Since the aquatic ancestors of fishes and tetrapods had no such connection, one might guess that this feature first evolved serving the function of enabling terrestrial locomotion. However, the earliest form of this connection as seen in Acanthostega evolved while these tetrapod precursors were still living in the water. Based on current evidence, Acanthostega appears to have been fully aquatic, so this connection likely evolved to function in something other than terrestrial locomotion.
Only later, as tetrapod ancestors moved onto land, was this trait co-opted for terrestrial support — and as it was, additional vertebrae were fused in the same way, providing further support. As the limbs and their connections to the rest of the skeleton evolved, limb bones took on distinct roles and many bones were lost. The humerus and the femur were already connected to two outer bones the radius and ulna in the forelimb, the tibia and fibula in the hindlimb.
This is something that evolved about 30 million years before vertebrates came onto land. However, muscular connections between these bones began to change on the road to land and allowed the limbs to be used for terrestrial locomotion. The ankle was originally composed of many small bones arranged in two rows, but gradually many of these small bones were lost.
The first animals to get close to walking on land had eight digits on each limb. Over time, some of these digits were lost, leading to animals with seven digits, then six, and then five, which is the common condition now seen in living tetrapods. As these animals evolved to live on land, other changes in the rest of their bodies evolved. Many would eventually lose their gills, which only work well for getting oxygen when wet, and their tail fins got smaller.
The environments of the animals shown in this evogram also changed through time. So the habitats that these animals occupy today are not necessarily the ones in which they have always lived, or in which they originally evolved. It is still unclear exactly where the transition from water to land took place ecologically.
Paleontologists have discovered fossils involved in this transition preserved from freshwater, brackish, and marine habitats. Regardless of where the transition occurred, eventually early ancestors of the first tetrapods came up onto land — although not all stayed. Some, like the whales, made the transition back into the water. The evolution of whales. Jaws to ears in the ancestors of mammals.
Subscribe to our newsletter. Email Facebook Twitter. Because the limb and digits are primitively ossified, one can directly observe the presence or absence of this feature in the fossil record. This character is well circumscribed and easily identified, and its homology is directly testable via phylogenetic analysis.
The origin of limbs is currently the focus of much research, so the clade of limbed vertebrates requires a name for ease of communication; why not use the name that is historically strongly tied to this clade and is already in wide use with this meaning? LAURIN: My literature survey shows that a majority of authors think that terrestriality is a defining feature of tetrapods, just as endothermy is a defining feature of mammals and birds.
Of course, in all these cases, there are other defining features hair and mammary glands for mammals; feathers, the wing, and flight for birds, the limb for tetrapods , and some of these features are behavioral flight, lactation, terrestriality. I would not consider terrestriality to be a process or a behavioral tendency; if terrestriality created selective pressures that led to further evolution of the digitized limbs after they originated in an aquatic environment, this scenario would not be drastically different from that of endothermy creating selective pressures for the appearance and subsequent evolution of hair in mammals and feathers in birds.
If you consider endothermy to be a defining feature of Aves and Mammalia, I see no reason to deny the connection between terrestriality and the name Tetrapoda. ANDERSON: Your literature search only showed an association between tetrapods and terrestriality, which I have never denied, but terrestriality was never explicitly stated in any of your chosen texts to be a key attribute. Simply mentioning terrestrial next to tetrapod is insufficient proof of its diagnostic utility in prephylogenetic paradigms of taxonomy.
How do you define terrestrial? How is terrestrial recognized in the fossil record? Limbs, unlike terrestriality, were specifically mentioned as key attributes in the references you surveyed. For instance, Romer and Parsons , emphasis added stated, Legs, the diagnostic feature of the tetrapod [the first tetrapods were identified as Ichthyostegalia on page 62], may have thus been, to begin with, only another adaptation for an aquatic life.
