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Author Topic:   "Archaeopteryx; bird or reptile, or both?"
nator
Member (Idle past 370 days)
Posts: 12961
From: Ann Arbor
Joined: 12-09-2001


Message 1 of 34 (195285)
03-29-2005 6:10 PM


Admin thought this was needing to be moved into it's own topic.

I spose it should go into the Biological Evolution topic under a thread entitled "Archaeopteryx; bird or reptile, or both?"

xevolutionist writes:

quote:
Are you referring to Archaeopteryx? Hasn't it definitely, especially with recent research, been found to be a bird?

No.

Birds don't have teeth.

Archie's avian features:

Feathers

opposable hallux (big toe)

wishbone

elongated pubis which is directed backward

Archie's reptilian features:

No bill

trunk vertebra are free, not fused

bones are pneumatic

pubic shafts with a plate like transverse cross section

Cerebral hemispheres elongate, slender and cerebellum is situated behind the mid-brain and doesn't overlap it from behind or press down on it.

Neck attaches to skull from the rear as in dinosaurs not from below as in modern birds.

Center of cervical vertebrae have simple concave articular facets.

Long bony tail with many free vertebrae up to tip (no pygostyle).

Premaxilla and maxilla bones bear teeth.

Ribs slender, without joints or uncinate processes and do not articulate with the sternum.

Pelvic girdle and femur joint is archosaurian rather than avian (except for the backward pointing pubis as mentioned above).

The Sacrum (the vertebrae developed for the attachment of pelvic girdle) occupies 6 vertebra.

Metacarpals (hand) free (except 3rd metacarpal), wrist hand joint flexible.

Nasal opening far forward, separated from the eye by a large preorbital fenestra (hole).

Deltoid ridge of the humerus faces anteriorly as do the radial and ulnar condyles.

Claws on 3 unfused digits.

The fibula is equal in length to the tibia in the leg.

Metatarsals (foot bones) free.

Gastralia present.


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AdminJar
Inactive Member


Message 2 of 34 (195288)
03-29-2005 6:14 PM


Thread moved here from the Proposed New Topics forum.
  
mark24
Member (Idle past 3395 days)
Posts: 3857
From: UK
Joined: 12-01-2001


Message 3 of 34 (195297)
03-29-2005 6:46 PM
Reply to: Message 1 by nator
03-29-2005 6:10 PM


Hi Schraf,

Nearly all cladograms place Archeopteryx within clade Aves (at the base), meaning they are indeed birds. Just because modern birds have a certain suite of features, doesn't mean an organism must have those specific modern features in order to be considered a bird (this is why the notion of basal & crown groups were invoked). I am aware of one or two cladograms that actually place Archaeopteryx in the node below Aves, called Avialae. Strictly speaking in this scenarion they can't be considered birds. But since science is a consensus activity, Archaeopteryx is currently considered a bird using the cladistic classification method.

I think you are actually asking the wrong question. I think the question you should be asking is, "is Archaeopteryx a basal bird, if it is a bird at all?" The last part is answered in para 1, & the first part is as unequivocal as it is possible to get in science; yes, Archaeopteryx is as basal a bird as it is possible to get without being a non-avian therapod with feathers. Starting with therapods & ending with modern birds, Archaeopteryx just about makes it over the start line of bird-hood.

Mark


There are 10 kinds of people in this world; those that understand binary, & those that don't
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PaulK
Member
Posts: 15237
Joined: 01-10-2003
Member Rating: 3.3


Message 4 of 34 (195358)
03-30-2005 1:27 AM
Reply to: Message 3 by mark24
03-29-2005 6:46 PM


The real question is whether archaeopteryx should be considered a transitional. Given that it is considered a basal or near basal member of aves surely that answer is yes. The basal members, by definition, must include some 'primitive' traits from the clade they are derived from which are replaced by derived traits in the crown group.
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mark24
Member (Idle past 3395 days)
Posts: 3857
From: UK
Joined: 12-01-2001


Message 5 of 34 (195362)
03-30-2005 3:37 AM
Reply to: Message 4 by PaulK
03-30-2005 1:27 AM


Paul,

No argument from me!

Mark


There are 10 kinds of people in this world; those that understand binary, & those that don't
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nator
Member (Idle past 370 days)
Posts: 12961
From: Ann Arbor
Joined: 12-09-2001


Message 6 of 34 (195621)
03-31-2005 1:46 AM
Reply to: Message 3 by mark24
03-29-2005 6:46 PM


Wow, so it can be a bird if it has teeth, eh?

Wierd.

However, the real question is the transitional one, which I'll see if xevolutionist wants to play with.


