Monster Mouth

Dunkleosteus terrelli

Members of Order Arthrodira, meaning joint-necked, composed the majority of the Class Placodermi and, for that reason, are the most familiar to science. Truly the top carnivores of their time, aquatic monsters such as Dunkleosteus terrelli filled the role of the largest sharks or even crocodiles in our time. They are named arthrodires due to a peculiar craniovertebral joint behind the skull, allowing the chondrocranium to rise as the splanchnocranium drops, resulting in a monstrous maw (Linzey 2001, Murphy 2004). Also unique amongst the arthrodires is the presence of gnathals, a structure performing the same function as teeth in more advanced gnathostomes. This means that teeth arose at least twice, convergently, given that placoderms have no living relatives and some arthrodires have very unique gnathal structure (Smith and Johanson 2003, Stokstad 2003).

.
Major differences, however, are structure and replacement methods. In the latter, the general opinion is that arthrodires had no replacement, unlike most fish which are polyphyodonts, though that has been recently questioned (Smith and Johanson 2003). Instead of many small teeth for capturing or rending prey, which other fishes of the time had, the arthrodires had what can be likened to broad axes in their mouths rather than daggers. These semidentine blades, supragnathals on top and infragnathals below, had a biting power surpassed only by archosauromorphs while being able to open with lightning speed (Stokstad 2003). This creates a truly frightening picture of arthrodires like Dunkleosteus. Not only did the up to thirty foot long creature have a bite surpassing anything in the oceans today, but its quickly opening gape created a pressure gradient causing suction, meaning the smallest and swiftest of prey had as much to fear as other heavily armored arthrodires.

.
Because of its size, it is supposed Dunkleosteus was an ambush predator, not unlike crocodilians. However, unlikely evidence seems to point to otherwise. A specimen was recovered with intact skin cells, bringing attention to color as well as behavior. Black and red pigments were recovered from the dorsum, while the venter was covered in a silvery, reflection layer (Waggoner 2000, Linzey 2001). This could be a shared trait, similar to the iridocytes found in modern fish, gleaming from guanine crystals. Determining the first instances of color would be crucial in determining whether this is a homology or homoplasy. This discovery had two major implications. The first was that color vision could have been around at the time, given that Dunkleosteus would have blended in with the reddish sediment if viewed from above and looked similar to the surface if viewed from the sea floor. It would have needed some visual camouflage from members of its own species, given little else could have done it harm and gouges matching gnathals from the same species have been found on their four-foot-wide armored heads (Waggoner 2000). The second point is that if Dunkleosteus was a benthic, ambush predator, it would not have evolved ventral camouflage. Needing camouflage on the venter means if was viewed enough from the bottom, and was vulnerable, to be selected for. This indicates that arthrodires are much more active predators that what was once thought, swimming as much in the middle of the water column as on the bottom of the sea.

.

Arthrodires had a range covering most seas of the Devonian, from species of the genera Confractamnis, Atlatuidosteus, and Doseyosletts of Queensland, Australia, to species in Morocco (Young 2005). Individuals have also been found in Scotland, such as Cosmacanthus, which was initially confused with an acanthodian (Newman 2004). This means that placoderms have an interesting zoogeography, their total range is sub-Equatorial, from Euramerica to Australia. However, there appears to be no evidence of placoderms unearthed from Arabia or India, lands that are between Euramerica and Australia, though areas further south in what would become North Africa have fossils. Therefore, an argument for a Gondwanan distribution could be made, though this would be tenuous. Arthrodire placoderms were apex predators, filling the niche of sharks today (Young 2005).

.

----

Linzey, D. 2001. Gnathostome Fishes. Pages 91-128. Vertebrate Biology. McGraw Hill, New York.

.

Murphy, DC. 2006. “Devonian Times.” Retrieved 05 Apr 2008 from http://www.devoniantimes.org/index.html.

.

