posted 15. May 2004 20:28                   

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As previously stated, a succession of transitional fossils exists that link reptiles (Class Reptilia) and mammals (Class Mammalia).

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The most important of the Karoo reptiles were undoubtedly the mammal-like reptiles or Therapsida.

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Diagram showing the relationships between the various mammal-like reptiles. Mammal-like reptiles did not survive beyond the end of the Triassic period, but one group, the Cynodontia, gave rise to the first mammals at the end of the Triassic, about 200 million years ago.

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These pre-mammalian groups of synapsids have at times been called "mammal-like reptiles". This term is now discouraged because although many had characteristics in common with mammals, none of them were actually reptiles.

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Synapsid classification has undergone tremendous change in recent years; most of the traditional groupings have been discovered to be paraphyletic. To represent this, we have used multiple lines drawn to paraphyletic groups in the cladogram above. Except for the Mammalia, all synapsid groups are extinct.

Current hypotheses about early synapsid diversification suggest that "pelycosaurs" are the basal-most synapsids, and are certainly paraphyletic. This group includes familiar "sail-back" critters like Dimetrodon, but also includes a variety of lesser known early synapsids, such as the herbivorous Caseidae. All groups of pelycosaurs went extinct by the end of the Permian. The remaining groups in the above cladogram constitute the Therapsida, and most of them diversified in the Permian or Triassic -- and then disappeared as the dinosaurs came to dominate the terrestrial world.

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I've read through this thread three times in succession now, and for the life of me I can't figure out what the issue is beyond a taxonomic (labelling, classification) confusion on some people's part. Is there something I'm missing? If so, what?

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Actually, it's more sloppy terminology--a shorthand-- that's causing the confusion

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This is where the shorthand comes in. Both the Reptilia and Synapsida inherited the same basic jaw structure from their common ancestor, the quadroarticular structure. They also possessed at the time only one ear ossicle (bone), the stapes. Mammals, however, developed a simplified jaw structure (dentary-squamosal, from trends started in the Therapsida, with the expansion of the dentary bone in the mandible, and with two of the bones, the quadrate and articular, becoming part of the inner ear structure (the incus and the malleus bones, respectively). In fact, two defining characteristics of mammals are the single dentary bone forming the mandible, and the 3 ear ossicles. Reptiles have retained the quadroarticular jaw structure, and still possess only one ear ossicle (the stapes). So that is why biologists use the terms 'mammalian' and 'reptilian' jaw structures

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The origin of mammals has been characterized as a gradual process, a claim based primarily on a well-preserved series of extinct nonmammalian synapsids ("mammal-like reptiles") that span some 200 million years. In contrast to the origin of many other higher taxa, the origin of mammals from within cynodont-grade therapsids is not considered to coincide with a major morphological change, but rather to be simply the culmination of a series of more and more mammal-like transitional forms. To test these assertions, an asymmetrical cladogram extending from primitive "pelycosaurs" to morganucodontid
mammaliaforms was created. Three different methodologies were then used to compare the amount of morphological change between nodes on
this cladogram with the minimum missing time interval between each node, as inferred from sister taxon-based ghost lineages. In general,
a statistically significant positive relationship was found,indicating that greater numbers of derived features tend to be correlated with longer ghost lineages. A significant correlation between the number of
accumulated apomorphies and branching events was also found. Although the rate of character change was variable, in no case was a long ghost
lineage associated with few apomorphies. These correlations are consistent with the hypothesis that rapid accumulation of derived features occurred relatively infrequently within the synapsid lineage leading toward mammals and that gradual character evolution predominated.

