upper jaw, the premaxilla and maxilla (as well as other bones), and the Meckel’s cartilage is eventually entombed in lower jaw bones, including the tooth-bearing dentary bone. Thus, in humans our “mandible” is, in fact, the dentary bone.

But how did these bones originate? Some of the earliest jawed vertebrates known are collectively identified as placoderms. Placoderms came in a variety of body morphs and sizes, and the largest ones stretched well over 20 feet and possessed bony jaws studded with blade-like teeth. Traditionally, placoderms were considered primitive curiosities of the early radiations of jawed vertebrates but not very helpful in terms of understanding the evolution of our own face and jaws (Long, 2016). Instead, it was hypothesized that cartilaginous fishes retained the most primitive versions of the jawed vertebrate skull. This made intuitive sense given that our skeletons begin as cartilaginous models replaced later with bone. Thus, it was assumed that by studying the anatomy and development of cartilaginous fishes we could gain insights into the origins of our own facial and jaw skeletons.

But these traditional assessments of placoderms and cartilaginous fishes are in flux. For example, it has been discovered that cartilaginous fishes have the same genes to create a bony skeleton as all other jawed vertebrates do. Thus, it now appears that cartilaginous fishes were originally bony and lost bone during their evolutionary history (Zhu, 2014). Moreover, phylogenetic studies of placoderms suggest they are not a single, natural branch of the jawed vertebrate family tree but are instead basal (close to the common ancestor) members of the more familiar cartilaginous and bony fishes (Brazeau and Friedman, 2015). Thus, placoderm anatomy has regained its relevance in providing a window into understanding the origins of our face and jaw bones (Long, 2016).

Placoderms do have a head and jaws comprised of multiple bony elements much like our own. But the homologies of these jaw bones have been difficult to work out and there are some differences. In particular, the upper and lower jaw bones of placoderms had their biting, tooth-bearing surfaces towards the inside of the mouth and had no outer covering of dermal bone. In contrast, the premaxilla, maxilla, and dentary bones of Osteichthyes are covered by dermal bone and the tooth-bearing surfaces form a biting edge near the outside of these bones (Long, 2016). Thus, although the pattern and sequence of the jaw bones in placoderms was eerily similar to those of Osteichthyes, it was difficult to determine from the pattern alone if these bones were truly homologous.

Two new fossils from Quijing in Yunnan, China, have provided us with direct bony evidence that placoderms did indeed have a premaxilla, maxilla, and dentary (Zhu et al., 2013, 2016). One of these placoderms, Qilinyu, retains the typical placoderm condition where the tooth-bearing surfaces of the jaw bones are situated towards the inside of the mouth. However, something fascinating has occurred with its tooth-bearing lower jaw bone: a new layer of dermal bone is present that appears to reinforce the lower jaw externally! This is fascinating because we see in Qilinyu what is likely the incipient beginnings of our own jaw development. Moreover, another fossil from the same locality, Entelognathus, has even more extensive development of dermal bone around the premaxilla, maxilla, and dentary (Zhu et al., 2013). These fossils suggest that our premaxilla, maxilla, and dentary bones evolved as the external, bracing dermal bones allowed a shift in the orientation of the biting surfaces of the jaws.

Phylogenetic analyses of these fossils place them close the origin of both the cartilaginous fishes and the Osteichthyes (Zhu et al., 2016). This strongly suggests that the pattern of bones that make up our jaws originated before our ancestors parted ways with our cartilaginous fish brethren (Zhu et al., 2016)! The position of these fossils in the jawed vertebrate family tree coupled with their possession of a premaxilla, maxilla, and dentary further bolster the hypothesis that the ancestors of sharks and their relatives once had a bony skeleton but gave it up during their evolutionary history. Thus, placoderms, once considered an evolutionary dead-end, turn out to have had a significant role in the evolution of our jaws (Long, 2016).

Our understanding of the history of life on earth, and the origins of our own anatomy, is often dependent on a global understanding of the fossil record. As more scientists from around the world explore and report on the fossils that form their country’s natural heritage, we are gaining a richer and unprecedented look at our collective evolutionary history. Entelognathus and Qilinyu both show how a more global, inclusive vertebrate paleontology will enhance the patchwork of fossils that comprise our shared natural history.

Submitted by: Matthew F. Bonnan, Stockton University

References Cited

Brazeau, M. D., and M. Friedman. 2015. The origin and early phylogenetic history of jawed vertebrates. Nature 520:490–497.

Cerny, R., P. Lwigale, R. Ericsson, D. Meulemans, H.-H. Epperlein, and M. Bronner-Fraser. 2004. Developmental origins and evolution of jaws: new interpretation of “maxillary” and “mandibular.” Developmental Biology 276:225–236.

Cohn, M. J. 2002. Evolutionary biology: Lamprey Hox genes and the origin of jaws. Nature 416:386–387.

Compagnucci, C., M. Debiais-Thibaud, M. Coolen, J. Fish, J. N. Griffin, F. Bertocchini, M. Minoux, F. M. Rijli, V. Borday-Birraux, D. Casane, S. Mazan, and M. J. Depew. 2013. Pattern and polarity in the development and evolution of the gnathostome jaw: Both conservation and heterotopy in the branchial arches of the shark, Scyliorhinus canicula. Developmental Biology 377:428–448.

Liem, K., W. Bemis, W. Walker, and L. Grande. 2001. Functional Anatomy of the Vertebrates an Evolutionary Perspective. Thomson Brooks/Cole, Belmont (Calif.), 703 pp.

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Rücklin, M., P. C. J. Donoghue, Z. Johanson, K. Trinajstic, F. Marone, and M. Stampanoni. 2012. Development of teeth and jaws in the earliest jawed vertebrates. Nature 491:748–752.

Shu, D.-G., S. Conway Morris, J. Han, Z.-F. Zhang, K. Yasui, P. Janvier, L. Chen, X.-L. Zhang, J.-N. Liu, Y. Li, and H.-Q. Liu. 2003. Head and backbone of the Early Cambrian vertebrate Haikouichthys. Nature 421:526–529.

Zhu, M. 2014. Bone gain and loss: insights from genomes and fossils. National Science Review 1:490–492.

Zhu, M., P. E. Ahlberg, Z. Pan, Y. Zhu, T. Qiao, W. Zhao, L. Jia, and J. Lu. 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354:334–336.

Zhu, M., X. Yu, P. E. Ahlberg, B. Choo, J. Lu, T. Qiao, Q. Qu, W. Zhao, L. Jia, H. Blom, and Y. Zhu. 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature 502:188–193.
Posted: 12/19/2016 12:01:00 AM by matthewbonnanadmin | with 0 comments
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