Although we certainly know that jaws were incredibly important to the evolution of early vertebrates, the evolution of the jaw itself is not very well understood. It's one of paleontology's big unanswered questions. How did jaws evolve? One of the reasons this remains a puzzle is that there is no transitional fossil that is partially jawed. Things are either completely jawless like osteostracans or have fully formed mandibular arches. The mandibular arches are very similar in their overall appearance to another set of arches in gnathostomes, the gill arches. And these are the arches that support the gills in living fishes and in some aquatic tetrapods. Because they look similar, the jaws and gill arches have long been hypothesized to be related. In 1870, the German anatomist Carl Gregenbaur suggested that the jaws and the hyoid arch that supports the jaws were modified from gill arches. Gregenbaur thought that the ancestor of jawed vertebrates had gill arches present in the pharynx, in the mouth and all the way along the pharynx. He suggested that the first gill arch was modified into the jaw apparatus of the mandibular arch. The second gill arch became the jaw-supporting hyoid arch and the rest of the gill arches remained as functioning respiratory gill arches. Is this still thought to be correct? Let's turn to the closest relative of jawed vertebrates. Osteostracans are widely considered to be the sister group to gnathostomes. Gill arches are unknown in osteostracans. Impressions on the inside of their dorsal shields suggests they had gill pouches like lampreys and hagfishes. Gill pouches are muscular structures that contain the respiratory gills. Gill arches are skeletal supports for the gill tissue. What's more, the gills of gnathostomes and osteostracans are oriented differently. In gnathostomes the gill tissues are lateral to the gill arches. So the gill tissue is external to the skeleton. In lampreys, and probably also osteostracans, the gill tissue is internal or medial to a basket-like skeleton of cartilage. We call the supporting basket-like cartilage the branchial basket. So the skeletal support of gnathostomes doesn't appear to be homologous with that of agnathans. To make matters more complicated, our living agnathans have had millions of years to evolve and are really quite derived. So they may not be good models for the early fossil agnathans. Not only do we wonder what jaws evolved from, but why they evolved. Fully formed jaws would be useful but what could have been the precursor to a jaw and what would this proto-jaw have been used for? Which of these things do you think might be uses for something that is almost a jaw but not quite? A, grasping prey, B, pumping water across the gills, C, chewing, or D, pushing water out of the mouth. Because most fossil agnathans fed by filtering particles from the water, or by sucking up mud, one thing a proto-jaw could have done before biting was to pump more water across the gills. So B is the correct answer. There is more than just one proposal as to how pumping proto-jaws evolved. In the 1990s, an evolutionary biologist named John Mallet observed that fossil agnathans had cheeks and lips around the large oral cavity, and the first gill arch was directly posterior to this oral cavity. Mallet proposed that the gill arch could have been used to help suck in water to filter for food particles. Enlargement of the gill arches and development of muscles to move them would have resulted in more efficient movement of water through the pharynx, conferring a selective advantage. This means that selection would have favored any change to the gill arches and pharyngeal muscles that would result in more efficient water flow through the pharynx, including being able to open and close the mouth rapidly. By closing the mouth, the pharyngeal muscles would be able to force water across the gills. So selection would have favored animals that could close their mouths. In this way Mallet suggested the mandibular gill arch developed into a proto-jaw entirely because it enabled the fish to open and close its mouth rapidly to increase respiratory function. The grasping capabilities of the jaw would have been a side benefit, but one that would also be acted upon by selection, quickly becoming a more and more effective jaw. Mallet's suggestion follows Gregenbaur's gill arch hypothesis that the jaws were originally formed from a gill arch, but it adds to this theory by suggesting a functional benefit of the proto-jaw. Around the same time as Mallatt, paleontologist Philip Janvier proposed another scenario for the evolution of proto-jaws. Lampreys and hagfish have a structure called a velum which is like a curtain of soft tissue that separates the esophagus from the gill pouches. The vellum is supported and moved by paired velar cartilages connected to the rest of the skull by a hinge joint, similar to a jaw. According to Janvier's hypothesis, the jaws evolved from the velar skeleton. The jaws didn't develop from the gill arches. Janviar's vellum hypothesis has one problem. The vellum extends into the throat rather than anteriorly as the jaw would do. So it's difficult to see how the jaws evolved from the vellum. A new fossil discovery has provided some new information about the earliest vertebrate gill arrangement, and raised some new questions. Remember <i>Metaspriggina</i>, a Cambrian vertebrate from the Burgess Shale Formation? This animal may have had gnathostome-like