Jaws are crucial to the evolutionary success of many animals, yet their origins have long been shrouded in mystery. Now a new discovery is shedding light on how the jaws of ancient fishes are related to our own.
Prehistoric armoured fishes called placoderms were the first fishes to have jaws. They arose some time in the Silurian Period, about 440 million years ago, to become the most abundant and diverse fishes of their day.
Placoderms dominated the oceans, rivers and lakes for some 80 million years, before their sudden extinction around 359 million years ago. This is possibly due to the depletion of trace elements in our oceans.
But placoderm jaws bear no resemblance to those of any living animal. So the question was, how did they evolve, and what is their relationship to modern jawed animals?
A paper published today in Science by Zhu Min and colleagues at the Institute of Vertebrate Palaeontology and Palaeoanthropology in Beijing, shows how placoderm jaws evolved and then rapidly changed. The key is a newly discovered fish named Qilinyu (pronounced “chee-lin-you”), which lived some 425 million years ago and was discovered at a site in Qujing, China.
It has an unusual set of jaws that is similar to both those of traditional placoderms and those of modern bony fishes, or osteichthyans.
This is significant because we are all ultimately descended from osteichthyans, as this group includes the lobe-finned fishes (Sarcopterygii), from which all four-limbed land animals arose.
As Zhu Min explained to me:
With this finding, we can now trace our dermal jaw bones (dentary, maxilla and premaxilla) to the most primitive jawed vertebrates. The gnathal plates of conventional placoderms, such as the gigantic Dunkleosteus, are the homologues of the marginal jaw bones of bony fishes and tetrapods.
From simple jaws to complex ones, or vice versa?
Before this new discovery, we had a very poor record of Silurian placoderms.
But the latest fossil is just the latest in a series of intriguing discoveries made over the past decade at Qujing. These include the oldest known complete bony fish, Guiyu, as well as the oldest known complete placoderms.
One of these, Entelognathus is arguably one of the most significant transitional fossils found in the past century, bridging a huge morphological gap between the extinct placoderms and the living bony fishes.
Such discoveries have profound philosophical implications, as they finally link our tetrapod line directly back to placoderms, which were the first vertebrates to evolve the complex suite of anatomical features that are found in all living higher animals today.
These not only include jaws and teeth, but also paired hind limbs, complex inner ears with three semicircular canals, a pattern of paired plates protecting the brain, and an interesting way to reproduce using external genital organs.
We humans are the product of just one of countless evolutionary lines, all of them shaped by this underlying vertebrate body plan.
Entelognathus had a complex set of externally ornamented dermal bones covering its lower jaw, as well as structures called gular plates underneath its throat.
It had a lower jaw formed of dentary and infradentary bones, and an upper jaw comprising premaxilla and maxilla – a pattern typical of all early osteichthyans and most early tetrapods.
This posed a conundrum for evolutionary biologists. Were the first jaws really complex structures made of many external dermal bones, as in Entelognathus and osteichthyans, or were the simple blade-like jaws of most placoderms the primitive condition?
Why Qilinyu matters
Enter the newly discovered Qilinyu, a bizarre armoured fish with a head shaped rather like that of a dolphin. Yet its most significant feature is its jaws, which are subtly reinforced by a small external surface of dermal bone, a trend that is more extensively developed in Entelognathus.
Unlike Entelognathus, the lower jaw of Qilinyu does not have a complex set of external dermal bones. Instead it has a simple bent blade-like jaw with a small part participating in the palate, and a large part participating in the face.
In this respect it more closely resembles other “typical” placoderms, whose lower jaws are mainly developed along the palatal face for biting.
The upper jaws of typical placoderms, like Bruntonichthys shown above, are entirely inside the mouth, attached at the front to the palate, and have no external dermal bone exposed.
Qilinyu shows us that the typical upper jaw bones of most placoderms had to gain an external dermal bone lamina so as to become incorporated into the cheek bones in later animals. Another bone in the cheek of most living animals – called the jugal bone, or zygomatic bone in mammals – also first appears in these early placoderms.
The discovery of Qilinyu confirms that the upper jaw bones of placoderms are indeed the evolutionary equivalent of the premaxilla and maxilla of other animals. These two bones persist in evolution and are still present in most mammalian skulls, although in some cases, such as in humans, the premaxilla is incorporated into the maxilla.
Qilinyu also provides the first hard evidence that the earliest placoderms had paired pelvic fins. Recent work has shown that placoderms were the first vertebrates to have paired hind limbs and paired external genitalia called “bony claspers”, formed by the same development process as typical vertebrate limbs.
The next big question for early vertebrate palaeontologists is the issue of when and how teeth first evolved. Was it at the same time as the first jaws, or before or after?
Neither Entelognathus nor Qilinyu has teeth, and neither do the most primitive of all placoderms, the antiarchs. Some placoderms with teeth are controversial as to whether these constitute real teeth or not.
But we also know that certain placoderms called arthrodires had relatively advanced tooth structures, with a primitive kind of dentine and pulp cavities.
Why did teeth evolve in some placoderms and not others? And are our own teeth and those of our fellow tetrapods evolved from placoderms’ teeth, or did they arise separately? The only way to find the answers will be to find more fossils – so it’s back to the trenches.
John Long, Strategic Professor in Palaeontology, Flinders University and Brian Choo, Postdoctoral fellow in vertebrate palaeontology, Flinders University
This article was originally published on The Conversation. Read the original article.