Sunday, 14 June 2009
The following is a paper by Katy Connor, geology graduate here at MSU with a biology minor. Katy was an instrumental part of the team of science students that assembles whenever creationists come to campus. I still take pride in being the only non-science major part of that team. :D
For other science students who have offered their work here, see astronomy major Aron McCart's paper on gravitational lensing (as well as his paper on the Large Hadron Collider) Also, I'll be posting one of Stephen Fullerton's (anthropology) papers on here later this week. I'll also try to acquire some of fellow-Juggernaut Amber Culbertson-Faegre's work (psychology).
Remember as you read through this that it is the work of a mortal (though a particularly scholarly and sharp mortal). But Katy is not all-knowing, not even close. Even the most brilliant of us are ignorant of almost everything there is to know. So the question you should ask yourself is why none of the books claiming authorship by an all-knowing being come close to resembling this. How is it that mortals understand and explain the world more clearly than the author of first century allegoric literature?
I have an idea...
What Were They Thinking?
Land is Conquered by the Animals
The diversity of land animals today takes its roots as late as the Silurian Period (possibly the Ordovician Period) about 443 million years ago starting with arthropods. As some of the lobe-finned fish of the seas began to develop more sophisticated skeletal features such as humeral bones, they evolved into the early tetrapods and amphibians. Other developments made by the tetrapods not including their bone structure contributed to their further decline in reliance on water, such as lung development and the amniotic egg. Modern reptiles can give an idea of how the early tetrapods may have lived and are often used as analogies for their behaviors. The phylogeny of the early tetrapods branched out into the groups that would bring about the dinosaurs, first mammals, crocodiles, and birds.
Understanding the evolution of early tetrapods also brings about a better understanding of transitional fossils and their importance to the evolutionary world. Transitional fossils provide more detail to how animals can develop from one stage to another and close gaps that otherwise would keep that information hidden. Not all tetrapods remained on land however, as some eventually returned to the sea and continued to evolve into the ancient water reptiles, followed by the vast diversity of undersea life that is present today.
The main topic addressed in this paper is how early animals, arthropods and vertebrates, came up onto the land from the water and why doing so would have been beneficial. Plants had already begun to diversify on land by the time the arthropods were ready to come up during the Silurian period about 443 million years ago. By the time vertebrates were ready to take their first steps on land in the Late Devonian Period, arthropods were becoming very well adapted, and the plant diversification had increased.
The first steps to the conquest of land, especially by the vertebrates, were not the easiest. While they had begun to evolve certain characteristics that gave them the ability to pull themselves onto land for whatever purpose, they still couldn’t completely do without water. Several paleontologists including Jennifer Clack and Neil Shubin, discoverers of two important Devonian tetrapods, have studied the fossil evidence left by the ancient vertebrates in order to better understand what their advantages and disadvantages were at the time. Paleontologists have also uncovered new transitional forms, which give an even better understanding of the connections between different evolutionary groups.
In general, my thesis can be best articulated as such: Through learning how to use natural resources and their newly developing body structures like limbs, long rib cages, and good pelvic girdles, animals were able to evolve to survive and diversify in the new terrestrial environment. Through the early tetrapods, several groups branched out and evolved into the variety of species that live on Earth today.
The first animals to come up onto the land were the early arthropods. The phylum Arthropoda, which means “jointed feet,” includes groups such as chelicerates (arachnids, horseshoe crabs, etc.), myriopods, and eurypterids. Myriapods include centipedes and millipedes. The name means “10,000 feet,” which is strictly a hyperbole since no recorded myriapod has had more than 750 feet. Eurypterids are more commonly known as sea scorpions and could sometimes reach lengths over six feet.
Since arthropods had their roots in the marine environments, it stands to be curious as to how the new terrestrial groups were able to adapt to breathing pure air as opposed to using a form of filtration system like gills. In an article by Carsten Kamenz, he and his colleagues discuss evidence of what is called a book lung that were lined with lamellar spines and trabeculae. “First, trabeculae and lamellar spines provide conclusive evidence that these were the lungs of fully terrestrial, air-breathing animals. Neither structure is seen in the book gills of horseshoe crabs, and thus the lamellae… are undoubtedly those of a functional lung and not, for example, a gill within a brachial chamber (Kamenz, 2008).” Having possession of a finely modified breathing system would have been advantageous to early arthropods and would have allowed for a greater ability to journey further on land than those would still needed to stay closer to the water.