gill arches with upper and lower arch elements. The first set of arches is somewhat enlarged relative to the others and is not associated with external gills. If these arches are homologous to gnathostome gill arches, this would mean that the branchial basket arrangement in lampreys is derived, and so could not have been a precursor for the gnathostome pharyngeal skeleton. Maybe the branchial basket is derived from the gnathostome-like gill arches instead of the other way around. The origin of the vertebrate jaw is still a very active area of research in vertebrate paleontology and evolutionary biology. It's probable that the real story was some combination of Gregenbauer, Mallet, and Janvier's hypotheses. We may not know for sure how the vertebrate jaw evolved, but we certainly have a clearer picture now, than we did 100 years ago. New fossil discoveries and new developments in molecular evidence from extant vertebrates continually add to our understanding of this evolutionary puzzle. In any case, we certainly know that jaws allowed early vertebrates to rapidly diversify into many new ecological niches. By the Late Silurian, gnathostomes had diversified into four major groupings, two of which remain today. These groups were the Placodermi, the Chondrichthyes, the Osteichthyes, and the Acanthodii. We'll discuss members of each of these groups in the rest of this lesson and in the next lesson of this course. All four of these groups possessed mandibular arches, jaws, a hyoid arch that helped connect the mandibular arch to the brain case, a third, horizontal, semicircular canal, and paired pelvic fins, in addition to the paired pectoral fins we saw in Osteostraci. Gnathostomes were already represented by large predatory fishes in the Late Silurian, as we have found fossils of large placoderms, acanthodians and osteichthyans. Chondrichthyans aren't well represented by fossils from the Silurian. But that's likely because chondrichthyans are cartilaginous, and don't have as many distinctive bony parts as the other three groups. Gnathostomes would continue to evolve and diversify, becoming the dominant vertebrates, throughout the Devonian, also known, as the Age of Fishes. Let's not forget our paleogeographic context. During the Early Devonian, Laurentia, Baltica and Avalonia completed their long collision and formed a supercontinent called Euramerica, also forming the Caledonian mountain range in the process. During the Devonian Period, the supercontinent of Gondwana and Euramerica would start to draw together. Eventually, all the continents would collide to form one giant supercontinent called Pangea, but not until the Permian Period. All of this tectonic activity meant that sea levels during the Devonian remained high, forming plenty of shallow sea habitat for reef building corals and relatives of sponges called stromatoporoids, as well as brachiopods, trilobites and coiled cephalopods called ammonoids. Agnathan vertebrates started to decline. But gnathostome vertebrates increased in diversity in both marine and freshwater environments. Our gnathostome fossil examples for this lesson come from two Canadian fossil localities, the Early Devonian locality of Man On The Hill or MOTH in the MacKenzie Mountains of the Northwest Territory, which we visited in lesson two, and the Late Devonian locality of Miguasha in Quebec. Miguasha is one of the most famous fossil localities in the world, having yielding thousands of spectacularly preserved fossil fishes since its discovery in 1842. Many of the fossils found in these rocks represent marine animals. The rocks include sandstone and mudstone, and some preserve ripple marks. In lesson one, we describe several types of paleoenvironments. What kind of paleoenvironment does Miguasha represent with its mix of terrestrial and marine characteristics? Is it A, lacustrine, B, continental shelf, C, fluvial, or D estuarine? Lacustrine, meaning lake, and fluvial, being river, are both fresh water environments. Continental shelf deposits are fully marine. This mix of marine and terrestrial characteristics suggests Miguasha was an estuarian environment. So D is the correct answer. Estuarian environments like those of the Devonian in Miguasha support some of the richest diversity of life on the planet, as nutrients from the sea mix with nutrients from the land. In addition, the waters at Miguasha like at MOTH were somewhat oxygen poor and the sedimentation rate was high. As soon as a fish died and settled to the bottom of the estuary, it would be rapidly buried. This protected the remains from the water current as well as preventing scavengers from reaching them. The mud around the carcass was even more anoxic than the water above, as evidenced by the presence of iron pyrite, similar to what we see in MOTH. Although they are different ages, between these two localities all four groups of gnathostomes are represented. We'll use examples from Miguasha, MOTH, and some spectacular examples from other localities, like the Gogo Formation of Australia, as we continue to travel along the vertebrate tree of life. This may be a good time to refresh your memory of the vertebrate phylogenetic tree. Open up the interactive zoomable tree, and take a look at the gnathastome node. Specifically take a look at how many branches there are between the gnathostome node and the branches with living gnathastomes. These branches represent the most primitive group of jawed fishes, the placoderms.
