Parabronchi

Summary of problems:

There are detailed, testable models of the evolution of dual-opening parabronchi in bird lungs from single-opening alveoli found in the reptilian ancestors of birds. Explore Evolution asks a number of questions about this transition, but then fails to offer students any means to answer any of them, or to discuss how a student or scientist might go about finding answers to these questions.

Full discussion:

Explore Evolution asks its readers:

What would the intermediate forms between the single openings (in-and-out) reptilian lung and a dual opening (flow through) avian lung look like? How would it happen in small yet advantageous steps? Can there even be a transition between a single-opening and a dual-opening system? How would the balloon-like alveoli transform into the tube-like parabronchi? How would the lung maintain function? Would the lung transformation happen before or after the development of air sacs? Would it be before or after the four stage breathing cycle?

EE, p. 137

Lungs of various amniotes, mapped onto the phylogeny of the respective organisms: Mammals are very distant from birds and neither the mammalian diaphragm nor the alveolar lung is thought to be an ancestral character for the lineage leading to birds. The crocodile hepatic-piston method of ventilating the lungs (muscles pulling the liver backwards and thereby expanding the chest cavity) is not homologous to the mammalian diaphragm, and neither basal reptiles nor birds have diaphragms, so it is incorrect to claim that it is "almost certain" that dinosaurs had diaphragms. Perforations (holes) between lung chambers, however, are shared by birds and crocodiles, and thought to be ancestral, so the alleged "topological" problem in producing the bird flow-through lung is imaginary. Sauropods are known to have air sacs from fossil evidence, so air sacs were attached to the lungs of the dinosaurian ancestors of birds for tens of millions of years before theropod dinosaurs and then birds arose.

Phylogeny diagram by Nick Matzke. Lungs modified from Figure 1, p. 152 of: Perry, Steven F. (1992). "Gas exchange strategies in reptiles and the origin of the avian lung". Physiological Adaptations in Vertebrates. Wood, S. C., Weber, R. E., Hargens, A. R. and Millard, R. W., Eds. New York, Marcel Dekker: 149–167. Perry's (1992) model for the origin of bird lungs: Perry's (1992) model for the origin of bird lungs, showing the relationship to the crocodilian lung. Archosaur and theropod lungs are hypothetical constructs. Extinct groups are marked with a "+", and perforations between chambers are marked with "*." Perry proposes that these perforations play a crucial role in the stepwise evolution of the avian parabronchi, as indicated in the detail sketches of theropod and avian-grade lungs. CrC and CaC are cranial [forward part of the trunk] and caudal [rearward part of the trunk] chambers, which are connected with the respective regions of the intrapulmonary bronchus. MvB and MdB are avian medioventral bronchi and mediodorsal bronchi, which are proposed to evolve from CrC and CaC, respectively, as indicated by small arrows. From Figure 6, p. 161 of: Perry, Steven F. (1992). "Gas exchange strategies in reptiles and the origin of the avian lung". Physiological Adaptations in Vertebrates. Wood, S. C., Weber, R. E., Hargens, A. R. and Millard, R. W., Eds. New York, Marcel Dekker: 149–167.

Having asked its audience of high school biology students these detailed questions about evolutionary biology, EE changes topics, without even suggesting the ways that someone might investigate those questions. These students are unlikely to know anything about the anatomy of bird or reptilian lungs, and little if anything about the anatomy of mammalian lungs. They have no experience forming or testing hypotheses about the evolution of anatomical structures, and Explore Evolution offers no references which might fill in that background. The average teacher is likely to be as stymied by these questions as the students. The authors of Explore Evolution seem to be little better informed, and are apparently comfortable leaving students and teachers with no guidance about how to answer the questions posed by the book.

Fortunately, scientists are not so incurious. The figure above demonstrates one set of hypotheses about the evolution of lungs and their anatomy. By considering not just two sets of lungs, but the full spectrum of variation in lung morphology, scientists can reconstruct the likely evolutionary pathways, and evaluate whether those intermediates might be functional.

Scientists like Steven Perry have proposed detailed models of the evolution of the internal lung morphology, models which answer many of the questions Explore Evolution asks. An inquiry-based textbook might describe this model and invite students to develop ways to test it against new data. Instead, Explore Evolution ignores actual research in order to preserve their creationist argument.