However, there is a gap in the fossil record that spans from 360 to 340 million years ago known as Romer’s Gap in which very few fossils from this time span have been found. Since both sides of the gap show different patterns of diversity in terrestrial organisms, it was thought that the gap was simply due to mysterious taphonomic processes or insufficient sampling. Both of those hypotheses, however, were put to rest. “The presence of two separate phases of arthropod diversification on either side of Romer's Gap, but with long-ranging taxa common to both intervals, argues against the gap being a taphonomic artifact or period of undersampling. New data from herbivory, such as the earliest example of insect folivory from the Late Mississippian preceding a reasonably ascribed culprit by 6 My, supports the inference from taxonomic range data that arthropods were present, but at low diversities, rather than representing a sampling artifact from an inadequate fossil record (Ward, 2006).” Oxygen content in the atmosphere was a major factor in how well the arthropods could develop and diversify on land. “Problems included not only acquiring O2, but the consequential problem of eliminating of CO2, a process more difficult in air than in water. However, benefits would have included an opportunity for greater body-size increases, perhaps modulated by the 10-My lag behind an atmospheric O2 rise for arthropods during the Late Silurian to Early Devonian; for larger-sized vertebrates, the origin of amphibious structures immediately before Romer's Gap; and for both groups, the subsequent and attendant consequences for occupying new land-based habitats by large-sized taxa during the Middle Mississippian to Lower Permian (Ward, 2006).”
Between the Silurian and the early Carboniferous Period, the oxygen concentration in the atmosphere reached from 17% to over 30%, which is over 10% higher than today’s oxygen levels. Since arthropods don’t rely on an indirect circulatory system to deliver oxygen to their tissues (they possess air tubes that take the oxygen there directly), they were able to reach massive sizes. For example, some myriapods could reach lengths of over three feet long. Carbon Dioxide levels declined as Oxygen levels rose, starting at a concentration of about 4500 ppm and falling to 800 ppm by the start of the Carboniferous Period. The climates of the Silurian and Devonian Periods were also very warm and very humid, much like rainforests, and reached average temperatures in the lower 70’s degrees Fahrenheit.
The Vertebrates and Early Tetrapods – The Devonian
While the arthropods were still developing their land legs, so to speak, during the Late Devonian Period, vertebrates from the marine environments were evolving new and unique characteristics that would allow them to develop into the first land vertebrates. A few of the most notable vertebrates from this time are Tiktaalik roseae, a transitional species, Acanthostega, and Icthyostega.
“It has long been clear that limed vertebrates (tetrapods) evolved from osteolepiform lobe-finned fishes, but until recently the morphological gap between the two groups remained frustratingly wide (Ahlberg, 2006).” The gap that Ahlberg refers to is the gap between the predatory fish Panderichthys and the primitive tetrapod Acanthostega. Panderichthys was preceded by the lobe-finned fish Eusthenopteron, which still possessed a more typical fish body structure (fig. 1)
Figure 1: Basic lineage of the early tetrapods of the Late Devonian Period and Carboniferous Period. The lack of a spinal diagram for Panderichthys is due to poor knowledge of its skeleton. (Ahlberg, 2006)
“Lobe-finned fish were common and diverse throughout the Devonian and Carboniferous. Their possession of paired fins with a strong skeletal axis and an open swim bladder enabled them to adapt to life near the bottom or in shallow, oxygen-poor water (Carroll, 2005).” Figure 1 shows that Panderichthys was the first in line to lose a few key fish features; dorsal and anal fins. “The absence of dorsal and anal fins and reduction of the dorsal and ventral lobes of the caudal fin, as well as the very low profile of the skull, suggest that Panderichthys was adapted to life in extremely shallow water (Carroll, 2005).” Dorsal fins were used by fish to prevent them from rolling over while turning in the ocean water. However, in a shallow water environment where there is less water pressure, the need for the prevention of rolling over during a sudden turn isn’t as high as it would be in the deeper marine environments. However, the issue of the poorly-oxygenated water still stands, and the question remains as to how the primitive tetrapods were able to maintain their metabolism and get oxygen to survive.
The shallow waters of the Devonian and Carboniferous were warmer and more capable of catching sunlight than the deeper oceans. Modern day reptiles, crocodiles especially since they share a similar body structure with the early tetrapods, demonstrate the ability to use radiant heat from the sun to build up their energy. “In addition, blood circulation to and from the limbs could have been subject to control (as in modern tetrapods) to increase flow to and from the rest of the body while basking, restricting heat loss from the body core when they entered cold water (Carroll, 2005).”
