Friday Rewind: The rise and fall of Alabamornis

I feel like I have been run over by a truck. Between blogging, working on my book, fieldwork, pitching freelance articles, and research, I just didn't have the energy to come up with something new today. Instead enjoy this post, written a little more than a year ago, about how the hip of a fossil whale was mistaken for the shoulders of an ancient bird. -- Brian

If you were a 19th century paleontologist and you wanted a skeleton of the fossil whale Basilosaurus, there was only one place to look; Alabama. Even though fossils of the ancient cetacean had been found elsewhere, their bones were most abundant in this state, and naturalists such as S.B. Buckley, Albert Koch, and Richard Harlan based their descriptions of Basilosaurus on specimens from the Yellowhammer State. Unfortunately, however, most of the skeletons were fragmentary. Vertebrae were common, but parts of the rest of the skeleton were extremely rare.

Then, in 1896, the paleontologist Charles Schuchert made a discovery that was altogether unexpected. Associated with a chain of Basilosaurus vertebrae were a pair of what appeared to be hips, and these bones were described by Frederic Lucas in 1900. It was difficult to tell whether they had been attached to the spine or suspended in the flesh of the body as in living cetaceans, which do have vestiges of a pelvis and sometimes limbs in their bodies, but this was the first recorded discovery of Basilosaurus hips. (It would later be found that Basilosaurus still had small external hind limbs but the hips were fully detached from the spine.)

The fact that the sediment around the skeleton was little disturbed led Lucas to suggest that the hips had been found in approximately their natural position, but the Austrian paleontologist Othenio Abel disagreed. In 1906 Abel published the paper "Uber den als Beckengurtel von Zeuglodon beschriebenen Schultergurtel eines Vogels aus dem Eocan Alabama" in which he suggested that the "hips" were really part of the shoulder of an enormous Eocene bird. He named it Alabamornis gigantea and proposed it was related to the giant flightless birds of the time like Gastornis.

Lucas could not accept this, and just as he refuted Abel's hypothesis that Basilosaurus was covered in armor he wrote to the leading journals of the day to make sure "Alabamornis" did not gain a foothold. In a letter printed in Science in February of 1908, he stated that it was impossible for the bones to be a bird shoulder, or at least belong to any type of bird known to science. (Lucas had waited so long to reply to Abel because he did not have time to reexamine the material himself. Eventually he had to rely on notes from his colleague C.W. Gilmore to confirm what he remembered of the bones.)

The key to whether the bones were part of the hip or the shoulder was decided by the number of surfaces on the bones that articulated with other bones. As Lucas pointed out, both hip bones had only one articular surface; the acetabulum, or the socket that articulates with the femur. That an incomplete femur (figured below) was found near the bones strengthened this conclusion. In Abel's hypothesis, however, the bones would have articulated with both the upper arm bone and the clavicles, meaning that there should have been more than one articular surface. There was only one and it was clear that it held the femur, thus ruling out that the bones were part of the shoulder. If these were truly the shoulder bones of a bird, Lucas wrote, "they represent a type of shoulder girdle entirely different from any with which we are at present acquainted, and the bird from which they come not only belongs to a new species and genus, but to a new order or superorder."

Surprisingly, however, Lucas wrote the note to strike down the hypothesis that the bones were bird shoulders, not to confirm that they were whale hips. At the end of the letter he wrote;

Now, I will not insist that the bones under discussion represent the pelvis of [Basilosaurus] nor deny that they are the coracoids of a bird; I will simply say that it seems to me doubtful that this last ascription is correct and wait for further discoveries to throw more light on the problem.

At some point Lucas must have been able to see the bones again or began to feel more confident in his original interpretation, though. A note about Abel's paper that appeared in Nature in 1910 included the following disclaimer;

Dr. Lucas wishes to state that there is no doubt whatever as to the correctness of his original determination, and that the bones in question have been mounted in their proper position in the [Basilosaurus] skeleton which is now exhibited in the U.S. National Museum. 'Alabamornis' must accordingly be deleted from the list of fossil bird genera.

The paleontologist J.W. Gidley agreed with Lucas, as well. In a 1913 report on "A Recently Mounted Zeuglodon Skeleton in the United States National Museum" he defended Lucas' interpretation of the bones;

Abel, in an article published in 1906, contended that these bones were the corocoids of a large bird to which he gave the name Alabamornis gigantea. A careful restudy of these elements, however, leaves no doubt as to their mammalian characteristics and no reason to assume that they do not properly belong to the skeleton with which they were found associated. ... Not having the actual bones to examine Abel doubtless was led to a wrong interpretation of the plates published by Lucas, because they do not show very clearly the essential characters of the bones. This is due to the fact that the bone surfaces are pitted and roughened through imperfect preservation and the reproductions, which are from photographs, are perhaps somewhat confusing.

If Abel made any effort to defend his hypothesis about the bones I have found no reference to it. As quickly as it appeared "Alabamornis" vanished, a forgotten footnote in the greater debate over the characteristics and evolutionary place of Basilosaurus.