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Fwd: Response to Ruben et al (1997)



I'm forwarding this reply to Ruben et al. by Leahy to the Dinolist. Enjoy. 

GSPaul
---------------------
Forwarded message:
From:   Leahyg%amds.EDW@mhs.elan.af.mil
Reply-to:       Leahyg%amds.EDW@mhs.elan.af.mil
To:     GSP1954@aol.com
Date: 98-01-23 15:52:57 EST

Form: Memo
Text: (249 lines follow)

        Though I'm no longer a member of the
list, (I only have Net access at work, 
and if I subscribed to the list I'd
never get any work done), I do browse 
the list digest periodically.  After
seeing some of the discussions 
regarding the recent Science paper on
dinosaur lungs by Ruben et al., I 
can't resist adding my two cents.  So,
since Greg Paul has kindly agreed to 
forward my contribution...

        Based
on an indirect analysis of  Sinosauropteryx specimens, Ruben et al 
(1997)
concluded theropods possessed "unmodified, bellowslike septate lungs 
that
were ventilated with a crocodilelike hepatic-piston diaphragm", and 
were
unlikely to have possessed either high metabolic rates or "avian style,

flow through septate lungs". This interpretation is likely incorrect,

however, as theropods do possess skeletal structures strongly suggestive
of 
lungs more similar to that of birds than crocodiles.
        The first of
these is that both birds and theropods (also sauropods and 
pterosaurs)
have thoracic ribs with a widely spaced capitulum and tuberculum 
oriented
in a vertical plane (Britt, 1993).  Such widely spaced rib heads 
stiffen
the rib cage, and in birds, reduce the amount of proximal flexion to

prevent collapse of the rigid avian lung (Britt, 1993).  The first three

thoracic ribs in crocodiles also possess widely spaced rib heads, probably

to support the pectoral girdle (Britt, 1993). However, all thoracic ribs

caudal to the first three have very closely spaced rib heads, which permit

much greater mediolateral motion than would be possible for most theropod

ribcages.  Basal theropods such as Coelophysis do have rib articulations

similar to crocodiles, but all more derived theropods have ribs with

avianlike, widely spaced
articulations.  With such differing rib
kinetics, it is very unlikely most 
theropods could ventilate their lungs
in the same manner as crocodiles 
(Britt, 1993). The rib articulations
suggest that theropod lungs occupied a 
dorsal position within the thorax,
as in birds.
        The second skeletal feature suggestive of avianlike lungs in
theropods is 
the extensive postcranial pneumaticity of theropod ribs and
vertebrae 
(Britt, 1993).  These pneumatic features are structurally and
histologically 
identical to those in bird skeletons (Britt, 1993; Reid,
1997). Postcranial 
pneumaticity in birds is the result of  lung airsac
diverticula invading and 
replacing bone (Britt, 1993; McLelland, 1989).
Because the theropod origin 
of birds remains the most parsimonius
cladistic interpretation (Sereno, 
1997), it is probable birds have
pneumatic vertebrae and ribs for the same 
reason their theropod ancestors
possessed them. That theropods, like birds, 
possessed somewhat avianlike
lungs, with airsac diverticula 
which invaded and pneumatized bone.
        Birds
have five separate airsacs; cervical, clavicular, anterior + 
posterior
thoracic, and abdominal (McLelland, 1989).  Interestingly, the 
airsac
diverticula exhibit a highly consistent pattern of association with

certain bones.  The cervical airsacs pneumatize the cervical & thoracic

vertebrae + ribs, the clavicular airsac aerates the pectoral skeleton, the

anterior & posterior thoracics do not invade any bones, and the abdominal

airsac aerates the pelvis hindlimb and caudal vertebrae (McLelland, 1989).
 
