The fish-tetrapod transition has been called the greatest step in vertebrate history (Long and Gordon GPE , 2004 DATE ) and even one of the most significant events in the history of life ( Carroll ORG , 2001 DATE ). Indeed, the morphological, physiological, and behavioral changes necessary for such a transformation in lifestyle to occur are astounding. The sum of these modifications occurring during the Devonian and Carboniferous EVENT led to the eventual filling of the terrestrial realm with vertebrate life, forever altering the structure and ecology of terrestrial communities. Long and Gordon PERSON ( 2004 DATE ) cited six CARDINAL critical questions relating to the evolution of tetrapods. These questions aimed to ascertain which sarcopterygian fish were basal to tetrapods, how morphological changes occurred sequentially, and when, where, how, and why these changes took place. Many researchers have described the morphological changes that occurred (Clack, 2002b DATE ; Eaton GPE , 1951 DATE ; Jarvik GPE , 1955 DATE ; Long GPE and Gordon GPE , 2004 DATE ; Thomson GPE , 1993 DATE ), and others have focused specifically on the development of limbs and digits (Clack, 2002b DATE ; Coates ORG and Clack ORG , 1990 DATE ; Coates ORG et al, 2002 DATE ; Daeschler ORG and Shubin ORG , 1995 DATE ; Shubin et al, 1997 PERSON ; Shubin et al PERSON , 2004 DATE ). As Long PERSON and Gordon PERSON ( 2004 DATE ) pointed out, the question that is the least well answered is the question of why these modifications occurred. Exactly what factors drove these changes to take place? Many researchers have posited theories over the years DATE attempting to answer this question, and the aim of this paper is to assess these arguments and suggest some possible common causes that could tie many of the proposed causal factors together. However, a brief description of known data pertaining to the time and place of tetrapod origins is first ORDINAL necessary in order to make valid statements regarding possible influential factors. The first ORDINAL tetrapods (defined as vertebrates with paired limbs and digits) appeared during the Late Devonian WORK_OF_ART , and it is now well-accepted that the panderichtyid fish are the sister group to tetrapods ( Carroll ORG , 1995 DATE ; Long GPE and Gordon ORG , 2004 DATE ). Prior to the past couple of decades, very few Devonian NORP tetrapod taxa were known: mainly Ichthyostega GPE and Acanthostega PERSON , both from the uppermost Famennian NORP . The discovery of possible tetrapod trackways in Australia GPE and Brazil GPE ( Bray ORG , 1985 DATE ; Warren GPE and Wakefield GPE , 1972 DATE ) stretched the potential range of tetrapods back into the Frasnian NORP , and these speculations were supported by the discovery of Elginerpeton GPE , the oldest known stem tetrapod, from the Late Frasnian ORG about 368 million CARDINAL years ago (mya) (Figure 1 CARDINAL ) ( Ahlberg GPE , 1995 DATE ; Carroll ORG , 1995 DATE ). Figure 1 CARDINAL . Stratigraphic appearances and interrelationships of early tetrapods. Adapted from Long and Gordon PERSON ( 2004 DATE ). However, it is important not to necessarily equate limbs and digits with terrestriality. Classically, the idea was that limbs developed to enable tetrapods to locomote on land. Jarvik PERSON ( 1955 DATE ) originally reconstructed Ichthyostega GPE in such a way that implied terrestriality, but a recent analysis indicates that Ichthyostega GPE was not well-adapted for terrestrial locomotion ( Ahlberg et al, 2005 PERSON ). It is now assumed that limbs with digits evolved completely for aquatic adaptation ( Ahlberg GPE and Milner GPE , 1994 DATE ; Ahlberg et al, 2005 GPE ; Carroll ORG , 1995 DATE ; Clack ORG , 2002b DATE ; Coates ORG and Clack ORG , 1990 DATE ; Daeschler ORG and Shubin ORG , 1995 DATE ; Lebedev, 1997 ORG ). Romer PERSON ( 1958 DATE ) even pointed this out, differentiating between the development of limbs giving the potentiality of terrestrial existence, and… PERSON the utilization of these limbs for life on land. The earliest fully terrestrial tetrapod appears to be Pederpes from the Tournaisian NORP about 354 to 344 CARDINAL mya (Figure 1 CARDINAL ) (Clack, 2002a DATE ; Long GPE and Gordon ORG , 2004 DATE ). Given the constraints imposed by the fossil record, it appears that the evolution of terrestriality took place in tetrapods between the Frasnian of the Late Devonian ORG and the Tournaisian NORP of the Early Carboniferous some time between 368 and 344 CARDINAL mya. An analysis of the environmental and ecological conditions imposed on creatures during this timeframe can help elucidate the major factors that drove terrestriality in tetrapods. Before making assertions about these environmental and ecological pressures, it