Hello All -
I am engaged upon a study of the evolution of head shape in horned lizards (
*Phrynosoma*) using the array of geometric morphometric functions from
geomorph. My sample consists of 3-D scans of 119 specimens from 16 of the
*Phrynosoma* species. There is a fair amount of head shape disparity among
these, a lot of which is due to differences in horn length.
I landmarked each specimen three times, in random order within species, and
species were not landmarked in any order. The initial landmark
configuration consisted of 72 landmarks covering the dermatocranium, mostly
placed upon suture intersections or suture termini, with each horn covered
by two landmarks on its base and one on the apex. I subject the entire
replicate data set to a Procrustes fit and extracted the symmetric portion,
subjecting it to the measurement error function gm.measurement.error, with
species identity included as an argument. The output was as follows:
[image: Picture1.png]
The magnitude of the interaction between systematic ME and groups is
dismaying, as I took some pains to mix up the order in which replicates
were taken; in addition, I didn’t group long-horned species or short-horned
species together as I landmarked. In the SNR plot for this analysis, the
individuals of each species cluster without a lot of overlap, and the
short-horned species are clustered in one area while the individuals of the
long-horned species are in another (non-overlapping) area. This is
discouraging when one is working on a phylogenetic analysis of head shape.
Horned lizards might as well be designed to potentially illustrate the
Pinocchio effect, so I redid the measurement error analysis with the same
data set, but with most of the landmarks designating horns removed before
Procrustes fitting etc. The results of that analysis:
[image: Picture2.png]
Still a large interaction effect.
There is only one published study so far using this method of assessing
measurement error in GM – Umulisa et al (2025) – and they were looking at
turtle shells, which are not complicated structures. I find it difficult to
place my findings in context. My question, for anyone who’s got some
perspective on this method, is whether I should consider the interaction
effect size I’ve been getting as a real hinderance to further examination
of phylogenetic shape change in these lizards.
Regards,
Larry Powell
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