Because theropods are the probable ancestors of birds, this pattern of

airsac/skeletal association was inherited from theropods.
Therefore, it
is reasonable the diverticula/skeletal associations seen in 
birds would
hold true for theropods.
        The occurrence of pneumatic cervical & thoracic
vertebrae & ribs in most 
theropods would, therefore, indicate the presence
of cervical airsacs.  No 
theropod has a pneumatic pectoral skeleton, so no
evidence of clavicular 
airsacs exists in theropods.  Though the thoracic
airsacs do not pneumatize 
bone, the avianlike rib structure suggests
thoracic airsacs were present.  
The occurrence of pneumatic sacral and/or
caudal vertebrae in segnosaurs, 
tyrannosaurs, ornithomimids,
Acrocanthosaurus and oviraptors suggests these 
theropods had abdominal
airsacs.  In coelophysids, the restriction of 
pneumatic foramina to the
cervical  region is consistent with their 
unavianlike rib articulation in
suggesting such basal theropods possessed 
cervical airsacs only.
 Pneumatic vertebrae are lacking in Eoraptor (Sereno 
et al 1993),
suggesting airsacs were absent in this basal theropod.
        The presence of
pneumatic foramina in the humeri of Confuciusornis (Hou, 
1995), suggests
this basal bird had developed clavicular airsacs.  Unlike 
birds and
pterosaurs, theropods do not possess pneumatic limb bones.  
However,
pneumatic limbs may be an adaptation for flight (Swartz et al 
1992), so
such absence in flightless theropods would not be surprising.
        Additional
evidence that theropods possessed at least a somewhat avian lung 
comes
from Archaeopteryx.  Archaeopteryx appears to have been capable of 
powered
flight, though the degree to which it was capable remains in dispute

(Shipman, 1997).  Powered flight has an extremely high metabolic cost; the

*minimum* rates of oxygen consumption in birds are at least 1.5 times the

*maximum* metabolic capacities of most terrestrial mammals (Thomas, 1987).
 
Importantly, the metabolic power during flight seen in bats and insects
is 
similar to that of flying birds (Thomas, 1987; Suarez, 1996).  The lung
of 
bats is structurally convergent towards the avian lung in many features

(Maina & King, 1984).  Thus, birds, bats and insects are not only highly

aerobic, they are *hyperaerobic* compared to most terrestrial mammals.

Though a few, such as pronghorns, are also hyperaerobic  (Lindstedt,
1991).  
Some flightless birds have secondarily reduced their aerobic
capacity 
(Kooyman & Ponganis, 1994), while a few cursorial birds retain
levels of 
aerobic power comparable to those of flying birds (Butler,
1991). The 
convergent evolution of such high metabolic capacities in
birds, bats and 
insects is unlikely to be coincidence.  Such strikingly
similar adaptations 
for elevated levels of aerobic power strongly suggests
that development of 
such rates of oxygen consumption is required in order
to develop powered 
flight.  
        The extremely limited aerobic capacity of
modern reptiles is far below that 
needed to sustain powered flight
(Bennett & Ruben, 1979), so the capacity of 
powered flight displayed by
Archaeopteryx suggests this protobird had 
evolved a level of aerobic power
far above that of modern reptiles.  the rib 
articulations and pattern of
skeletal pneumaticity seen in  Archaeopteryx 
are identical to those seen
in theropods (Britt, 1993).  This suggests the 
lung structure, and
therefore, the aerobic capacity, may have been similar 
in Archaeopteryx
and theropods.
        Because theropods were flightless, they may not have needed
the 
hyperaerobic capacities of modern birds, so it is possible their lung

