It is interesting that dinosaurs that had long slim legs, small lightweight bodies, and in general appearance look somewhat like birds, such as ostrich mimic Struthiomimus, were "lizard-hipped". Conversely, dinosaurs that look more like low-slung tanks than graceful birds (e.g. Ankylosaurus) were "bird-hipped". Therefore, one question that should be put to dinosaur experts is: If dinosaurs evolved into birds, wouldn't we expect to see "bird-hipped" dinosaurs looking more bird-like rather than the reverse?

Mr. Poling:

Here's where the problems begin. This is a highly misleading paragraph for two reasons.

1. Someone not familiar with the morphology of Dinosaurs could easily conclude that all "lizard hipped" dinosaurs were like Struthiomimus, and all "bird hipped" dinosaurs were like Ankylosaurus. This is nowhere near the case.

2. The rhetorical and, in my opinion, leading question asked in this paragraph is never answered. As a result, someone not familiar with the anatomy of birds and dinosaurs could conclude that the "bird hips" of Ornithischians and the "bird hips" of Aves are identical. This, too, is not the case. There is a similarity in the positioning of the bones, but in terms of the actual morphological characteristics of the bones the "bird hips" of Aves have more in common with the "lizard hips" of Saurischians than with the "bird hips" of Ornithischians. These facts are not addressed anywhere else in the article and, as a result, the readers are pointed in a direction they should not be going. The unfortunate and misleading (and, ultimately, incorrect) naming of the two main subgroups of Dinosaurs should not be used to try to debunk evolution. It's a matter of early scientists incorrectly reading and naming the anatomy, not true evolutionary progression.

Addendum:

Birds do not, in fact, have "bird-hips." They have "lizard-hips," albeit highly derived ones. The following figures illustrate some of the hips found in the Dinosauria (red denotes the ilium, blue the pubis, and green the ischium; figures are not to scale).

Fig. 1: Theropod "lizard-hip"	Fig. 2: Deinonychus "lizard-hip"	Fig. 3: Archaeopteryx "lizard-hip"
Fig. 4: Sauropod "lizard-hip"	Fig. 5: Hypsilophodont "bird-hip"	Fig. 6: Crow "lizard-hip"

A cursory glance at the figures reveals a resemblance between the "bird-hip" (fig. 5) and the bird's hip (fig. 6). However, careful examination should reveal differences between the "bird-hip" and the bird's hip, and corresponding similarities between the "lizard-hips" (figs. 1-4) and the bird's hip, including:

• shape of the ilium
• shape of the pubis
• size and shape of the ischium
• the prepubic process

The figures also strikingly illustrate the evolution of the bird's hip from the primitive "lizard-hip:"

• Fig. 1 is the hip of Tyrannosaurus rex, the largest of the bird-like non-avian dinosaurs.
• Fig. 2 is the hip of Deinonychus, a close relative of Velociraptor, and very closely related to birds. It clearly shows the beginnings of the bird-like condition.
• Fig. 3 is the hip of Archaeopteryx, the most primitive bird known. The hip is clearly on its way to a modern-like configuration, yet still retains "primitive" features such as the typical theropod "boot" on the end of the pubis. Recent studies have shown that the pubis of the type specimen of Archaeopteryx is broken, and when properly articulated as shown in Fig. 3 the pubis is even more dromaeosaur-like than previously thought.¹
• Fig. 6 is the hip of a modern crow, and shows the neornithine (modern bird) condition.

Further evidence for the evolution of the bird's hip from the "lizard-hip" comes from ontogeny. Ontogeny often recapitulates phylogeny, or the developmental stages of an embryo show the evolutionary history of the organism. In modern bird embryos the pubis initially points forward as in "lizard-hips," then rotates backwards as the embryo develops.²

Clearly, careful comparison of the hips, a good progression of hip fossils showing the evolution of the bird hip, and embryological study shows that a bird's hip is a "lizard-hip," not a "bird-hip."

A logical question to ask, then, is "Why the heck did paleontologists use 'lizard-hip' to describe theropod hips when clearly they are birds' hips?" To answer this question we must travel back to the very dawn of dinosaur paleontology.

The first dinosaur bones recognizable in scientific literature as dinosaur bones were discovered in the 17th century. By 1842, enough material had been found for Sir Richard Owen to recognize a new order of animals which he named Dinosauria ("terrible lizards," referring to their large size rather than supposed demeanor). By 1887 enough good material had been found that paleontologists could clearly distinguish two different groups within the Dinosauria on the basis of their hips. Harry G. Seeley named the groups the Saurischia ("lizard-hips") and Ornithischia ("bird-hips") because their hips looked like the hips of extant lizards and birds, respectively.

The 19th century was the dawn of comparative anatomy and paleontology as sciences. Early scientists noticed the similarities between extant birds and extinct dinosaurs. One of these similarities, as discussed above, was the hip of ornithischian dinosaurs. Based largely on this similarity, early paleontologists thought birds descended from ornithischian dinosaurs. This would change over the next century, as refinement and changes in the methodology of paleontology and comparative anatomy, coupled with spectacular new discoveries, advanced the sciences.

Among the greatest discoveries of the twentieth century are the maniraptoriforme dinosaurs. Early in the 20th century Barnum Brown discovered the fearsome Tyrannosaurus rex, a very large yet very gracile dinosaur with many bird-like characteristics. In the 1920s Roy Chapman Andrews discovered Velociraptor mongoliensis, and in the 1960s John Ostrom described a very close relative of Velociraptor, Deinonychus anthirropus. Later, in the course of studying specimens of Archaeopteryx, Dr. Ostrom made the startling discovery that the hand of Archaeopteryx is an almost exact scaled-down version of Deinonychus's hand. These animals share so many characters that Archaeopteryx, all other birds, Deinonychus, Velociraptor, and other closely related forms are included in the bird clade Maniraptora ("hand robbers").

