The differences between mammals and reptiles are considerable. A chief difference is that reptiles have at least four jaw bones and one middle ear bone while mammals have one jaw bone and three middle ear bones. To make matters worse, two bones in the fetal reptile that turn into jaw bones turn into ear bones in developing mammals. Other key differences. Reptile have undifferentiated teeth while mammals have incisors, canines, premolars, and molars. Reptile teeth are continuously replaced, mammals teeth are replaced at most once. Reptile teeth only have a single root while mammal molars have two roots. Reptiles lack a diaphragm. Reptiles have their legs sprawled out to the sides while mammals have their legs underneath. The pelvis of a mammal is fused. They have different numbers of bones in their toes. Reptiles are cold blooded while mammals are warm blooded.
A list of transitional animals with limited comments (still long and still a cut and hatchet job but editted to reduce length):
Paleothyris - A reptile
Protoclepsydrops haplous
Clepsydrops
Archaeothyris - Showed a slight change in teeth
Varanops - Lower jaw shows first changes in jaw musculature...lower-limb musculature starts to change Too late to be a true ancestor, and must be a "cousin".
Haptodus - Teeth become size-differentiated, with biggest teeth in canine region and fewer teeth overall...Vertebrae parts & joints more mammalian.
Dimetrodon, Sphenacodon or a similar sphenacodont - More advanced pelycosaurs, clearly closely related to the first therapsids (next). Dimetrodon is almost definitely a "cousin" and not a direct ancestor... Teeth further differentiated, with small incisors, two huge deep- rooted upper canines on each side, followed by smaller cheek teeth, all replaced continuously. Fully reptilian jaw hinge. Lower jaw bone made of multiple bones & with first signs of a bony prong later involved in the eardrum..
Biarmosuchia - Upper jaw bone (maxillary) expanded to separate lacrymal from nasal bones, intermediate between early reptiles and later mammals. Canine teeth larger, dominating the dentition. Variable tooth replacement: some therocephalians (e.g Scylacosaurus) had just one canine, like mammals, and stopped replacing the canine after reaching adult size. Jaw hinge more mammalian in position and shape, jaw musculature stronger (especially the mammalian jaw muscle)...more mammalian femur & pelvis. The toes were approaching equal length, as in mammals, with #toe bones varying from reptilian to mammalian.
Procynosuchus - The first known cynodont -- a famous group of very mammal-like therapsid reptiles, sometimes considered to be the first mammals. Lower incisor teeth was reduced to four (per side), instead of the previous six (early mammals had three). Jaw hinge still reptilian. Scapula beginning to change shape. A diaphragm may have been present.
Dvinia - First signs of teeth that are more than simple stabbing points -- cheek teeth develop a tiny cusp. The dentary bone was now the major bone of the lower jaw. The other jaw bones that had been present in early reptiles were reduced to a complex of smaller bones near the jaw hinge.
Thrinaxodon - Functional division of teeth: incisors (four uppers and three lowers), canines, and then 7-9 cheek teeth with cusps for chewing. The cheek teeth were all alike, though (no premolars & molars), did not occlude together, were all single- rooted, and were replaced throughout life in alternate waves. First sign of the mammalian jaw hinge. Scapula shows development of a new mammalian shoulder muscle. All four legs fully upright, not sprawling. Number of toe bones is intermediate between reptile number and mammalian . The specialization of the lumbar area (e.g. reduction of ribs) is indicative of the presence of a diaphragm, needed for higher O2 intake and homeothermy. The eardrum had developed in the only place available for it -- the lower jaw, right near the jaw hinge, supported by a wide prong (reflected lamina) of the angular bone. Cynodonts developed quite loose quadrates and articulars that could vibrate freely for sound transmittal while still functioning as a jaw joint, strengthened by the mammalian jaw joint right next to it.
Cynognathus - Teeth differentiating further; rate of replacement reduced, with mammalian-style tooth roots (though single roots). TWO JAW JOINTS in place, mammalian and reptilian. Limbs were held under body. There is possible evidence for fur in fossil pawprints.