Bird diaphragms

Summary of problems:

The claim that air sacs in evolving birds would put a hole in the a diaphragm and lead to a nonfunctional, fatal intermediate, is based on selective quoting of a single sentence from a scientific publication from 1997; the conclusions in that publication are more complicated than one might guess from reading that single out-of-context sentence. Furthermore, Explore Evolution ignores more recent findings that have overturned the idea that the dinosaur ancestors of birds even had diaphragms to damage.

Full discussion:

On p. 137 of Explore Evolution, the authors argue that the radical transformation of the lung from reptilian to avian seems improbable. Part of the argument goes like this.

Finally, what happens to the diaphragm? The reptiles thought to be the ancestors of birds almost certainly had a diaphragm breathing system (footnote 8). According to many evolutionary biologists, changing from a diaphragm lung system to a flow-through lung would require changing and increasing the musculature of the reptile's chest. At the same time, the diaphragm would need to gradually go away. This poses a fundamental problem. Evolutionary biologist John Ruben points out that the earliest stages of this transformation would have required a hole or hernia in the reptile's diaphragm. This would have immediately compromised the entire system and led to certain death for any animal unfortunate enough to possess this non-functioning intermediate structure.

Explore Evolution, p. 137.

Footnote 8 refers to Ruben et al. (1997), Science 278:1268-1269 (actually 1267-1270), and quotes from the article.

8. "Therapod dinosaurs like modern crocodiles, probably possessed a bellows-like septate lung, and that lung was probably ventilated … by a hepatic-piston diaphragm."

John A. Ruben, Terry D. Jones, Nicholas R. Geist, W. Jaap Hillenius, (1997) "Lung structure and ventilation in theropod dinosaurs and early birds," Science 278:1268-1269. Explore Evolution, p. 140, note 8, quoting Ruben, et al. 1997.

Note that "Therapod" is a misspelling of "theropod." Also, the correct page numbers for the article are pages 1267-1270. The authors of Explore Evolution managed to cram about five typos into their short quote of Ruben, et al., so for the sake of correctness, as well as including the ellipsed text and the rest of the sentence, here is the exact quote from the original Science article, which might be enough to trigger a question in the mind of a student in a truly inquiry-based activity.

These observations, combined with the occurrence among theropods of a distinct, relatively vertical, crocodile-like, highly elongate pubis (Figs. 4 and 5), as well as well-developed gastralia, provide evidence that theropod dinosaurs, like modern crocodiles, probably possessed a bellows-like septate lung and that the lung was probably ventilated, at least in part, by a hepatic-piston diaphragm that was powered by diaphragmatic muscles that extended between the pubic bones and liver.

The authors of Explore Evolution have miscast the conclusions. Ruben, et al. are basically arguing that the theropod dinosaurs are not the earliest ancestors of birds. This position was highly unpopular in the scientific community in 1997, and is extremely unpopular now — the number of holdouts against the idea that birds are descended from theropod dinosaurs can be counted on one hand. In the 1997 Science paper, Ruben, et al. argued that there is a logical problem with an intermediate form between purported ancestors (theropod dinosaurs, which allegedly possessed hepatic-piston diaphragms) and modern birds. But they also argued that theropod lung physiology was not consistent with endothermy [warm-bloodedness], a character that might be important in creatures (like birds and their ancestors) which are capable of flight. Here is the meat of Ruben, et al.'s conclusion section:

Recently, conventional wisdom has held that birds are direct descendants of theropod dinosaurs. However, the apparently steadfast maintenance of hepatic-piston diaphragmatic lung ventilation in theropods throughout the Mesozoic poses fundamental problems for such a relationship. The earliest stages in the derivation of the avian abdominal air sac system from a diaphragm-ventilating ancestor would have necessitated selection for a diaphragmatic hernia in taxa transitional between theropods and birds. Such a debilitating condition would have immediately compromised the entire pulmonary ventilatory apparatus and seems unlikely to have been of any selective advantage.

In other words, Ruben, et al. are not saying that this poses an insurmountable obstacle for any theory that postulates evolution of the bird lung. They are merely saying that this logic, as well as the arguments against endothermy in putative ancestors, argues against the specific theropod-bird ancestral connection. Birds (with their unique lungs and high oxygen requirements) must, by this logic, be descended from other ancestors. And even that conclusion generated almost immediate controversy. In November of 1998 three critiques of this paper, along with a rebuttal by Ruben, et al., appeared in Science (281(5373):45-48). Interestingly, these focused primarily on the conclusions about endothermy, rather than on the idiosyncratic diaphragm anatomy issue highlighted by the authors of Explore Evolution. Evidence has continued to accumulate against Ruben, et al.'s claim that theropod dinosaurs had diaphragms (see below).