is first ORDINAL necessary to locate where tetrapods were likely evolving. This question of place involves at least two CARDINAL aspects: ( 1 CARDINAL ) where geographically they were evolving, and ( 2 CARDINAL ) where ecologically they were evolving, i.e. whether they were evolving in marine or freshwater conditions. Geographically, the first ORDINAL early tetrapod specimens collected were from the Old Red Sandstone of North America ORG and western Europe LOC (Clack, 2002b DATE ; Jarvik GPE , 1955 DATE ), and the majority of Late Devonian NORP tetrapods have been concentrated in localities on the southern coastal belt of the Euramerican NORP plate, in what is modern-day Scotland GPE , Greenland GPE , eastern North America LOC , and the Baltic NORP states (Clack, 2002b DATE ; Daeschler ORG and Shubin ORG , 1995 DATE ; Milner GPE , 1990 DATE ). Some authors hypothesized an East Gondwanan GPE origin of tetrapods based on the Australian NORP trackways ( Milner PERSON , 1993 DATE ), but the discovery of Frasnian ORG -age panderichthyids and tetrapods in Latvia GPE and Russia GPE offer strong support for a Euramerican NORP origin of tetrapods ( Ahlberg GPE , 1995 DATE ; Clack 2002b ORG ; Daeschler ORG and Shubin ORG , 1995 DATE ). However, it is clear that by the end of the Famennian NORP , tetrapods had achieved a broad geographic distribution in equatorial regions from Euramerica ORG all the way to Australia GPE and even China GPE ( Daeschler ORG , 2000 DATE ; Daeschler PERSON , et al, 1994 DATE ; Long GPE and Gordon GPE , 2004 DATE ; Milner GPE , 1993 DATE ; Zhu PERSON , et al, 2002 DATE ). As for marine versus freshwater considerations, it has traditionally been hypothesized that tetrapods evolved in freshwater conditions and that seasonal drying of these water bodies had driven terrestriality (Clack, 2002b DATE ; Gordon PERSON and Olson ORG , 1995 DATE ; Long GPE and Gordon GPE , 2004 DATE ; Milner GPE , 1990 DATE ; Thomson ORG , 1993 DATE ). Some authors have argued that certain factors in freshwater conditions that could have driven terrestriality would have exerted an even stronger influence in marine conditions. For instance, Packard ORG ( 1974, 1976 DATE ) argued that anoxia would be even more of a problem in marine habitats than it is in freshwater habitats. However, other authors have argued that the intertidal habitats for early vertebrates proposed by Schultze ORG ( 1999 DATE , quoted in Graham PERSON and Lee PERSON , 2004 DATE ; Long and Gordon GPE , 2004 DATE ) would not have exhibited a strong enough selective force to initiate air breathing or the invasion of land ( Graham PERSON and Lee PERSON , 2004 DATE ). Most modern amphibians are unable to live in salt water (Clack 2002b), and most amphibian fossils have been discovered in what appear to be freshwater environments (Bendix-Almgreen, et al, 1990 DATE ; Clack ORG , 2002b DATE ; Daeschler GPE , 2000 DATE ; Long and Gordon ORG , 2004 DATE ). However, Bray ORG ( 1985 DATE ) and Clack PERSON ( 2002b DATE ) noted that it is not always easy to distinguish between fluvially-influenced and tidally-influenced sediments. Bray ( 1985 DATE ) hypothesized that tetrapods evolved in marginal marine rather than freshwater conditions, arguing that there was less of a salinity gradient between fresh and salt water in the Devonian NORP than there is today DATE . He noted that the density of terrestrial plants at that time was likely less than what we have today DATE , which would allow weathering to occur at a higher rate, thus increasing the dissolved ion concentration in freshwater. Recent fossil finds have included some early tetrapods from possible tidal, lagoonal, marginal marine, and/or brackish water sediments ( Carroll ORG , 2001 DATE ; Clack GPE , 2002b DATE ; Daeschler ORG and Shubin ORG , 1995 DATE ; Janvier ORG , 1996 DATE ; Long GPE and Gordon GPE , 2004 DATE ), as well as evidence that many early sarcopterygians dwelt in marine habitats (Clack, 2002b DATE ; Thomson GPE , 1993 DATE ). Some authors have even suggested that the apparent widespread geographic range of early tetrapods (from modern-day DATE North America LOC to Australia GPE and China GPE ) could only be a result of dispersal through epicontinental seas ( Carroll ORG , 2001 DATE ; Daeschler ORG , 2000 DATE ; Thomson GPE , 1993 DATE ). While the evidence is not necessarily conclusive of either freshwater or marine origins, recent evidence seems to indicate that tetrapods likely arose in marginal marine and possibly lowland freshwater environments, and it is possible that they could have been tolerant of both marine and freshwater conditions, as are many modern vertebrate types (Clack, 2002b DATE ; Daeschler ORG and Shubin ORG , 1995 DATE ). So it appears