Smith, MM & Z Johanson. 2003. “Separate Evolutionary Origins of Teeth from Evidence in Fossil Jawed Vertebrates.” Science, Vol. 299 (5610), 1235.

.

Stokstad, E. 2003. “Primitive Jawed Fishes had Teeth of Their Own Design.” Science, Vol. 299 (5610), 1164.

.
Waggoner, B. 2000. “Introduction to the Placodermi.” Retrieved 29 Mar 2008 from http://www.ucmp.berkeley.edu/vertebrates/basalfish/placodermi.html.

.

Young, GC. 2005. “A New Middle Devonian Arthrodire (Placoderm Fish) from
the Broken River Area, Queensland.” Records of the Australian Museum, Vol. 57
(2), 211-220.

Popular Dissent

It is an odd thing that evolution is disputed so much. Physics and chemistry do not have nearly so much dissent, especially from a religious or organized group. Astronomy and geology have had some problems, but they have been overlooked by the masses. For instance, the Coperinican heliocentric solar system spat on the belief that the Earth is the center of the Universe. In geology, it is not hotly debated that the formation of canyons and mountains must take place on the scale of millions of years. Perhaps it is because those things seem so distance, impersonal, and dead to most people to be relevant. Biology, on the other hand, takes the brunt of the populace's largely misinformed critiques.

.

The reason why, as I alluded to above, is that biology is something personal both to the individual and to humans as living beings. Sure, most people have a general idea about how the human body works, with basic structures, systems, and so forth. However, that limited knowledge, without thorough, or sometimes even remedial, knowledge of the mechanisms and history behind them adds a sense of mystery and awe. That cannot be argued with. Biologists working for decades are probably still as enraptured with life as ever. The layman, however, seems to have a special, dual-pronged belief about the human body that refutes evolution before any scientist can explain. The first is the religiosity of the United States, in which includes the tenet that people are on a special level compared to the rest of the material of the universe. We are too complex, they might say, to have not been created by an outside force. The argument from incredulity is the basis for the intelligent design movement, or as some have called it, 'creationism dressed up in a cheap tuxedo.'

.

The second idea about life is almost opposite to the first. It is that life is simple. Everyone has seen nature shows and watches Discovery Channel and fancies themselves knowledgeable on life. However, many creationists and intelligent design proponents laugh at the idea of Australopithecus, jesting that it appears to be some kind of joke based on the old 'Planet of the Apes' movies. Obviously, they say, this is a farce. They think they know how evolution occurs - which is usually completely random events in a progressive fashion, as they would understand it. Based on that limited, erroneous information, of course evolution is a fraud. The problem is that it is so much more complex than that. The genetic basis of mutations in the chromosome and genome up to climatic changes and ecology are all crucial to evolutionary theory.

.

Personal incredulity, therefore, actually has two bases. The first is that the body is too complex to have come about naturally, invoking a deity. The second is that life is so simple that anyone can understand it, making everyone a personal authority on it. It cannot be overstated that evolution by natural selection can be simply stated as genetic change in a population over time, but there are many, many facets of evolutionary study that such a maxim barely does it justice. The fact of the matter is that evolution is supported so thoroughly by science that it should not even be a question. Sure, there is debate about punctuated equilibrium versus gradualism, but that is one aspect of a massively supported theory.

.

Consider string theory, a hypothesis in physics which I will not even pretend to understand. There is no public outcry, however, about string theory appearing in school texts. Perhaps it is because it does not speak to the very humanity of the individual that he or she thinks they understand. It certainly does not include any tenets of religion, which may be why evolution is a prime target. Both are complex theories. The problem is with perceived understanding, which the general population claims to have much of regarding biology and little regarding physics. In reality, both are probably rather low. A little less arrogant anthrocentrism may go far in public understanding of evolution, though it will be more difficult to convince people that they may know less than they really do on their own histories, complexities, and indeed life as a whole. Evolution is not as simple as they think.