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Evolutionary Origins of the Mammalian Middle and Inner Ears
Homology of the mammalian middle-ear ossicles with accessory jaw bones on the mandible of other vertebrates is a classic example of comparative anatomy and vertebrate evolution. Previously it was not clear how the accessory mandibular (postdentary) bones of cynodonts (close relatives to mammals) could migrate from the mandible to the cranium in mammals. Luo and Crompton (1994) show that the phylogenetic transformations of several features of the quadrate (incus) made it possible for the postdentary bones to establish new sites of attachment to the cranium. Through the cynodont–mammal transition, the simple changes in the quadrate and its surrounding structures have allowed a fundamental reorganization of the basicranium while a presumably functional tympanum was maintained in the mandible. New data from Luo and Crompton (1994) support Allin and Hopson’s (1992) hypothesis on the postdentary origin of the mammalian tympanic membrane.

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Cynodonta contains the most mammal-like of the non-mammalian therapsids, which are sometimes termed 'mammal-like reptiles'. The most derived cynodonts are found within a taxon called Eucynodontia, which also contains the members of Mammalia.

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The Therapsida, the basal members of which are traditionally called "mammal-like reptiles" or "paramammals," are the advanced synapsids, and include the mammals. The traditional Linnaean classification groups the therapsids into several suborders - usually Phthinosuchia/Biarmosuchia, Dinocephalia, Anomodontia, and Theriodontia, this last often subdivided. A suggested cladistic tree is shown in the Cladogram

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posted 16. May 2004 00:08                    

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There isn't a smooth transition between Class Reptilia and Class Mammalia because Class Mammalia is not a monophyletic subset of Class Reptilia.

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What the transitional fossils do show is a transition between earlier synapsids, which morphologically were reptile-like, and mammals.

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Then why don't we call them "reptile-like mammals" ?

I disagree with the claim that morphologically the synapsids are reptile-like. It is their lack of a reptile-like morphology which leads to their placement in a clade which is not within the Reptilia.

regards

[ 16. May 2004, 00:10: Message edited by: Scott ]

posted 16. May 2004 12:38                   

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"Mammal-like reptiles" (poo poo on said terminology ) got their "reptilian" traits from the same place we did, the birds did, the dinosaurs did, etc., that is, from the common ancestor of all amniotes. These traits are apamorphies of the Amniota, and thus are symplesiomorphies for clades derived from Amniota. Thus, the traits which caused scientists to group "mammal-like reptiles" as members of Reptilia are not useful for determining phlyogenetic relationships, since all primitive amniotes should share them, regardless of where their clades were "going" evolutionarily. What we need are synapomorphies (derived traits), and "mammal-like reptiles" share more derived traits with mammals than they do with Reptilia (sensu Gauthier et omnia cladistica(?)). Hence they are members of the Synapsida, the sister stem to Sauropsida (upon which Reptilia resides), and which includes the
Mammals.

So all amniotes did indeed descend from an animal with "reptilian traits", but the Reptilia, as defined today, includes only some of these animals, based on a different group of derived states which unit the Diapsids and the Anapsids (including the turtles), if I am not mistaken.
Recall, however, that a clade is neither defined by it's constituant taxa, nor by the characteristics they posses (although it can be defined by shared derived characters, this is potentially destabilizing, and should be discouraged (Holtz 1996, and the references therein)). A Clade is defined as the most recent common ancestor of two (is it always two? I need to read up a little more...) animals and all of it's decendants (a node-based clade), or all animals more closely related to one taxon than to another.
So, while they look like "reptiles" as we know them today, the "mammal-like reptiles" are a group of amniotes which exhibit some plesiomorphic characters, but are more closley related to us than they are to any modern "reptile" or "bird" youcare to mention

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"Mammal-like reptile" is a pretty loose term once commonly used for a broad series of paraphyletic grades of non-mammalian Synapsida.
Synapsids are one of the two main clades of the Amniota.