The other issue that had to be addressed was how to get oxygen in such an oxygen-deprived environment. There were certain physiological developments and physical changes on the primitive tetrapods that gave them more options to get oxygen into their systems. Upon examining the transition to more tetrapod-like features, especially those seen in Tiktaalik, Ahlberg (2006) states that “It turns out that the distal part of the skeleton is adapted for flexing gently upwards – just as it would if the fin were being used to prop the animal up. Although these small distal bones bear some resemblance to tetrapod digits in terms of their function and range of movement, they are still very much components of a fin.” Fish do not possess the ability to move their heads up and down, either. Primitive tetrapods likely had the ability to move themselves as close to the surface as possible and lift their heads above the surface to get a lungful of air before going back underwater. Also, as noted by Carroll (2005), “The choanate or osteolepiform fish, including Eusthenopteron and Panderichthys, were unique in the possession of internal nostrils, which may have enabled them to respire with the mouth closed, making use of a buccal pump.” Buccal pumping can be most easily observed in modern frogs. The movement of their throats bulging out and sucking back in is the external appearance of buccal pumping (fig. 2).
Figure 2: Process of buccal pumping in modern amphibians. Some of the pre-tetrapods were able to get oxygen through this process to survive in oxygen-poor water. (www.wikipedia.org)
Tiktaalik roseae – Missing Link
The transitional fossil Tiktaalik roseae was discovered in Nunavut, Canada, in 2004 by Neil Shubin. For several years, there had been a substantial gap between Acanthostega, another primitive tetrapod, and the Panderichthys. Upon its discovery, Tiktaalik was soon believed to be the proverbial “missing link” between the two animals. While Tiktaalik wasn’t completely separated from Panderichthys by physical appearance, it does show some of its own unique developments and features that its predecessor didn’t have. These differences are what define it as a transitional fossil. But what were these differences?
Tiktaalik had evolved features that were more tetrapod-like than those of its predecessors. “The bony gill cover has disappeared, and the skull has a longer snout… A longer snout suggests a shift from sucking towards snapping up prey, whereas the loss of the gill cover bones (which turned the gill cover into a soft flap) probably correlates with reduced water flow through the gill chamber (Ahlberg, 2006).” The overall body structure became more robust, especially in the rib region. Having more robust ribs served as beneficial to supporting its own weight whenever Tiktaalik came out on land. It is also important to note that while Tiktaalik still had fish-like fins, the bones inside closely resembled primitive humeral bones and shoulder limbs.
The early tetrapod Acanthostega was officially discovered in 1987 by Jennifer A. Clack. It was the first tetrapod to have recognizable limbs and digits in its feet as it started evolving less fin-like limbs. “In Acanthostega, the ulna extends well beyond the radius, precluding an effective wrist joint, and most of the carpus remains unossified (Carroll, 2005).” While Acanthostega was developing decent humeral bone structure in its arms and legs, it still lacked effective wrist and ankle joints since the bones in the forearm weren’t evenly set. Lacking these structures was impractical for much movement on land since the body weight couldn’t be supported properly at the feet or the elbows.
Acanthostega did possess primitive lungs, like Tiktaalik, but the rib cage wasn’t nearly long enough to provide any kind of support for them if the tetrapod wanted to come up on land (fig. 3). “Although clearly a tetrapod, Acanthostega retains a variety of aquatic adaptations, including flipper-like appendages, internal gills, and a broad finned tail (Shubin, 2004).” Acanthostega remained a marine tetrapod. However, it still remained in the shallow waters since, like Tiktaalik, it lacked dorsal fins. The shallow waters also gave it the ability to sun itself, giving it the energy it needed to hunt or avoid being hunted.
Figure 3: Rib structures spanning upward from Eusthenopteron to Ichthyostega. Acanthostega is at C. Note the short rib cage. (Carroll, 2005)
A close relative of Acanthostega, Ichthyostega was a late Devonian tetrapod discovered in 1931 in eastern Greenland. Like Acanthostega, Ichthyostega possessed a finned tail (fig. 3), though less conspicuous, but its ribcage was longer and far more robust than its relative’s. That alone gave it more of an advantage to walk on land more effectively. Also, the forelimbs and shoulders of Ichthyostega had changed morphologically. According to Carroll (2005), “The shoulder girdle is also decoupled from the skull by the loss of the dermal elements of the operculum and the suprascapular bones that link the back of the skull table to the dorsal portion of the shoulder girdle.” The better the girdle structure, the better the muscles will be to hold up body weight in a lack of buoyancy. Furthermore, Ichthyostega had almost completely forgone the need for gills, as its lungs were the primary tools for breathing.