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Chiroptera
Member
Posts: 6708
From: Oklahoma
Joined: 09-28-2003
Member Rating: 5.8


Message 7 of 34 (195709)
03-31-2005 10:17 AM


Actually, if I recall correctly, Aves is defined (in cladistics) to be the common ancestor of any modern bird and Archaeopterix, and all the descendents of that common ancestor. So Archaeopterix is bird, but mainly because the definition of bird has been consciously made to include it.
  
mick
Member (Idle past 3186 days)
Posts: 913
Joined: 02-17-2005


Message 8 of 34 (195854)
03-31-2005 7:02 PM
Reply to: Message 4 by PaulK
03-30-2005 1:27 AM


The real question is whether archaeopteryx should be considered a transitional.

Cladistics treats all known species as terminal nodes on a phylogeny. If by transitional you mean an internal node on the phylogeny (i.e. a node that branches into daughter species), then the answer is no. This is because phylogenetics only infers the existence of internal nodes based on the fact that we don't know what they are. If we knew what species were represented by these internal nodes, then we would treat them as terminal nodes of a tree with different topology.

This is a bit of nitpicking for the cladists. If you want my real opinion, I would guess that Archaeopteryx is about as "transitional" as any other species.

just my two cents.

mick


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PaulK
Member
Posts: 15237
Joined: 01-10-2003
Member Rating: 3.3


Message 9 of 34 (195928)
04-01-2005 2:09 AM
Reply to: Message 8 by mick
03-31-2005 7:02 PM


I recognise the limits of cladistics - although it seems to me that the reason behind it is a generalisation that is not always true. (That is there is a small proportion of cases where we can identify that one species did descend from another).

Archaeopteryx ought to be considered more transitional than most species because birds are so greatly modified from their dinosaurian ancestors and because it shares such a mix of avian and dinosaurian features (and at least one feature - brain sixe - that appears to be intermediate in itself).


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mark24
Member (Idle past 3395 days)
Posts: 3857
From: UK
Joined: 12-01-2001


Message 10 of 34 (195960)
04-01-2005 4:29 AM
Reply to: Message 8 by mick
03-31-2005 7:02 PM


Mick,

Cladistics treats all known species as terminal nodes on a phylogeny.

Strictly speaking, true. A cladogram does not infer ancestors. But when fossils from different times are considered there is no reason why this can't be the case, although the cladogram becomes a phylogenetic tree (which can infer ancestors), rather than a cladogram. The principles are the same.

Mark


There are 10 kinds of people in this world; those that understand binary, & those that don't
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mick
Member (Idle past 3186 days)
Posts: 913
Joined: 02-17-2005


Message 11 of 34 (196074)
04-01-2005 4:09 PM
Reply to: Message 10 by mark24
04-01-2005 4:29 AM


Hi Mark,

This is a reply to you and PaulK.

I know I made a rather petty point, but i think it's worth making simply because the question "why are so many transitional forms missing" is such a common criticism on the part of creationists of the fossil record. This question misunderstands the fact that, according to the evolutionary theory, all species and all individuals are transitional.

Anyway, l'm not so sure whether it's possible to decide whether a species is transitional or not based on fossil record or anything else. A good example is the platypus. It has characteristics that are kind of intermediate between reptiles and mammals (it's oviparous, it lactates but has no nipples, it is endothermic). Molecular phylogenies also put this species basally to the rest of mammalia. So it's transitional in the sense that it looks intermediate. But as far as I know nobody has ever suggested that eutherian and marsupial mammals evolved FROM the platypus, and there is no evidence of this. so it isn't transitional in what I understand to be the evolutionary sense.

does the archaeopteryx represent a transitional form en route from being a reptile to beign a bird? I think the only reasonable answer is "maybe", due to lack of data. I don't think that apparently intermediate morphology tells us anything about the evolutionary process leading from one group to another.

cheers,

mick


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mark24
Member (Idle past 3395 days)
Posts: 3857
From: UK
Joined: 12-01-2001


Message 12 of 34 (196089)
04-01-2005 4:41 PM
Reply to: Message 11 by mick
04-01-2005 4:09 PM


Hi Mick,

Anyway, l'm not so sure whether it's possible to decide whether a species is transitional or not based on fossil record or anything else. A good example is the platypus. It has characteristics that are kind of intermediate between reptiles and mammals (it's oviparous, it lactates but has no nipples, it is endothermic). Molecular phylogenies also put this species basally to the rest of mammalia. So it's transitional in the sense that it looks intermediate. But as far as I know nobody has ever suggested that eutherian and marsupial mammals evolved FROM the platypus, and there is no evidence of this. so it isn't transitional in what I understand to be the evolutionary sense.

You make a valid point.

