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re: Interpreting the autopodia of tetrapods - comment



A recent paper by Hone, Sullivan and Bennett (2009) concluded that “IP [interphalangeal] hinge lines, as conceived by Peters (2000a), can often be superimposed on 2-D drawings of tetrapod mani or pedes but have little or no value as a tool in palaeontological and biological research."

Thanks a lot guys! That paper has been cited fewer than a dozen times and you're trying to knock it down?

The observation of parallel interphalangeal lines (PILs) was first offered by Peters (2000a) as I reported on the virtually parallel lines that I found could be drawn through the interphalangeal joints and across the ungual tips of virtually every tetrapod manus or pes, including wings and flippers. That such lines were present appeared to indicate that the phalanges worked in sets, whether for walking (extension) or climbing (flexion). One can see the knuckles line up in the grip of the human hand, for instance, but that lineup changes when the hand is flat (not being used). I considered these lines obvious, but overlooked, understudied and potentially useful. Several dozen examples were presented from Devonian forms to the present. More to the point of the paper, I was looking for a way to understand and perhaps distinguish digitigrady from plantigrady in pterosaurs and I thought I had succeeded.

Contra the Hone, Sullivan and Bennett (HSandB) conclusions, the odd thing about this paper is how often HSandB report that PILs can indeed be drawn and may have some usefulness. They report:

"IP [interphalangeal] hinge lines can often be superimposed on 2-D drawings of tetrapod mani or pedes."

“Some hinge lines clearly are valid, in the sense that straight lines can indeed be drawn precisely through three or more joint centres or ungual tips for some tetrapod digits in some positions.”

“In general, there are likely to be a number of specific cases in which an IP hinge line does indeed operate as an axis of rotation, as in the metatarsus of Iguana during initial elevation of the metatarsus (Brinkman 1980). In such cases, the concept of a hinge line is useful from a functional perspective, and likely to be helpful in describing the observed motion.”

“At best, it might be possible to use the hinge line criterion to identify a broad envelope of possible configurations for a given foot or hand, and to exclude as implausible the inverse set of configurations on which hinge lines could not be drawn.”

Unfortunately, what HSandB demand is for IP hinge lines to intersect exactly at the middle of IP joints and to do so parallel to the joints themselves. While it is remarkable how often this does happen (some joints are even angled apparently just for this) it doesn't happen all the time with every joint. That’s because the IP lines are applied to imperfect animals that are in the process of evolving. A higher level of precision was never expected or encountered. In fact, as radiographic images of a moving pes indicate (Peters 2000a), the lines shift during every step cycle. Moreover the medial and transverse line sets seem to have evolved for extension while the transverse set seems to have evolved for grasping parasagittal branches and other similar substrates. So they don’t work together. Nothing about these lines was ever meant to be as precise as Hone, Sullivan and Bennett (2009) appear to demand. In fact, some clades are specified by hinge lines that intersect the middle of certain phalanges and unguals. That breaks all the so-called “rules,” but that’s okay, or so I have observed.

HS and P tested the IP hinge lines on an ornithomid pes drawing taken from Romer. First of all, this hypothesis was never meant to be proven on one-, two- and three-toed taxa. It applies only as a vestige. Second, the drawing they chose is one with parallel digits, as if in the recovery phase of the step-cycle, not spread out in use.

HS and P tested the pes of Smilodon, which I had copied from a plantigrade drawing. My bad. They used cylinders(?) created in a 3-d program to show what would have happened in a digitigrade configuration. Unfortunately they didn't look at a genuine Smilodon mount. The metatarsals don't all line up transversely. They arc. The medial and lateral ones fall a little behind. That's important for the lineup of the rest of the digits. Plus cat toes are never in a straight line and should never be tested that way. When it's configured correctly, like most museum mounts, the IP hinge lines are there.

HS and P note that in a few taxa, like ratites, the pads don't line up with the phalanges. True. Counting the number of pads per digit is the solution to reveal inconsistencies. If there's one for every phlalanx, use it. Somehow they didn't think of that.

With regard to pterosaurs HS and P reported, “there is no reason to think that all Pteraichnus trackways were made by ctenochasmatids, or even that any Pteraichnus trackways were necessarily made by ctenochasmatids.” Peters (2000) provided evidence with pad counts that matched phalanx counts and digit proportions that matched ichnite to pes. No counter evidence was provided. No other pterosaur pes was matched to the track. If a statement is false they really should have falsified it.

Hoping to distance pterosaurs from my digitigrade reconstruction of Cosesaurus (matched to the digitigrade ichnite Rotodactylus) HSandB report, "Moreover, the suggestion of a close relationship between pterosaurs and prolacertiforms such as Cosesaurus is contradicted by a considerable weight of opposing evidence.” Their evidence consisted of six cladistic analyses in which Cosesaurus was not included as a tested taxon and one analysis (Hone and Benton 2008) in which only a quarter of its characters were scored.

Red herrings don't make good arguments.

I sincerely hope you all don't take this paper at its word and forget about PILs. It's ideal for reconstructing scattered phalanges but should be used in accord with articulated sister taxa.

David Peters
St. Louis


Hone, D.W.E. Sullivan, C. and Bennett, S.C. 2009 Interpreting the autopodia of tetrapods: interphalangeal lines hinge on too many assumptions. Historical Biology iFirst article, 2009, 1–11 DOI: 10.1080/08912960903154503

Peters D. 2000a. Description and interpretation of interphalangeal lines in tetrapods. Ichnos. 7:11–41.