Diademodon - Mammalian toe bone numbers, with closely related species still showing variable numbers.
Probelesodon - Teeth double-rooted, as in mammals. Second jaw joint stronger. Hip & femur more mammalian.
Probainognathus - Additional cusps on cheek teeth. Still two jaw joints. Mammalian number of toe bones.
Exaeretodon - Mammalian jaw prong forms, related to eardrum support. Three incisors only (mammalian). More mammalian hip related to having limbs under the body. This is probably a "cousin" fossil not directly ancestral, as it has several new but non-mammalian teeth traits.
Oligokyphus, Kayentatherium - Alternate tooth replacement with double-rooted cheek teeth, but without mammalian-style tooth occlusion. Skeleton strikingly like egg- laying mammals (monotremes). Double jaw joint. Scapula is now substantially mammalian, and the forelimb is carried directly under the body. Various changes in the pelvis bones...this animal's limb musculature and locomotion were virtually fully mammalian. There is disagreement about whether the tritylodontids were ancestral to mammals or whether they are a specialized offshoot group not directly ancestral to mammals.
Pachygenelus, Diarthrognathus - Alternate replacement of mostly single- rooted teeth. This group also began to develop double tooth roots -- in Pachygenelus the single root of the cheek teeth begins to split in two at the base. Pachygenelus also has mammalian tooth enamel. Double jaw joint, with the second joint ...fully mammalian. Reptilian jaw joint still present but functioning almost entirely in hearing. Highly mobile, mammalian-style shoulder. These are probably "cousin" fossils, not directly ancestral.
Adelobasileus cromptoni - Currently the oldest known "mammal."
Sinoconodon - The next known very ancient proto-mammal. Mammalian jaw joint stronger. This final refinement of the joint automatically makes this animal a true "mammal". Reptilian jaw joint still present, though tiny.
Kuehneotherium - A slightly later proto-mammal, sometimes considered the first known pantothere (primitive placental-type mammal). Teeth and skull like a placental mammal. The three major cusps on the upper & lower molars were rotated to form interlocking shearing triangles as in the more advanced placental mammals & marsupials. Still has a double jaw joint, though.
Eozostrodon, Morganucodon, Haldanodon - Truly mammalian teeth: the cheek teeth were finally differentiated into simple premolars and more complex molars, and teeth were replaced only once. Tiny remnant of the reptilian jaw joint. Thought to be ancestral to all three groups of modern mammals -- monotremes, marsupials, and placentals.
Peramus - A "eupantothere" (more advanced placental-type mammal). The closest known relative of the placentals & marsupials.
Endotherium
Kielantherium and Aegialodon
Steropodon galmani - The first known definite monotreme.
Vincelestes neuquenianus - A probably-placental mammal with some marsupial traits.
Pariadens kirklandi - The first definite marsupial.
Kennalestes and Asioryctes - Canine now double rooted.
Cimolestes, Procerberus, Gypsonictops - Primitive North American placentals with same basic tooth pattern.
So we have a finely divided set of fossils going from purely reptile to purely mammal with intermediate features seen gradually changing throughout the sequence.
To read more see http://www.talkorigins.org/faqs/faq-transitional/part1b.html
and http://www.talkorigins.org/faqs/comdesc/section1.html#morphological_intermediates_ex2
The latter has drawings of the jaw in transistion to see what it looked like. The former has the full text of most of what I posted above.
Still think there are no transistionals?
No Transitionals?
Discussion in 'Creation vs. Evolution' started by UTEOTW, Jul 1, 2003.
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UTEOTW , since you're discussing evidence, I doubt you'll get many YEC's posting here, other than an occaisional blanket statement.
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I looked at it. I haven't followed it through yet for lack of time, but I would have the following questions first off:
1. Were these specimen in appropriate stratas for the claimed evolutionary sequence?
2. Was each in some approximation geographically to the area of the one 'before' and 'after' it?
3. Who put together this sequence?
4. Is this a sequence which is generally accepted in paleontology? (For instance, I know the horse sequence put up on Talk Origins is so full of holes you could use it to strain food!)