When I suggest that Archaeopteryx lithogaphica is a transitional, I am not specifically pointing to that individual, or even that species, but a morphologically simlilar taxon that is closely related. I would agree that Archaeopteryx lithogaphica isn't necessarily the direct descendent of all birds, it may be a sister species or genus of one that was. As far as we know, one of the seven specimens may have been the last living example of A.lithographica, & an as yet unknown sister clade went on to spawn the rest of the birds. It seems a more than reasonable inference to state that A.lithographica was a taxon at least representative of an intermediate form, if it wasn't the intermediate itself.

The platypus example is a bit different, it isn't a fossil found at a relevant point in the geological record. It is an extant species, & a cladogram would simply show the divergence of it's clade relative to other mammalian clades, rather than infer that it is itself a transitional between modern & ancient taxa (which it can't be, obviously, since it is itself a modern taxon). The actual organism at the monotreme node was probably very different.

Mark


There are 10 kinds of people in this world; those that understand binary, & those that don't
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Palaeos
Inactive Member


Message 13 of 34 (214084)
06-04-2005 1:25 AM


Dromaeosaur
My answer to the original question:

Archaeopteryx is what you would call a arboreal theropod. Specifically speaking, it's a dromaeosaur (think Velociraptor) because it shares a number of anatomical characters only w/ dromaeosaurs. Other features are found only in the two forms and some basal groups of birds. Some of the Archaeopteryx-dromaeosaur characters are as follows:

-Nasal depressed nasal and snout upturned
-Dorsal process of maxilla almost reaches preorbital bar
-Preorbital bar slender & straight preorbital in lateral view
-Dorsal depression on the ectopterygoid
-Diamond shaped supraoccipital
-Strongly twisted paraoccipital process (noted by Currie)
-Highly modified tail with hyperdorso-flexible base (condition approached in troodonts)
-Middle finger most robust
-Ilium parallelogram shaped (also basal birds)
-Pubic peduncle very large & reversed
-Ilio-pubic articulation inverted V shape
-Pubic shafts are flat plates oriented 140 degrees to each other
-foot is functionally two toed, with a short toe II that is hyperextendable.

Charater list cited from Dinosaurs of the Air : The Evolution and Loss of Flight in Dinosaurs and Birds by Gregory S. Paul

Yet again another image for reference. Provided by my good friend Scott Hartman of Wyoming:

This message has been edited by Palaeos, 06-04-2005 12:32 AM


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randman 
Suspended Member (Idle past 3099 days)
Posts: 6367
Joined: 05-26-2005


Message 14 of 34 (214096)
06-04-2005 3:46 AM
Reply to: Message 12 by mark24
04-01-2005 4:41 PM


I brought this up on the PNT, but thought it might be an appropiate comment here as well. I understand that by transitional, you do not mean that it is the ancestot of any later species, but simply shows the evolution of forms that later species had, but that the idea is these forms could have stemmed from a different species.

However, presumably there is the assumption of a common ancestor prior to the transitional species whose evolutionary line died out, the old bush illustration rather then the tree, right?

But cannot similar traits arise also from convergent evolution, and not a shared common ancestor passing a common trait down? How can you tell if the commonalities of so-called transitional species are not due to just species out there being subject to commonalities such as similar environment, physical laws, etc,...as occurs with convergent evolution?

Moreover, since similarities can evolve independently of a mutal common ancestor passing those similar traits down, does that not undercut the whole assumption used in examining fossils, namely that if they are similar, they got their similar traits from a common ancestor?

This message has been edited by randman, 06-04-2005 03:48 AM


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mark24
Member (Idle past 3395 days)
Posts: 3857
From: UK
Joined: 12-01-2001


Message 15 of 34 (214139)
06-04-2005 10:04 AM
Reply to: Message 14 by randman
06-04-2005 3:46 AM


randman,

Moreover, since similarities can evolve independently of a mutal common ancestor passing those similar traits down, does that not undercut the whole assumption used in examining fossils, namely that if they are similar, they got their similar traits from a common ancestor?

But the pattern of common descent isn't just homologous characters, it manifests itself over time as well. As evidenced by the correlation between stratigraphy & cladograms.

Moreover, for convergent evolution to take place similar selective pressures must be in evidence. Why do birds show so many homologies with therapods when their lifestyles are so different? Why do molecular homologies also point to a reptile avian ancestor when modern birds & reptiles have very different lifestyles, & therefore different selective pressures? There are mammals that share a more similar lifestyle to birds; are under more similar selective pressures than reptiles, yet there are many more homologies amongst Aves, therapods, & modern reptiles than mammals.

Common descent explains all of this.

Mark

This message has been edited by mark24, 06-04-2005 10:24 AM


There are 10 kinds of people in this world; those that understand binary, & those that don't
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