that tetrapods evolved in some sort of coastal wetland environment around the margins of the Euramerican NORP plate during the Late Devonian FAC . An analysis of terrestrial flora, fauna, climate, and geography at this time could help elucidate some of the factors that would have favored terrestriality in these earliest tetrapods. The advent of land plants had important evolutionary consequences for terrestrial life. During the Silurian NORP , the first ORDINAL terrestrial plants (mainly lichens, liverworts, and moss-like plants) evolved and were able to grow in habitats near the shore ( Kenrick PERSON and Crane ORG , 1997 DATE ). True vascular plants with stomata evolved by the end of the Silurian NORP (Clack, 2002b DATE ), and by the end of the Devonian NORP , many other advanced characteristics had already evolved as well, including leaves, roots, sporangia GPE , seeds, and secondary growth allowing plants to have a tree-like habit ( Algeo PERSON and Scheckler PERSON , 1998 DATE ; Edwards GPE , 1998 DATE ; Kenrick PERSON and Crane ORG , 1997 DATE ). Frasnian NORP floras were dominated by progymnosperms, including Archaeopteris PERSON trees with trunks in excess of a meter QUANTITY in diameter ( DiMichele and Hook ORG , 1992 DATE ; Edwards GPE , 1998 DATE ). At the Frasnian NORP -Famennian boundary, an extinction event occurred that resulted in significant changes to the constituent flora (Clack, 2002b DATE ). As the plants recovered during the Famennian NORP , species diversity and structural complexity of floral communities increased; multi-storied forests developed, and different plant groups evolved down distinct ecological lines ( Algeo PERSON and Scheckler PERSON , 1998 DATE ; DiMichele and Hook ORG , 1992 DATE ; Kenrick PERSON and Crane ORG , 1997 DATE ). These developing forests generated oxygen as a photosynthetic waste product, thus increasing its abundance in the atmosphere and making the land a much more suitable place for animal life ( Bray ORG , 1985 DATE ; Clack ORG , 2002b DATE ). By the Late Silurian NORP and Early Devonian GPE , there was already a complex terrestrial ecosystem in place, which included arthropod populations. Centipedes ORG , millipedes, arachnids, mites, scorpions, and other terrestrial arthropods were all present by this time ( DiMichele and Hook ORG , 1992 DATE ; Gordon PERSON and Olson ORG , 1995 DATE ; Jeram ORG , et al, 1990 DATE ; Kenrick ORG and Crane ORG , 1997 DATE ). They appear to have been mainly predators and detritivores ( Kenrick PERSON and Crane ORG , 1997 DATE ), thus establishing themselves as a major link between animals and plants (DiMichele and Hook, 1992 DATE ). The radiation of these terrestrial invertebrates likely had a strong influence on the later radiation of terrestrial vertebrates. The classical idea regarding climate in the Late Devonian NORP was that it was a time of warm, arid conditions with only seasonal rainfall ( Barrell GPE , 1916 DATE ; Bendix-Almgreen et al, 1990 GPE ; Clack 2002b ORG ; DiMichele and Hook ORG , 1992 DATE ; Ewer ORG , 1955 DATE ; Long GPE and Gordon GPE , 2004 DATE ; Orton ORG , 1954 DATE ; Romer ORG , 1945 DATE , 1958 DATE , 1966 DATE ; Warburton PERSON and Denman PERSON , 1961 DATE ). The red beds in which the early tetrapods were found were thought to be indicative of arid conditions. However, Inger ( 1957 DATE ) cited Krynine PERSON ( 1949 DATE ) as demonstrating that red beds often form in non-drought conditions; thus, red beds in and of themselves are not necessarily indicative of aridity. Romer PERSON ( 1958 DATE ) responded to Inger PERSON 's arguments by citing evidence of aridity in these strata other than the red color, including associated evaporites and evidence of subaerial deposition. The consensus today DATE is that at least some areas appear to have been semi-arid with seasonal rainfall, especially those areas that were around the equator ( Gordon PERSON and Olson ORG , 1995 DATE ), but it is clear that not all Devonian NORP rocks indicate arid conditions (Clack, 2002b DATE ). Figure 2 CARDINAL . Late Devonian NORP ( Famennian NORP ) paleogeographic reconstruction from Scotese NORP and McKerrow PERSON ( 1990 DATE ) in Daeschler PERSON and Shubin PERSON ( 1995 DATE ). Filled circles indicate tetrapod body fossils, and open circles represent trackways. During the Devonian, Euramerica ORG (also known as Laurussia GPE ), consisting primarily of Laurentia GPE and Baltica ORG , is hypothesized as being in an equatorial position ( Clack ORG , 2002b DATE ; Daeschler ORG and Shubin ORG , 1995 DATE ; Daeschler et al, 1994 DATE ; DiMichele and Hook ORG , 1992 DATE ; Gordon PERSON and Olson ORG , 1995 DATE ; Scotese NORP and McKerrow LOC , 1990 DATE ; Thomson ORG , 1993 DATE ). Gondwana