5. Given the sequence proposed for a change in tooth and jaw structure, do these animals show concordant changes in other areas which would bring them from reptile to mammal?
6. Since tooth eruption is NOT determined genetically (not when it starts, but how far the tooth erupts and why it stops when it does is not connected to any known genetics), how is it proposed that each of these organisms managed to have a change in tooth structure/jaw structure and still had the teeth behave properly for that organism?
Just a few questions off the top of my head.
You see, anyone can take ONE set of criteria and 'show' a progression. However it's a whole lot more difficult to show that this might have really happened given all of the other things involved.
I could take dogs starting with a miniature poodle and going up to a great Dane and show how one had to be descended from another as an increase in size and characteristics -- but we know that didn't happen.
Lists like the one in the lead post here are actually fairly easy to come by, or even to make up if one 'needs' to. That is not the point at all, though. The point is to show that this actually did happen in terms of one sort of things gradually changing into another sort. Lists are not really evidence, I'm afraid.
What sort of mutations are being proposed here? Where in the genome did they take place to produce the 'changes' listed, or implied above? There are so many unanswered questions that the list is simply a list and really does not mean anything evolutionarily. -
Let's deal with the technical ones, first:
The simplification of the lower jaw made a more powerful bite and prolonged chewing possible. At about the same time, we see an enlargement of the opening in the skull that gives synapids their name. The relatively weak reptillian jaw adductor was reconfigured in that enlarged space to form the mammalian massater. The remains of the lower part of that opening became the zygomatic arch, through which the muscle attached to the dentary.
At roughly the same time, teeth began to differentiate. The major change were elaborations to the cheek teeth to make them more efficient at tearing/grinding food to make it smaller, which meant it was more digestable, making the animal more efficient at utilizing available resources.
At roughly the same time, we see a progressive growth of the bony shelf in the mouth of some primitive therapsids, to form a secondary palate.
No longer do such animals have to quickly gulp down food; they can chew and breath at the same time.
At the same time, the single occipital condyle at the base of the skull first widens, and then splits into two parts, allowing a stronger joint, and more power for tearing meat or pulling vegetation.
At the same time, the thoracic muscles enlarge and extend to form a sheet of tissue that becomes the diaphragm. (we know this one, because this is accompanied by a reduction in abdominal ribs in animals having a diaphraphm; the ribs are no longer needed to attach "bellows") which means that the animal can walk and breathe at the same time. It can be more quick and agile, because it can now have a higher metabolism, due to more efficient feeding.
And all these occured more or less in concert. Some more rapidly in some forms than others, but each one made the advancement of the other possible.
http://tolweb.org/tree/eukaryotes/animals/chordata/synapsid_lichen/synapsida_synapomorphies.html
The early reptillian teeth were merely for holding prey long enough to kill/swallow. Carnassals can still hold, even if their primary function is to slice. Notice, though, that the first "mammalian" teeth were doglike canines, with the molars developing later, after more secure "holding" teeth evolved.
The evolution of mammalian teeth is complex and well-documented, (often the teeth are the only things that survive, especially in small mammals) if you want to get more detail. -
Most of the time, the evolution of horses is so finely detailed that we can see changes within genera. Even Gould was compelled to admit that the fossil evidence shows that horse evolution was gradual. -
First, I cut off a bunch of data to shorten things up. Mainly, I tried to simplify the changes to a few specific areas instead of all the changes, just to keep it on a simpler level and to keep it short enough to maybe be read.
Here is an more detailed read, if you wish. Everything summarized in a nice, easy to follow chart and drawings showing the step by step teeth changes.
http://www.gcssepm.org/special/cuffey_05.htm
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Sorry...The 140 miilion years above should have been 240 million years. Apparently I have forgotten how to do simple math.