PAST - Case study 5 - Heterochrony in a fossil rhynchosaur reptile

Case study 5 - Heterochrony in a fossil rhynchosaur reptile

Heterochrony involves differences in the timing of developmental processes; thus detailed, preferably statistical, analyses of ontogenetic series are of considerable value in unravelling and identifying the track and mode of heterochronic trends in phylogeny. The rhynchosaur reptiles were abundant and widespread during the mid and early late Triassic. Benton & Kirkpatrick (1989) have studied heterochrony in the rhynchosaur Scaphonyx.

Two growth stages of the rhynchosaur Scaphonyx (from Benton & Kirkpatrick 1989, text-fig. 10 with permission from the Palaeontological Association).

Nine measurements were made (in mm) on a small sample (N=13) of the rhynchosaur Scaphonyx fischeri: SKULL, SKULW, ORBIT, FRONW, FRONL, PARW, PARL, UTLEN and UTWID.

Location of measurements on the skull of Scaphonyx (from Benton & Kirkpatrick 1989, text-fig. 3 with permission from the Palaeontological Association).

Statistical description of the variates

Open the file rhynch.dat. Choose one column at a time, and select 'Univariate statistics' in the Statistics menu. Discuss the high variances in the sample and explain the negative skewness in many variates.

Investigate the parametric and non-parametric mutual correlations of the variates. Try the Rho/Tau correlation (non-parametric) on pairs of variates at a time, and then try 'Correlation' (parametric) on the whole data matrix. Discuss the strength of correlation between variates in this sample.

Allometric growth

Assess the mode of growth of the skull of the rhynchosaur Scaphonyx, using the 'Linear' fitting function in the Model menu. For selected pairs of variates assess the suitability of either the allometric (a is different from 1 when log-transforming both variates) or isometric growth equation. The figure below shows a plot of SKULL vs. FRONW. Discuss the allometric effects in this sample.

More information about linear regression can be found in the manual.

Benton & Kirkpatrick (1989) used some of the allometric trends in their sample to illustrate a peramorphic evolutionary cline towards a broader skull type (for example SKULL vs. FRONW and PARW). Does your study support these conclusions?