According to the current cladistic definition of "Reptilia", synapsids are excluded. Thus, we mammals arguably aren't really descendants of
"reptiles" after all, [that is, if "reptile" is to be based on the taxon "Reptilia" , which now does include birds, however]:

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The Therapsida, the basal members of which are traditionally called "mammal-like reptiles" or "paramammals," are the advanced synapsids, and include the mammals. The traditional Linnaean classification groups the therapsids into several suborders - usually Phthinosuchia/Biarmosuchia, Dinocephalia, Anomodontia, and Theriodontia, this last often subdivided. A suggested cladistic tree is shown in the Cladogram.

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These considerations cast a whole new perspective on what has been called convergent evolution. The morphological similarities that
exist, especially in the skeletal systems, between placental wolves and bears and their marsupial counterparts, to my mind, defy any
explanation based on the accidental evolution of these similarities through chance events.

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Nevertheless, the Darwinians continue to insist that evolution is the result only of chance events.

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Stephen J. Gould has recently compared evolution to a drunk reeling back and forth between the
bar room wall and the gutter (Gould 1996 page 149).

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Gould's second main point is that "evolutionary progress" is a bias of human-centric observers. Gould brings up the Drunkard's Walk to illustrate that there is no progress in a statistically random set.

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Gould argues that, even if there were empirical evidence for a large-scale trend, that does not imply that there is a driving force behind the trend [16, 17]. Gould argues that evolution is performing a random walk in complexity space, but there is a constraint on the minimum level of complexity. When the complexity of an organism drops below a certain level (e.g., the level of prokaryotes), it can no longer live. Gould’s metaphor is that evolution is a drunkard’s random walk, but with a wall in the way (i.e., a bounded diffusion process). This wall of minimum complexity causes random drift towards higher complexity. This random drift does not involve any active selection for complexity; there is no push or drive towards increased complexity.

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posted 16. May 2004 14:56                   

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Q: How can chance mutations result in adaptations that seem to be purposefully designed?

A: It's a common phrase that natural selection is chance and necessity. It goes back to the title of a book by the great French biologist Jacques Monod. But it's not really correct, because the chance part is not natural selection. Natural selection is actually a locally deterministic force.

If you want to say the Darwinian evolution has a component of chance, and a component of local necessity, that's quite accurate. But the basic argument goes like this: Because natural selection doesn't make anything, natural selection is an eliminative force. Natural selection can only differentially preserve certain variations in a spectrum of variation within a species. Now some other process produces that variation; ultimately it's mutation. And mutation is spread around through recombination and sexual organisms.

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One of the central tenets of standard evolutionary theory is that characteristics
of evolved populations can be explained by the process of adaptation, and thus that phenotypic differences, for the most part, have an adaptive value. That all of biological diversity is due to adaptation has been challenged in modern expositions of Darwinian theory. Kimura [4], for example, proposes that there is a strong component of chance in evolution, while Gould and
Lewontin [3] stress the importance of history and contingency. Effects of chance are usually due to genetic drift and random mutations without value to the organism. History can become important if certain genetic changes
(of adaptive value in the past) constrain or promote some evolutionary outcomes over others.

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He (Gould) argues, correctly I think, that the process of natural selection is in itself not directional. There is nothing in it which necessarily pushes evolution towards, for example, more complex, more sophisticated, or more intelligent organisms. Evolution and adaptation in the face of changing environmental conditions, he argues, can equally be towards (roughly speaking) simpler organisms. There is no preferred direction or vector of change.

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ABSTRACT Due to the segmental organization of their body plans, arthropods can be considered
the paradigmatic modular organisms. In the past two decades, genetic studies of the homeotic
(Hox) genes in Drosophila melanogaster have provided initial insight into the molecular mechanisms that govern the establishment of segmental identity. In this review, we will address the question of the possible role of four Hox genes: labial (lab), proboscipedia (pb), Deformed (Dfd), and Sex combs reduced (Scr) in the morphological evolution of arthropods, particularly with respect to
the evolution of the head and head structures in insects. Overall, these preliminary studies illustrate the role that some of the Hox genes expressed in the insect head have played in the morphological evolution of hexapods and likely arthropods in general.

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