Today, so many characters have been found in common among birds and dinosaurs that there is little doubt in the minds of dinosaur paleontologists that birds are dinosaurs and bird hips are not "bird-hips" but "lizard-hips." These characters simply were not known to the 19th century paleontologists who, in the infancy of the science, created the names we use today. There is a recommendation by Dr. Robert Bakker to rename the Ornithischia to Predentata in recognition of the Ornithischian pre-dentary bone, a feature truly unique to the Ornithischia. However, science is slow to change its nomenclature. Paleontology had the opportunity to rid itself of this old nomenclatural baggage when phylogenetic taxonomy became the norm and all groups were redefined; however, in their zeal to use as many names from the old Linnean system as possible, they missed the chance to sink Ornithischia once and for all. Ornithischia now has priority over any other name for the same clade, and the confusing names of "bird-hipped" dinosaur and "lizard-hipped" dinosaur will continue to be used unless another name completely supplants the official name. To this end, from this point forward, I will be using the name Predentata, rather than Ornithischia, to denote Triceratops and all taxa sharing a more recent common ancestor with Triceratops than with birds.

If birds have "bird-hip"-like "lizard-hips" while other dinosaurs had true "bird-hips" (see why I advocate changing the names?), a logical question is "why do these two different dinosaur lineages have similar hips?"

Similar characters in two separate species can either be inherited or evolve independently. Similar characters that are present in two separate species and a common ancestor are most likely inherited. Similar characters that are present in two separate species but not a common ancestor are most likely not inherited. Similar characters due to inheritance are called homologies. Similar characters that evolve independently are called homoplasies.

The last common ancestor of saurischians and predentatans was the very first dinosaur. This dinosaur was a biped more like a theropod than a Triceratops. Its hip was very much like a saurischian hip, meaning that birds and predentatans did not inherit their similar hip configurations from a common ancestor. Therefore, bird hips and "bird-hips" are homoplasies.

The evolution of homoplasies is called convergent evolution. Birds and predentatans have similar hips because of convergent evolution.

It is well known that convergent evolution usually occurs when different species are subjected to similar environmental pressures. The end result of such similar pressures is disparate species that share many similar characteristics, not just one or two, resulting in animals that look very much like each other. So why do birds and predentatans not both look like neornithes? Because the pressures that led to convergence were likely very different for both lineages.

Predentatans evolved mainly as quadrupedal, graviportal plant-eaters. A vegetarian diet requires special adaptations to absorb the nutrients from tough plant tissue. These special adaptations usually come in the form of greatly expanded digestive tracts. One of the drawbacks to the forward pointing pubis of the first dinosaurs, which were carnivores, is that it limits the size of the digestive tract. Sauropods likely solved this problem by using gizzards lined with gastroliths to help preprocess plant material before passing it to the gut. Predentatans solved this problem by evolving a pubis rotated back against the ischium which allowed enlargement of the gut. In some later Predentatans the pubis nearly disappeared completely.

There are two possible explanations for why birds evolved their opisthopubic hips:

1. Flight. Birds started out as bipedal, cursorial theropods. At some point these ancestral theropods took to the trees, where they evolved feathers and learned how to fly. Flying as a primary mode of locomotion poses many problems from a bio-engineering standpoint. One problem is the stresses flight places on the body. In this respect, the long back of dinosaurs is a serious problem. Evolutionary pressure led to a compaction of the typical theropod body plan, shortening the back of birds. Naturally, when the back shortens, the ribs and everything else must either get smaller or move towards the rear of the animal. Given the large energy requirements of flight, shrinking the lungs and digestive track would not be feasible. However, as with limiting the size of the digestive track, the forward pointing pubis of theropods limits how far the internal organs can move rearward. To solve this problem the pubis rotated rearward as birds evolved.

2. Locomotion. The primary thrust muscle of early theropods was a long muscle, the caudofemoralis longus, that started from the leg and stretched for much of the length of the tail. In maniraptorids the primary thrust muscles are located on the hips and base of the tail. Decoupling the tail from thrust allowed the tail to be used as a dynamic stabilizer for greater agility and control while running, jumping and climbing. It also made it possible for birds to lose the tail completely when their primary means of locomotion became flight. Rotating the pubis back allows the pubis to be more closely associated with the tail, providing attachment points for the thrust muscles. ³

It is also possible that both of the above hypotheses are true. Both could have occurred as adaptations for flight or arboreality, or one could have led to, or allowed, the other.

With this background information in mind, we can now turn to Mr. Buckna's article. Mr. Buckna states that

...one question that should be put to dinosaur experts is: If dinosaurs evolved into birds, wouldn't we expect to see 'bird-hipped' dinosaurs looking more bird-like rather than the reverse?

The answer, of course, is no. As shown above, the similarities between bird hips and "bird-hips" are superficial, at best. Birds have "lizard-hips," not "bird-hips." Further, the "bird-hipped" lineage does not look bird-like because the convergent evolution of bird and predentatan hips was due to very different evolutionary pressures. By asking this question without giving the necessary background information, and without giving the proper answer, Mr. Buckna perniciously plants the seeds of unreasonable doubt in the minds of an uninformed public.