Interestingly enough, though, it is hypothesized that only the juveniles could walk freely on land. While having a more robust and longer rib cage is essential for body cavity support, the bones became heavier as the animal aged. The hind limbs of Ichthyostega were shorter than the front too. The forelimbs would have been useful in pulling the tetrapod up onto land to sun itself, however, which was still a key behavior to survival.
The fossil evidence of Ichthyostega shows something interesting about the early tetrapod, as well as its relative Acanthostega. There was variation among different members of the two groups. “In both the Ichthyostega and Acanthostega material, there is incongruence between size and ontogenetic state that presumably implies either individual or sexual variation in growth patterns (Callier, 2009).”
“Ichthyostega, with its morphological peculiarities and observed intragenic variation, further adds to the picture that the early evolution of tetrapods in the Famennian involved considerable exploration of morphology, function, and ecology rather than merely a linear progression towards being fully terrestrial (Blom, 2005).”
While the Devonian tetrapods weren’t gifted with the best features for terrestrial life, they still made the first steps onto the land. They used what could have been a very new and possibly dangerous environment to their advantage (sunning, catching nearby arthropods for food, etc.). However, at the end of the Devonian Period came the Late Devonian Extinction. Whatever the exact cause of the extinction was, it took a toll on the diversity of the tetrapods and no doubt forced a major setback to the development of Carboniferous tetrapods and amphibians.
In the strictest sense, while some Devonian tetrapods such as Ichthyostega did resemble primitive amphibians, they aren’t included in the group. Amphibians didn’t truly arrive on land until the Carboniferous Period. Some of the amphibian groups of that time included the Labyrinthodonts and Lepospondyli, both of which still had a heavy reliance on water. However, the tetrapods would manage to survive and evolve past that dependence to the point where all water was needed for was to keep from dying of thirst.
Return to Water
Obviously, not all tetrapods evolved to stay on land, as mentioned above. Several of them stayed in the water to never again come up to land for anything. It’s possible some of them may have stemmed from Acanthostega and other fish since they never really adapted to land themselves. As time progressed past the Paleozoic Era and into the Mesozoic Era, these tetrapods would provide the means for the evolution of the great marine reptiles, including Plesiosaurs, Mososaurs, ancient turtles, sharks, and Ichthyosaurs. Gaps still remain in the fossil record, however, that hide the exact steps they took to evolve that way. Hopefully as research and field work continue on, those gaps can be filled. Further analyses can promote our understanding of past morphology, not to mention present morphological processes.
Even though not all tetrapods went back into the water, some of those who remained on land still heavily relied on it (i.e. Amphibians), making the best of both worlds. One of the main reasons they still needed water was to lay their eggs. They also needed a place to cool themselves off after getting sun as they probably weren’t capable of regulating their own body temperatures.
The Land Lovers
A key development to the weaning of tetrapods away from water was the evolution of the amniotic egg. Amniotic eggs have hard, solid shells that don’t require water to be incubated. Up until that point, tetrapod eggs were very gel-like and had to stay underwater, much like today’s amphibian eggs. Without having to be raised in water, the babies didn’t need to develop the same reliance on it that their ancestors did.
During the Carboniferous, tetrapods evolved key physical features that would significantly aid them in their ability to walk on land; elbow and wrist/ankle joints. During the Devonian, the tetrapods only had stiff arm bones that didn’t allow for much give in these two regions. While it’s true the humeral bones gave them a leg to stand on at least, they weren’t enough to let the creatures walk efficiently while supporting their body weight. Joints allow for give and easier movement when walking. If anything else, having moveable joints would be more comfortable to walk on.
The possible presence of individual size and sexual variation, especially in the Ichthyostega and Acanthostega fossils, could also be visible evidence of the animals’ abilities to adapt to their own different environments. This could be one of the first steps as to how these groups helped bring about the following diversity of future tetrapods, as adaptation is one of the keys to surviving natural selection. Since juveniles of Ichthyostega were more able to walk on land, being able to look at the different sized fossils and determine which could have been the juveniles would bring about a better understanding of why they did better on land than the adults. Also, it’s possible that due to the individual size differences, some of the adults may have been small enough to still walk on land themselves past the juvenile stages.