structure was somewhat less derived, and not fully identical to the

parabronchial lung of modern birds.  The lung of crocodiles and fetal
birds 
exhibit numerous similarities (Perry, 1989), and hypothetical
evolutionary 
scenarios depicting transitional lung types have been
plausibly
constructed (Perry, 1992).  That the avian respiratory system
contains 
redundancies in regards to gas exchange is verified by
experimental data 
where various air sacs have been selectively blocked
(Brackenbury, 1991), 
and neither oxygen consumption nor  ventilation were
significantly affected.
        Ruben et al (1997) argue that possession of
 jointed sternal ribs and a 
large sternum are critical  in order to
ventilate an avian grade lung, and 
since theropods lacked these features,
their lungs could not have been of 
this type.  However, birds such as
moas, elephant birds and kiwis exhibit a 
very small, theropod-sized
sternum, with only 2-4 pairs of sternal ribs.   
In some juvenile precocial
birds, the sternum is very small, entirely 
cartilaginous, and the sternal
ribs do not yet articulate with the thoracic 
ribs )Fig. 7 in Olson, 1973),
yet these birds are fully capable of 
ventilating their lungs.  Neither
jointed sternal ribs nor a large sternum 
appears essential to operate an
avian-style lung.
        Ruben et al (1997) postulate the pubis of theropods was
similar to that of 
crocodiles, and served as an attachment site for
diaphragmatic muscles 
needed to power a crocodile-like hepatic-piston
lung.  However, the pubes of 
crocodiles are mobile and excluded from the
acetabulum, features not 
characteristic of any known theropod.  Other
archosaurs, such as 
ornithosuchids, prestosuchids, poposaurids,
prosauropods, sauropods and some 
basal birds also share a pubis similar in
design to theropod and crocodile 
pubes (Parrish, 1997; McIntosh,
Brett-Surman & Farlow, 1997). Ruben et al 
(1997) did not demonstrate that
theropod pubes are structurally closer to 
crocodilian pubes than those of
these other archosaurs.  The pubes of 
crocodylomorphs such as
Terrestrisuchus are similar to those of some 
theropods such as Coelophysis
and Sinornithoides (Parrish, 1987; Russell & 
Dong, 1993), but in these
cases the pubes are slender rods lacking any 
distal expansion.  It is
questionable whether crocodilian-like diaphragmatic 
muscles could have
attached to such slender pubes.  The development of 
mobile pubes excluded
from the acetabulum in more derived crocodilians 
indicates crocodilian
pubic design became less, rather than more, 
theropod-like over time.
        The
pubes of alvarezsaurids may present additional problems with this

hypothesis.  If alvarezsaurids are birds, then the theropod-like pubes of

Patagonykus (Novas, 1997) indicate any purported similarity of theropod

pubes to crocodilian pubes cannot be diagnostic of a crocodile-like
hepatic 
piston, since Patagonykus, as a bird, would lack this structure.
 If 
alvarezsaurids are theropods, then the pubes of Parvicursor remotus
(Karhu & 
Rautian, 1996), are inconsistent with the presence of
crocodilian-like 
lungs.  The pubes of this animal are highly retroverted,
attached to the 
ischium, and do not appear to form a 
pubic
symphysis.
        Ruben et al (1997) have interpreted the pubes of Archaeopteryx
as highly 
retroverted, like those of modern birds.  This interpretation is

inconsistent with the pubic orientation of the most recently described

specimen of Archaeopteryx (Fig. 13 in Wellnhofer, 1993).  The pubes of
this 
specimen are articulated, and clearly exhibit a near vertical
orientation, 
similar to theropods such as Unenlagia (Novas and Puerta,
1997).
        A similar pubic orientation is also seen in the undescribed
"sickle-clawed" 
bird from Madagascar though the pubis is not preserved,
the orientation of 
the ischium of the early bird Iberomesornis suggests a
vertical orientation 
as well (Sanz & Bonaparte, 1992).  The well preserved
position of the pubes 
in the newest Archaeopteryx specimen indicates the
pubic orientation 
postulated by Ruben et al (1997) is
untenable.

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GUY LEAHY
95 AMDS/SGPZ
208 W. Popson Ave.,
Bldg 2204
EDWARDS AFB, CA 93523
(805)
277-8392
leahyg%AMDS.edw@mhs.elan.af.mil

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