The earliest amphibian was little more than a four-legged fish. Life on land would have been the farthest thing from its thoughts if it had any. However, similar statements could be made for birds and feathers or the wing or for mammals and the mammary glands.
There is probably a link to the etymology of these terms. Thus, the main key character of tetrapods is the limb, but in many of the sources that I have consulted, the link between tetrapods and terrestriality was about as strong as that between endothermy and birds or mammals. If there is a difference, it is in degree but not in kind. Other databases were not used to avoid getting duplicates, and the Current Contents database includes all of the life sciences, so it is probably representative of the scientific literature in which the word tetrapod can be found.
A search for the word tetrapod yielded references, of which included the word tetrapod in a biological meaning the other meanings described, among other things, molecular structures with four axes. The word Ichthyostega was present in the text of a few of these articles. The same survey has direct bearing on your claim that to preserve continuity of usage, an apomorphy-based definition of the name Tetrapoda should be adopted.
A careful reading of the abstracts indicates that in the recent literature which reflects the current usage , the word tetrapod has been used in an ambiguous way that is neither clearly apomorphy based nor crown clade based in about half 59 of these studies Fig.
The meaning of the word tetrapod is clearly associated with a morphological concept in only 12 studies. A crown group concept was seldom unambiguously and explicitly associated with the word tetrapod in seven abstracts , but in the studies in which the meaning of this term was not clearly stated but could be implied, it was closer to a crown clade concept in the vast majority of cases 31 of 34; see the Appendix.
Examination of the entire text does not markedly change these results Fig. This is not surprising because the literature is replete with statements about the evolution of genes, proteins, hormones, behavior, or other features of organisms that do not fossilize. Most of the authors who have discussed the evolution of such features did not apparently make a distinction among the various meanings of the word tetrapod, and indeed most of them were probably unaware of these nomenclatural considerations, but by necessity the vast majority of such statements can only be known to apply to crown clades.
Thus, if their literal rather than intended meaning is surveyed, most statements about tetrapods actually concern the crown group. In the following exchange, I always refer to the literal meaning. Literal meaning of the word tetrapod in the scientific literature.
These articles covering the period January 1, to August 5, included the word tetrapod in either the title, key words, or abstract. The y -axis indicates the number of articles. The initial score is based only on the abstract of these papers, whereas the revised score is based on the full text. Categories as in a and b. The scientists working on nonfossilizable characters outnumber those working on the mineralized parts of hard tissues mostly paleontologists and a few histologists.
The greater part of the scientific literature dealing with any taxon is not paleontological Figs. Discussions of characters that cannot usually be preserved in fossils could use well-known names, rather than having to use more obscure names when they are available or having to explain that these characters may not have been present in the first extinct members of the taxon. Another solution would be to adopt new names for the crown clades and use a definition closer to the paleontological tradition for widely used names, but it seems unrealistic to expect that most physiologists, molecular biologists, and other scientists working exclusively with extant taxa would adopt the new names.
This survey of the biological literature Fig. The name Neotetrapoda, erected by Gaffney , which coincided more or less with the crown group of tetrapods under the phylogeny that he accepted, has not been used by other workers. Other names that have been proposed for crown groups, such as Neornithes as used by Cracraft, have not been used often. Gauthier and de Queiroz reported that the words Aves or birds were used in , publications listed in the Zoological Record for the period —, whereas the word Neornithes was used in only seven publications for the same period.
Furthermore, the adoption of new names for crown clades would not solve the problem of the numerous statements made about tetrapods, birds, mammals, amniotes, etc. Whatever the proponents of Phylogenetic Nomenclature decide, the literature on extant taxa will still contain many statements about the characteristics of tetrapods, birds, and mammals, and if the definition of these names does not correspond with the crown clades, many of the statements in that literature will be inaccurate.
