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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.
Date: 98-01-23 15:52:57 EST
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...
on an indirect analysis of Sinosauropteryx specimens, Ruben et al
concluded theropods possessed "unmodified, bellowslike septate lungs
were ventilated with a crocodilelike hepatic-piston diaphragm", and
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
lungs more similar to that of birds than crocodiles.
The first of
these is that both birds and theropods (also sauropods and
have thoracic ribs with a widely spaced capitulum and tuberculum
in a vertical plane (Britt, 1993). Such widely spaced rib heads
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
the extensive postcranial pneumaticity of theropod ribs and
(Britt, 1993). These pneumatic features are structurally and
identical to those in bird skeletons (Britt, 1993; Reid,
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.
have five separate airsacs; cervical, clavicular, anterior +
thoracic, and abdominal (McLelland, 1989). Interestingly, the
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.
is reasonable the diverticula/skeletal associations seen in
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,
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,
this basal bird had developed clavicular airsacs. Unlike
pterosaurs, theropods do not possess pneumatic limb bones.
pneumatic limbs may be an adaptation for flight (Swartz et al
such absence in flightless theropods would not be surprising.
evidence that theropods possessed at least a somewhat avian lung
from Archaeopteryx. Archaeopteryx appears to have been capable of
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
similar to that of flying birds (Thomas, 1987; Suarez, 1996). The lung
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,
Some flightless birds have secondarily reduced their aerobic
(Kooyman & Ponganis, 1994), while a few cursorial birds retain
aerobic power comparable to those of flying birds (Butler,
convergent evolution of such high metabolic capacities in
birds, bats and
insects is unlikely to be coincidence. Such strikingly
for elevated levels of aerobic power strongly suggests
that development of
such rates of oxygen consumption is required in order
to develop powered
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
Because theropods were flightless, they may not have needed
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
exhibit numerous similarities (Perry, 1989), and hypothetical
scenarios depicting transitional lung types have been
constructed (Perry, 1992). That the avian respiratory system
redundancies in regards to gas exchange is verified by
where various air sacs have been selectively blocked
and neither oxygen consumption nor ventilation were
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
Ruben et al (1997) postulate the pubis of theropods was
similar to that of
crocodiles, and served as an attachment site for
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
pubic design became less, rather than more,
theropod-like over time.
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
piston, since Patagonykus, as a bird, would lack this structure.
alvarezsaurids are theropods, then the pubes of Parvicursor remotus
Rautian, 1996), are inconsistent with the presence of
lungs. The pubes of this animal are highly retroverted,
attached to the
ischium, and do not appear to form a
Ruben et al (1997) have interpreted the pubes of Archaeopteryx
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
specimen are articulated, and clearly exhibit a near vertical
similar to theropods such as Unenlagia (Novas and Puerta,
A similar pubic orientation is also seen in the undescribed
bird from Madagascar though the pubis is not preserved,
the orientation of
the ischium of the early bird Iberomesornis suggests a
as well (Sanz & Bonaparte, 1992). The well preserved
position of the pubes
in the newest Archaeopteryx specimen indicates the
postulated by Ruben et al (1997) is
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