[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Dinogeorge Digest #9

Subj:   Re: scales to feathers (was Dinogeorge Digest #7)
Date:   98-07-07 13:32:41 EDT
From:   Dinogeorge
To:     bettyc@flyinggoat.com

In a message dated 98-07-06 13:37:02 EDT, bettyc@flyinggoat.com

<< As I've mentioned to James C. off list-the very first featehrs would
 most likely have been symmetrical as Nature LIKES bilateral symmetry.  I
 couldn't tell you at what point asymetrical feathers would have first
 shown up, meaning before gliding, before flight, during flight
 (concentric development) or after flight was already in use.  >>

Actually, nature >hates< symmetry: >perfect< symmetry, that is. Every
bilaterally symmetric organism has asymmetrically arranged internal organs or
organelles, and every bilaterally symmetric organism is externally at least
slightly different from left to right.  And this is not to mention the
molecular asymmetry of life's building blocks, such as amino acids, DNA,
proteins, sugars, and so forth. Undoubtedly, this stems from the simple fact
that there are many, many more ways to be asymmetric than to be >perfectly<
symmetric, so even when the environment works to constrain the evolution of
asymmetry, it doesn't go away completely.

When I note that the theropod hand is asymmetric, I mean that it is
asymmetric from front to back, and from top to bottom, and much more so than
is usual among tetrapods. Unfortunately, there's no clear way to decide >how<
asymmetric is "much more" asymmetric, if you see what I mean. We can see, for
example, that a dog's hind foot is not as asymmetric as a theropod's hand:
the dog's foot bones match one another in size fairly well across the foot's
plane of symmetry, whereas there is no such matching at all, and indeed no
perceptible plane of symmetry, in the theropod hand. But assigning numerical
values to such symmetries and asymmetries, so that you can compare is not
very easy. The amount of asymmetry in a given structure seems too subjective
a quality to be of much use.

Subj:   Re: scales to feathers (was Dinogeorge Digest #7)
Date:   98-07-07 13:32:38 EDT
From:   Dinogeorge
To:     bettyc@flyinggoat.com

In a message dated 98-07-06 13:37:02 EDT, bettyc@flyinggoat.com writes:

<< would feathers have direved from scutes, as we've seen on many LARGE
 dinosaurs, or more traditionally reptilian overlapping scales as in the
 tummy of snakes (which seem very similar to the larger scales across the
 front of birds' feet), or flat scales such as on snakes' backs (and on
 most of the rest of the birds foot)? >>

At this point there is simply no way to tell. One might think, since lizards
and snakes are covered with scales and birds are covered with feathers, that
scales are somehow "more primitive" than feathers. But scales and feathers
appeared independently--scales in lepidosaurs, feathers in archosaurs--so
that neither is a derived state of the other. It is entirely possible that
some kind of keratinous "pre-pre-feather" appeared in the most primitive
archosaurs and subsequently became modified into "things that look like
scales but aren't" in pre-avian archosaurs, "things that look like feathers
but aren't" in such animals as _Longisquama_, spikes and plates in sauropods
and ornithischians, pre-feathers in the earliest avian dinosaurs, and true
feathers in later theropod dinosaurs and birds.

It is also possible that scales and feathers developed independently from the
same kind of dermal structure in the common diapsid ancestor of both lizards
and birds. I suppose that tests of the chemical composition of scales, croc
scutes, and feathers in extant reptiles would shed some light on the issue.

Subj:   Re: BCF
Date:   98-07-07 11:08:16 EDT
From:   Dinogeorge
To:     gbabcock@best.com
CC:     Dinogeorge

In a message dated 98-07-07 07:44:05 EDT, gbabcock@best.com writes:

<< What evidence would it take to establish that BCF (George Olshevsky's
Birds Came First scenario) was supported by a preponderance of evidence over
BAMM (Birds are Modified Maniraptorans)?  >>

Birds--that is to say, extant modern birds and their Cenozoic
predecessors--are indeed modified maniraptorans, according to both BCF and
the "standard model." The already available evidence is too good for this not
to be the case. Where the "standard model" and BCF diverge is at the
beginnings of flight. BCF contends that the Triassic and pre-archaeopterygid
Jurassic ancestors of birds were far more birdlike in their arboreal
lifestyle and ability to fly--that is, to control their passage through the
air--than the "standard model" says. According to the "standard model," the
Triassic and Jurassic bird-ancestors were ground-dwelling, bipedal theropods;
according to BCF, the known ground-dwelling, bipedal theropods were
secondarily flightless descendants of the smaller, more birdlike,
quadrupedal, arboreal climbers and fliers that eventually evolved into birds.
In BCF, certain small, primitive archosaurs adopted an arboreal lifestyle,
perhaps to escape ground-dwelling predators or just to push their habitat
envelope, and stayed arboreal, becoming ever better climbers and, eventually,
fliers. From time to time, a lineage of these arboreal archosaurs returned to
a ground-dwelling existence and diversified into what we know as a group of
theropods. In BCF, such theropod characteristics as short forelimbs and
obligatory bipedal stance occurred repeatedly in each lineage because the
forelimbs had become too winglike--or at least, too strongly adapted for
climbing and leaping in trees--for use in walking (much like human forelimbs
and bipedality). And so on.

Testing BCF against the "standard model" in real terms will commence with the
discovery of good specimens of very small theropods from the Triassic and
Jurassic. Some, at least, will have long, winglike forelimbs if BCF is right,
none if the "standard model" is right.

Subj:   Re: dino listing
Date:   98-07-06 19:50:17 EDT
From:   Dinogeorge
To:     bigelowp@juno.com, Kimba4evr

In a message dated 98-07-06 19:25:32 EDT, bigelowp@juno.com writes:

<< And  I have to add one little caveat:  Keep in mind that some
 Maastrichtian sites are early Maastrichtian, not latest Maastrichtian.  There
is evidence that some Maastrichtian dinosaur taxa died out prior to the end of
the Maastrichtian. So you will only have a rough estimate of the dinosauria
alive at the terminal Cretaceous boundary. >>

An article by Robert Sloan in the latest Dinofest volume notes 22 dinosaur
genera during the late Maastrichtian from the Hell Creek Formation, dropping
to 19 and then to 12 right before the extinction event. These genera aren't
tabulated in the paper, but you might check the references therein for papers
that do.

Alternatively, we could throw this question out to the list at large: What
might the 12 genera right at the Hell Creek/Cenozoic boundary be? I'm pretty
sure _Tyrannosaurus_ and _Triceratops_ qualify, also _Pachycephalosaurus_,
_Stygimoloch_, _Ornithomimus_, and maybe _Ankylosaurus_. How
about_Thescelosaurus_? _Ricardoestesia_? _Torosaurus_? It's pretty easy to
compile a list of North American Maastrichtian dinosaurs (see, for example,
the stratigraphy section of _The Dinosauria_), but figuring out which of
those might be the very, very last ones known is a bit trickier. (Unless of
course you find the right paper.)