The meaning attributed to the word tetrapod in the paleontological literature seems to differ from that in the neontological literature. In most paleontological studies, a morphological concept of the word tetrapod is clearly dominant Fig. This difference has not often been discussed Lee, because most neontologists do not appear to be concerned with the exact meaning of the word tetrapod and they probably think that early limbed stem tetrapods such as Ichthyostega are tetrapods, but the way that they use the name tetrapod is inconsistent with this position, as de Queiroz and Gauthier pointed out.
Thus, although the apomorphy-based concept that you advocate appears to be widespread in paleontology, it does not appear to prevail in most biological fields.
This conclusion is reached if literal, rather than intended, usage of the word tetrapod is studied as is done here. Most neontologists seem to accept an apomorphy-based definition of the name Tetrapoda, but their use of this name is inconsistent with such a definition and reflects the widespread use of fuzzy tree-thinking the use of a single name for more than one clade, usually two or more nested clades.
ANDERSON: First, I am not advocating the use of a term in a paleontological sense because all biologists used Tetrapoda as referring to limbed vertebrates historically; Goodrich himself, who coined the name, was a neontologist. Second, it is not just neontologists who did not adopt Gaffney's Neotetrapoda. Third, you criticize my statement about the use of Tetrapoda as a taxon name, but your entire study of current usage in biology is based upon a search of the vernacular word and not the technical nomen.
It is little wonder that the vast majority of times that tetrapod is used in the literature is ambiguous because there is little dispute over what the vernacular term means—limbed vertebrates—so why should these authors define this in the abstract let alone specifically mention Ichthyostega? A search of the ISI Web of Science formerly Current Contents for the nomen Tetrapoda using the same parameters as your review yielded a total of seven citations.
Two of these are my papers Anderson, , , and one is yours Laurin, , so the reader can probably infer how the term was used given our current discussion. In one article, Tetrapoda was used parenthetically in the title of a specimen description Reisz and Sutherland, and was not discussed further ambiguous usage. The remaining three citations are molecular studies; however, the authors' intended meaning can be understood from the text.
Lovsin et al. Sanchez et al. Mezhzherin referred to superclass Tetrapoda in the traditional sense. Searches for Tetrapoda support my previous statement. Nonetheless, you raised some ancillary issues that should be considered further. However, to improve upon this speculation, I surveyed the corresponding authors of the 51 studies you examined in detail in your tetrapod search.
After multiple studies by the same author were combined, I garnered a total of 46 currently valid e-mail addresses, to which I sent my survey of what the authors think is meant by tetrapod and Tetrapoda. To date, I have 21 respondents: nine people who work exclusively on extant organisms, five who work exclusively on extinct organisms, two who work on both but primarily extant organisms, and five who work on both but primarily extinct organisms.
The considerable overlap in taxa of interest demonstrates the difficulty of partitioning the use of names by living status extinct or extant ; many biologists work on both living and fossil taxa and presumably are sensitive to issues related to the study of either group. Three of the respondents using the crown group meaning of Tetrapoda work exclusively on extant organisms, and one works primarily on extinct organisms yourself.
This survey demonstrates that a wide majority of workers mean limbed vertebrates when they use the term tetrapod regardless of the living status of taxa studied, and a slight but not significant majority believes it is important that the vernacular term equate with the technical term. The ambiguous usage you found in the literature probably actually reflects a consensus among biologists of what the name means—why should they restate the obvious?
However, you intended to survey how people literally used the term, regardless of what the author intended. This view assumes that features currently unavailable for study in fossil taxa will always remain so, which is probably not correct.
Cases of extraordinary preservation of ancient biomolecules have already been reported e. Second, the authors of the papers you searched are obviously not extending the results from, say, molecular studies to fossil taxa or they would probably more frequently use the taxon name Tetrapoda.
They do frequently extrapolate their results to fossil taxa displaying the gross feature of interest without any conceptual problem, but it is clear that their discussion is inferential or speculative.