PERSON lay southward, with the Iapetus Sea LOC separating the two CARDINAL (Figure 2 CARDINAL ). While there is not necessarily a consensus as to how much ocean separated the two CARDINAL major continents during the Late Devonian (Daeschler 2000 EVENT ; Dalziel, et al, 1994 DATE ; Milner GPE , 1993 DATE ; Thomson ORG , 1993 DATE ; Van Der Voo PERSON , 1988 DATE ), it is agreed that the Iapetus Sea LOC was in the process of closing up as Gondwana GPE and Laurussia GPE were moving closer, ultimately coming together in the Carboniferous (Clack, 2002b DATE ; Gordon PERSON and Olson ORG , 1995 DATE ; Van Der Voo PERSON , 1988 DATE ). This tectonically active region would have had a great effect on the lives of early tetrapods as their habitats were being resized, reshaped, and eventually eliminated. Over the years DATE , many authors have considered these ecological and environmental factors and posited theories as to why tetrapods evolved into fully terrestrial creatures. Barrell PERSON ( 1916 DATE ) was one CARDINAL of the earliest to propose that adverse climatic conditions were the driving factor in the origin of terrestriality. He cited the red beds in which early tetrapods had been found as evidence of aridity and hypothesized that shrinking pools of water during the dry season would have pushed amphibious tetrapods out onto land in order to survive. Romer PERSON ( 1945, 1966 DATE ) advanced this theory, postulating that tetrapods evolved limbs in order to remain in the water. When these small pools dried up, those creatures with the stoutest limbs and most efficient terrestrial locomotion would be more likely to make it to another body of water and survive. He noted that amphibians and crossopterygians NORP lived in the same habitat and argued that amphibians would have an obvious advantage if their habitat evaporated. He proposed that these short treks on land would eventually increase in duration as some amphibians would possibly linger on land to eat. For many years DATE , this was the popular theory, and many authors proposed nuanced versions of this basic idea. During the 1950s DATE , a series of papers was published on this topic. Orton ORG ( 1954 DATE ) espoused the possibility that limbs may have been a digging adaptation. She cited extant amphibians digging aestivation burrows to stay moist when surrounding conditions got dry rather than dispersing to find another body of water. However, even she noted that there are many burrowing animals that are able to do so without any limbs at all. Ewer ( 1955 DATE ) suggested that early tetrapods did not leave the shrinking ponds simply because their habitat was shrinking; rather, he noted that the receding habitat would have greatly increased population pressure, which would have triggered migration if environmental conditions were adequate. Gunter ( 1956 DATE ) held to the basic Romer/Ewer ORG theory, but he argued that the tetrapod limb had to be formed prior to these excursions. He emphasized that this was a gradual process, with the limbs first ORDINAL acting as props under water and the tetrapods making very short excursions onto land to escape predators or seek nearby food. The longer terrestrial excursions to escape the drying conditions were the final step of the process towards terrestriality. Goin and Goin ( 1956 DATE ) theorized that competition for food was the major driving factor, citing the presence of arthropods in the shallows and on the shore that could have served as an untapped food source for early tetrapods, even though Romer PERSON ( 1958 DATE ) argued that these food sources were not nearly adequate. In Inger's ( 1957 DATE ) paper offering a different climatic interpretation, he noted that the hypothesized aridity would have caused a great desiccation problem for migrating amphibians; a humid climate would have offered more favorable migration conditions for early tetrapods. Warburton PERSON and Denman PERSON ( 1961 DATE ) pointed out that in order to be a successful frog, one first must be a successful tadpole. They postulated that protoamphibians NORP laid their eggs in shallow pools away from competition with larger lungfish and predators. Terrestrial locomotion would have been necessary for these larvae to get back to the water, and they pointed out that in this case, selection would be operating on a large number of individuals. This view was echoed by Gordon PERSON and Olson PERSON ( 1995 DATE ) as well. Thomson PERSON ( 1993 DATE ) considered the whole pool-drying scenario to be logically inadequate, instead claiming that ecological conditions had to be the driving factor. It was the emergence of wetlands that fostered the origination of terrestrial tetrapods, offering a moist environment with