The tetrapods that remained on land branched out into several different groups of terrestrial animals. As they progressed into the Mesozoic Era, they began evolving into the Thecodonts, which branched into the dinosaurs. Other sections of the phylogenic tree moved off to lizards and snakes, crocodiles, and the birds. And most importantly, especially for us humans, one of their branches led off to mammals. Given at least the phylogenic evidence, many believe that humans evolved from the water. While there may be some debate over this issue, first from lack of solid fossil evidence, and second from certain religious factions, it’s very possible we can call the Devonian tetrapods some of our first ancestors. It is very important, in order to learn more about and understand human ancestry, that more solid fossil evidence of the transitions from semi-aquatic tetrapod to mammal be found.
Why Come On Land?
The question that remains is why land had any appeal to it whatsoever. The marine environments had been familiar to the tetrapods up to this point, whereas land was completely unknown.
As mentioned earlier, the shallow waters that the tetrapods lived in during the Devonian were very oxygen-poor. Even though the oxygen content in the atmosphere at the time wasn’t as high as it is today (only 75% of present levels), the air above the water surface was more abundant in it than the water was. Since tetrapods had begun to evolve their lungs alongside their gills, having the ability to come up on land far enough to get a gulp of air would have proved very beneficial to their survival in their shallow marine territories.
The fact that arthropods were already on land too may have worked out to their advantage. Since there weren’t any natural predators to the tetrapods on land, being able to snap up a passing arthropod from above the surface without putting their own lives in danger would have aided in their survival chances. Though it’s possible that some tetrapods ate other tetrapods, as fossils do contain evidence of pointed carnivorous teeth, they would have been safer from most of their other enemies that could only live in deep waters.
Finally, the previously mentioned ability to sun themselves was a very useful advantage that other marine life wouldn’t have had since they were in the deeper waters. Radiant heat from the sun provides a high amount of kinetic energy in the body, which was something they would have needed in order to escape predators, hunt for prey, or even just to move around. Building up that much energy also gave a good boost to their digestion and metabolic rates.
The beginnings of land conquest by arthropods in the Silurian and vertebrates in the Late Devonian and Carboniferous Periods brought about much of the land diversity that is seen in the present time. Considering the challenges the new terrestrial environment posed to the early tetrapods, not to mention the extinction event at the end of the Devonian, they branched out and evolved into very efficient survivors. Several fossils have been found and studied thoroughly to understand how they came from their watery homes. Using body structures, bone developments, and analogies to modern tetrapods, we have come to a better understanding of how that happened.
Even though there isn’t much solid fossil evidence showing these evolutionary processes, many feel safe enough to agree that much of the life we have on the planet today came from the water.
Ahlberg, P.E., and Clack, J.A., 2006, A firm step from water to land: Nature, v. 440, p. 747-749.
Blom, H., 2005, Taxonomic Revision of the Late Devonian Tetrapod Ichthyostega from East Greenland: Palaeontology, v. 48, Part 1, p. 111-134.
Callier, V., Clack, J.A., Ahlberg, P.E., 2009, Contrasting Developmental Trajectories in the Earliest Known Tetrapod Forelimbs: Science, v. 324, doi: 10.1126/science.1167542.
Carroll, R.L, Irwin, J., Green, D.M., 2005, Thermal physiology and the origin of terrestriality in vertebrates: Zoological Journal of the Linnean Society, v. 143, p. 345-358.
Kamenz, C., Dunlop, J.A., Scholtz, G., Kerp, H., Hass, H., 2008, Microanatomy of Early Devonian Book Lungs: Biology Letters, v. 4, p. 212-215.
Shubin, N.H., Daeschler, E.B., Coates, M.I., 2004, The Early Evolution of the Tetrapod Humerus: Science, v. 304, p. 90-93.
Ward, P., Labandeira, C., Laurin, M., Berner, R.A., 2006, Confirmation of Romer's Gap as a low oxygen interval constraining the timing of initial arthropod and vertebrate terrestrialization: PNAS, v. 103, doi: 10.1073/pnas.0607824103.
Buccal Pumping image: www.wikipedia.org