I do not believe that we should decide to radically change the widely accepted meaning of an established name because the distribution of certain shared primitive traits of the crown can only be known in the crown. An apomorphy-based Tetrapoda, however, has a synapomorphy with explicitly stated homology to anchor the name. Furthermore, an apomorphy-based definition for Tetrapoda is fully consistent with historical and current use of both the technical and vernacular term.
Contrary to your allegation above, it is a crown clade rather than an apomorphy-based definition for Tetrapoda that will lead to widespread inaccuracy interpreting the literature of the past 70 years and to widespread confusion in communication; most biologists currently mean the same thing when they use tetrapod or Tetrapoda: limbed vertebrates.
LAURIN: You began your remarks by emphasizing that I made a search on the vernacular word tetrapod rather than on the formal name Tetrapoda and concluded them by stating that biologists give the same meaning to both words.
I agree with your last argument; communication in biological sciences would be difficult if we could no longer consider vertebrates as synonymous with Vertebrata or annelids as synonymous with Annelida, to give but two examples. I also agree that most biologists mean limbed vertebrates when they use the word tetrapod, but unfortunately, that is not what they literally say; in most cases, they describe the crown while stating that they describe limbed tetrapods.
This contradiction simply proves that fuzzy tree-thinking is very widespread in biology. The dangers of fuzzy tree-thinking are shown by your interpretations of the meaning of the word Tetrapoda in the literature, because in my opinion the intended meaning of the name Tetrapoda in at least some of the seven references that you found in the ISI Web of Science is far from clear. However, the literal meaning is clearly not limbed vertebrates, which shows how ambiguous or variable the meaning of the name Tetrapoda is in this literature.
For example, you concluded that Lovsin et al. The date used by Lovsin et al. Feng et al. Because MYA coincides with the beginning of the Devonian, presumably this event refers to the divergence between dipnomorphs a group that includes lungfishes and tetrapodomorphs a group that includes tetrapods because the oldest members of this clade date from that time.
Thus, Lovsin et al. Mezhzherin referred to Tetrapoda without defining it, but he clearly was describing the crown, because all his data were from extant taxa. Thus, in my opinion, statements about Tetrapoda in the papers that you mention do not appear to describe the clade of limbed vertebrates.
This brief discussion also shows how difficult it is, in many cases, to know which clade is actually described under the name tetrapod or Tetrapoda in the literature.
I do not assume that features currently unavailable for study in fossil taxa will always remain so. I simply mean that statements that are made about such characters that are said to be tetrapod features apply to the crown because in the vast majority of cases the data on which such statements are based are restricted to the crown. The discovery that a character applies to a more inclusive clade than the crown would not falsify a statement that tetrapods even if restricted to the crown possess this character the character would simply not be unique to tetrapods.
The reverse situation is more problematic because if a character were stated to be characteristic of tetrapods defined as limbed vertebrates and was later discovered to be restricted to a part of that clade the crown, for instance , the statement would be falsified. This distinction was not widely appreciated in previous studies. I agree that crown clades cannot be viewed as better corroborated than more inclusive clades simply because their diagnosis often includes nonosteological synapomorphies.
However, if we wish to maximize veracity of the scientific literature, there is an advantage to using well-known names for crown clades because many statements in the neontological literature can be tested for crown clades but cannot be tested for more inclusive clades. Thus, these statements are scientific in the context of crown clades but not in the context of larger clades. Worse, if some of these statements that cannot be currently tested were to become testable later through development of new techniques or discovery of exceptionally preserved fossils as you argue, many of them could turn out to be false if widely used names were attributed to clades more inclusive than the corresponding crown group.
Any vertebrate with four limbs. Any vertebrate such as birds or snakes that have evolved from early tetrapods; especially all members of the superclass Tetrapoda. Concrete structures with 'arms' used to arrest wave energy along the shore in sea defence projects.
Having four limbs or feet. Origin of tetrapod. From tetra- "four" and -pod, from podos "foot". Tetrapod Sentence Examples.
Related articles.
0コメント