This is the image (in Rieppel 1992) that led me astray in 1998.

After I went to visit the holotype of Cosesaurus (Fig. 1) in Barcelona in 1998 (the Monica Lewinsky scandal was going on at the time), I stopped in to visit the original author, Paul Ellenberger, who we discussed earlier here, here and here.  At the time I was naive, inexperienced and untutored, but I did notice the obvious strut emerging anteriorly from the anteriorly expanded ilium (Fig.1). I recognized that was an odd structure. I had never seen anything like it before on other fossils. And nothing like it since.

Cosesaurus prepubis

Figure 1. Cosesaurus prepubis in situ and reconstructed. The strut anterior to the ilium is one of the most distinct features here.  Here is is now interpreted as the stem of the nascent and displaced prepubis.

Ellenberger (1993) noticed the strut as well and also considered it an anterior process of the anterior ilium.

Further Confirmation
Then I noticed Rieppel (1992) described something similar in the T4822 specimen of Macrocnemus and illustrated it (Fig. 2). So, to my mind there was a sister taxon with a similar structure and that made it ‘valid.’ So I accepted and echoed that interpretation in Cosesaurus (Peters 2000). Boy, was I wrong!

Figure 1. Macrocnemus pelvis, specimens identified. Cocked to the right is the Rieppel (1992) interpretation that caused me to think there actually could be an anterior process on the ilium. The process is actually the ventral process, where the pubis connects.

Figure 2. Macrocnemus pelvis. Cocked to the right is the Rieppel (1992) interpretation that caused me to think there actually could be an anterior process on the ilium. The process is actually the ventral process, where the pubis connects. I misinterpreted his pre-actabular process as an anterior process of the ilium blade because Rieppel did not illustrate the actual acetabulum, but he did illustrate the indentation below the reinforcing lateral diagonal ridge also seen in the IVPP specimen (green arrows).

I have regretted it ever since
because that strut in Cosesaurus turned out to be the stem of the Cosesaurus prepubis (Fig. 1), a trait I overlooked in the long line of pterosaurian traits that were otherwise present there.

So I was curious
What did Rieppel (1992) actually see? Here (Fig. 2) I finally found out helped by a more recently discovered and described Macrocnemus, the IVPP V15001 specimen, in which the left pelvis is laid out perfectly in lateral view (Fig. 2) to show exactly what a Macrocnemus pelvis should look like, as opposed to the T 4822 specimen in which the ischium is largely concealed. The IVPP specimen shows the ilium includes a diagonal ridge leading toward the pubis (preacetabulur) process. Rieppel (1992) saw the same ridge in T 4822, but did not illustrate the acetabulum or the other pelvic elements. I have always found that illustration of his confusing, but now I understand it. That’s a relief.

So this is how we fix things in Science.
Correcting mistakes is what we do. I hope to publish this corrected data someday. Currently a manuscript is under review.

Ellenberger P 1993. Cosesaurus aviceps . Vertébré aviforme du Trias Moyen de Catalogne. Étude descriptive et comparative. Mémoire Avec le concours de l’École Pratique des Hautes Etudes. Laboratorie de Paléontologie des Vertébrés. Univ. Sci. Tech. Languedoc, Montpellier (France). Pp. 1-664.
Li C, Zhao L-J and Wang L-T 2007. A new species of Macrocnemus (Reptilia: Protorosauria) from the Middle Triassic of southwestern China and its palaeogeographical implication. Science in China D, Earth Sciences 50(11)1601-1605.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Rieppel O 1992. The hind limb of Macrocnemus bassanii (Nopcsa) (Reptilia, Diapsida): development and functional anatomy.

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