Finally, since 1997 many spectacular fossils (both of birds and dinosaurs thought to be ancestral to birds) have been discovered, but none of these more recent findings are discussed in Explore Evolution, even though many of them (e.g. O'Connor & Claessens, 2005. Nature 436 (7048): 253-256) provide evidence that further argues against the conclusions of the 1997 paper of Ruben, et al.

Dinosaur diaphragms

Summary of problems:

The dinosaur ancestors of birds probably did not have diaphragms. The one researcher cited to oppose this view also rejects the evidence that birds evolved from dinosaurs; his views on both topics have been widely refuted.

Full discussion:

Paleontologist Matt Wedel explains:

Non-avian dinosaurs did not necessarily have the same pulmonary anatomy as crocodilians or extant birds. As hypotheses of pulmonary anatomy in dinosaurs, "croc lungs" versus "bird lungs" is a false dichotomy. It is more informative to identify the derived features that non-avian dinosaurs share with their extant relatives, and to determine the hierarchical distribution of these characters in archosaurian phylogeny.

Wedel, Matt (2007) Postcranial pneumaticity in dinosaurs and the origin of the avian lung, Ph.D. dissertation, University of California, Berkeley, p. 112.

That is exactly the approach that evolutionary biologists and textbooks about evolutionary biology take in addressing the evolution of organisms and particular parts of organisms. An inquiry-based textbook could include exercises allowing students to undertake the same process of investigation. Explore Evolution does not use this comparative approach, and discourages students from further investigation in areas of ongoing biological research, or even areas where the research has already been conducted.

By comparing fossils of dinosaurs to modern birds, it is possible for paleontologists to produce and test hypotheses about the evolution of the dinosaur lung. Wedel describes his approach:

Instead of focusing on particular [traits] that are either not present in all birds (large sternum, uncinate processes of the ribs) or not clearly necessary for air sac ventilation (ossified rather than cartilaginous sternal ribs), it may be more productive to identify the skeletal movements that take place during avian respiration and the effects of these movements on the shape and volume of the thoracic cage, and then to ask whether the skeletons of non-avian dinosaurs were able to produce similar movements.

Wedel (2007), p. 119

He concludes that "the respiratory movements in non-avian dinosaurs would have had a similar effect on the volume of the thorax as those of extant birds. … there is no basis for inferring that non-avian dinosaurs could not have ventilated an air sac system, based simply on the absence of some avian features" (pp. 120-121).

Wedel's analysis of the fossils shows that a functional intermediate could have existed without the need for the full suite of avian (bird-like) adaptations. Those adaptations may improve the efficiency of bird breathing, but their absence would not be fatal, despite Explore Evolution's claims. Explore Evolution misrepresents evolution as a linear path from reptilian anatomy to avian anatomy, and considers it problematic if a straight line cannot be drawn from ancestral to modern conditions. This is not how evolution works.

As to the particular issue of a diaphragm in the common ancestor of birds and dinosaurs, paleontologists are skeptical. The paleontological evidence used by Ruben to support the claim that the ancestors of dinosaurs had a diaphragm is weak at best. He takes the coloration of rock within a fossil to reflect the location of the liver in the living organism, and then suggests that a liver in that position requires the sort of diaphragmatic breathing found in crocodiles. Even if he were correct that the color in the rock originated in the liver, and if that liver hadn't shifted as the organism decayed, it would still not support his final claim, since living birds have livers in exactly the same position, and do not have diaphragms (see discussion in Wedel, 2007, p. 128).

While crocodilians do breath using a diaphragm, many reptiles do not use a diaphragm, and neither do the amphibians which are ancestral to reptiles, mammals, dinosaurs and birds. By examining the full range of paleontological evidence, scientists can reconstruct probable anatomies not seen in modern species, but which would provide the sort of functional intermediates which evolutionary theory predicts should exist. By contrast, Explore Evolution and the sources it cites take an "approach to inferring soft tissue anatomy, function, behavior and physiology [which] tends to force extinct animals into the reduced spectrum of animals available to us today, without considering substantial evidence of mosaic change in related extinct forms. It lacks an evolutionary component, produces only conundrums, and explains very little" (Kevin Padian and John R. Horner. 2002. "Typology versus transformation in the origin of birds," Trends in Ecology and Evolution, 17(3):120-124).