abundant new food sources and protection from predators. Sayer and Davenport ( 1991 DATE ) conducted a study of modern-day amphibious fishes and found that they leave water under a variety of factors. Environmental degradation, including decreased oxygen content, increased temperature, and drastically fluctuating salinity, often causes fishes to evacuate the water. Biotic factors, such as competition for food and space, predation, feeding, and reproduction, can have a large influence as well. It is clear that a large number of potential factors could have played a role in the evolution of terrestriality, and because of this, the discussion of tetrapod origins has been unusually-theory laden ( Thomson ORG , 1993 DATE ). Yet it is important to try and understand what conditions may have driven such an important evolutionary event. By analyzing the available evidence and assessing the validity of the many theories put forth, it may be possible to elucidate a small number of major factors that could have driven this critical occurrence in the history of life. Long and Gordon PERSON ( 2004 DATE ) cited that both evolutionary pushes and pulls likely influenced the evolution of terrestriality. The pushes-the factors that encouraged tetrapods to leave the water-included poor environmental conditions, predators, competitors, diseases, and parasites. The pulls-the factors that encouraged tetrapods to come onto land-included favorable conditions, empty niches, abundant food resources, and a lack of predators, competitors, diseases, and parasites. The influences of all of these factors seem logical, so how can one disentangle them and determine the most important factors? Maybe it is not possible to sort out these factors and give some of them priority over others; they might all have been of equal importance. But if one CARDINAL or two CARDINAL primary causes could be determined that would have amplified the effects of these ecological factors, one could assign primary importance to these causes. The evolution of plants could have been one of these primary causes. In order for animals to move on to land, it first ORDINAL had to be habitable for them, and, as described earlier, the evolution of land plants drastically altered the composition of the atmosphere and formed the basis of new terrestrial ecosystems. The emergence of coastal wetlands offered an array of habitats previously unseen in earth LOC 's history ( Thomson ORG , 1993 DATE ) and encouraged the evolution of terrestriality in arthropods. Despite Romer's PERSON ( 1958 DATE ) objections that these food sources would not have been adequate, it is probable that even piscivorous fishes would have fed on these new prey items (Clack, 2002b DATE ; Goin and Goin, 1956 ORG ; Thomson GPE , 1993 DATE ). There were no vertebrate predators on land, so this would basically have been an unexploited niche. Rather than competing with fish in the sea, they could have an untapped source of food on land as long as they could get to it. Yet, in addition to these evolutionary pulls, plants exerted some pushes on tetrapods as well. The evolution of deciduousness in plants could have played a crucial role. Not only would mass senescing of leaves have enhanced the terrestrial ecosystem by enriching soil development (an evolutionary pull), it also likely caused anoxia in near-shore waters ( Algeo PERSON and Scheckler PERSON , 1998 DATE ; Clack GPE , 2002b DATE ). As the plant matter decayed in the water, oxygen levels in the water would have decreased. This situation could have encouraged air breathing in some fish (Sayer and Davenport, 1991 DATE ), which was a requisite step in the transition to life on land. Certainly, of course, the fish could have merely come up to the surface to breathe air and survived that way, but as Sayer and Davenport ( 1991 DATE ) pointed out, many modern-day fish do leave anoxic waters. The evolution of land plants clearly played a critical role in the evolution of terrestriality. They enhanced the terrestrial ecosystem and offered wide open niches, abundant invertebrate food resources, protection from predators, and an oxygen-rich atmosphere as opposed to anoxic waters. However, aside from the anoxia in the water, all of these would be considered evolutionary pulls rather than pushes. There had to have been some factors in their aquatic environment that made a move onto land-and all of the requisite changes-beneficial. The tectonic activity occurring in and around the Iapetus Sea LOC at this time could have enhanced the effects of many various evolutionary pushes. As discussed previously, early tetrapod evolution appears to have been concentrated along the southern coast of Euramerica GPE during the Late Devonian. During this time, Euramerica ORG and Gondwana PERSON were converging, eventually forming Pangaea ORG during the Permo-Carboniferous ORG . This closing of the Iapetus Sea LOC would have affected aquatic tetrapods living in this region in several ways. As the continents came together, the major direct effect would have been habitat loss, and this could happen in several ways. The convergence of continents would decrease the amount of coastlines and lower global sea level (Clack 2002b). The arrangement of the continents also led to a short period of global cooling and glaciation during the Famennian NORP in Gondwana GPE ( Algeo PERSON and Scheckler PERSON , 1998 DATE ; Johnson PERSON , et al, 1985 DATE ; Van Der Voo PERSON , 1988 DATE ). The uptake of water by glaciers would have lowered sea level as well. For tetrapods living in coastal habitats, these compounding factors would have led to a great decline in available habitat. As the amount of habitat decreased, previously separated populations of animals would be brought together into more of a confined space. In such a situation, the competition would be very intense. This recalls Ewer ORG 's ( 1955 DATE ) emphasis on the importance of population pressure, as well as Goin and Goin's ORG ( 1956 DATE ) focus on competition, in tetrapod evolution. Clack ( 2002b DATE ) also noted that when previously separated populations are forced to share a common environment, the biodiversity would actually decrease, while distribution of the remaining species would increase. This intense competition would have been a strong evolutionary push for tetrapods to find another suitable habitat. When Inger ( 1957 DATE ) was contesting the interpretation of red beds as indicative of an arid climate, he argued that discerning the stimulus that pushed terrestriality is dependent on one's climatic interpretation. And it is clear that there is not consensus about the climate in which early tetrapods evolved. But at the heart of Romer PERSON 's classic scenario of tetrapods escaping drying pools is the loss of habitat. It has been suggested here that tectonic activity and its effects could have caused the habitat of early tetrapods to be lost; thus, an arid climate need not necessarily be a critical component in theories of the evolution of terrestriality. The evolution of terrestrial tetrapods has certainly sparked much discussion over the years DATE , and deservedly so, for a rich terrestrial vertebrate fauna of about 360 million years DATE is contingent on this event. During the Late Devonian WORK_OF_ART , the first ORDINAL tetrapods made their way onto land. As their habitat was shrinking and causing fierce intra- and interspecific competition for resources in the shallows, a new habitat with abundant resources had been brought about by plants in the terrestrial realm. The filling of these new niches available on land forever changed the course of life on earth. To understand the full breadth of evolution, it is crucial to try and understand these landmark events in the history of life on this planet. The origin of new species depends on a complex combination of environmental conditions, ecological factors, and chance. The environmental conditions of the Late Devonian NORP certainly made life difficult for aquatic organisms, as evidenced by the mass extinction event in the marine community (DiMichele and Hook, 1992 DATE ; Johnson PERSON , et al, 1985 DATE ; Long GPE and Gordon ORG , 2004 DATE ); but had the opportunity for the colonization of land never been presented by the changes brought about by plants, the early aquatic tetrapods may have never survived long excursions in the terrestrial realm. The chance coincidence of increasingly poor quality and decreasingly abundant aquatic habitats, an emerging high quality terrestrial ecosystem, and the acquiring of morphological adaptations by the first ORDINAL tetrapods set the stage for one CARDINAL of the most important steps in the history of animal life. The Michigan Corpus of Upper ORG -level Student Papers ( MICUSP ORG ) is owned by the Regents of the University of Michigan (UM), who hold the copyright. The corpus has been developed by researchers at the UM English Language Institute FAC . The corpus files are freely available for study, research and teaching. However, if any portion of this material is to be used for commercial purposes, such as for textbooks or tests, permission must be obtained in advance and a license fee may be required. For further information about copyright permissions, please contact PRODUCT . The recommended citation for MICUSP ORG is: Michigan Corpus PERSON of Upper-level Student Papers WORK_OF_ART . ( 2009 DATE ). Ann Arbor PERSON , MI: The Regents of the University of Michigan ORG .