Markey and Marshall. 10.1073/pnas.0701706104. |
Fig. 6. Cross-sectional measurements of the interfrontal (IF), interparietal (IP), interpostparietal (IPP), frontoparietal (FP), and nasofrontal (NF) sutures in Eusthenopteron (blue), Acanthostega (red), and Phonerpeton (green). Values are means ± SD. One slice through the IF suture was available for both Acanthostega and Eusthenopteron; therefore, those data points lack error bars. For all other taxa and sutures, N equals at least 2 (see SI Table 1). (A) Index of interdigitation (dimensionless). (B) Bone thickness (mm). (C) Beveling (equals overlap) (mm). (D) Size of the largest interdigitation (mm). These data show that there is not a clear increase or decrease in sutural complexity across the fish-tetrapod transition; instead, certain sutures may appear more complex depending on the metric examined (see Surface Versus Cross-Sectional Sutural Shape).
Table 1. Cross-sectional (indices 1-4) and ectocranial (equals surface; index 5) measurements of sutural complexity in Eusthenopteron, Acanthostega, and Phonerpeton
Species | Specimen φ | Suture | N | Interdigitation index† | Bone thickness, mm | Beveling amount, mm | Largest interdigitation size, mm | Sinuousity†† |
Eusthenopteron° | SMNH p. 222 | IF | 1 | 1.36 | 0.71 | 0.13 | 0.10 | 1.05 |
Eusthenopteron° | SMNH p. 222 | IP | 2 | 2.63 ± 0.59 | 1.37 ± 0.03 | 0.17 ± 0.00 | 0.50 ± 0.34 | 1.01 |
Acanthostega ‡ | MGUH f.n. 236 | IF | 1 | 1.41 | 2.70 | 2.12 | 0.35 | 1.03 |
Acanthostega ‡ | MGUH f.n. 236 | IP | 3 | 2.33 ± 1.05 | 1.30 ± 0.11 | 0.76 ± 0.40 | 0.53 ± 0.50 | 1.44 |
Acanthostega ‡ | MGUH f.n. 236 | IPP | 4 | 1.58 ± 0.28 | 1.57 ± 0.18 | 0.33 ± 0.29 | 0.23 ± 0.21 | 1.90 |
Acanthostega ‡ | MGUH f.n. 236 & 1305 | L FP | 3 | 1.97 ± 0.41 | 2.10 ± 0.99 | 1.81 ± 0.69 | 0.58 ± 0.33 | 1.02 |
Acanthostega ‡ | MGUH f.n. 1305 | L NF | 3 | 1.59 ± 0.20 | 1.03 ± 0.34 | 0.55 ± 0.47 | 0.17 ± 0.05 | 1.94 |
Phonerpeton * | MCZ 1414 | IF | 2 | 2.21 ± 0.79 | 1.10 ± 0.13 | 0.37 ± 0.15 | 0.36 ± 0.08 | 1.11 |
Phonerpeton * | MCZ 1414 | IP | 5 | 2.14 ± 0.47 | 1.19 ± 0.14 | 0.25 ± 0.17 | 0.44 ± 0.09 | 1.22 |
Phonerpeton * | MCZ 1414 | IPP | 3 | 2.65 ± 0.37 | 2.88 ± 0.33 | 1.04 ± 0.39 | 1.45 ± 0.51 | 1.26 |
Phonerpeton * | MCZ 1414 | L FP | 3 | 1.67 ± 0.17 | 0.94 ± 0.33 | 2.11 ± 1.31 | 0.13 ± 0.06 | 1.23 |
Phonerpeton * | MCZ 1414 | L NF | 3 | 3.48 ± 1.02 | 1.34 ± 0.26 | 0.83 ± 0.28 | 0.79 ± 0.52 | 1.24 |
Phonerpeton* | MCZ 1414 | R FP | 3 | 1.84 ± 0.30 | 0.89 ± 0.09 | 1.22 ± 0.87 | 0.10 ± 0.09 | 1.18 |
Phonerpeton * | MCZ 1414 | R NF | 2 | 2.52 ± 1.48 | 0.79 ± 0.04 | 1.74 ± 0.87 | 0.77 ± 0.36 | 1.12 |
For the interdigitation index, bone thickness, beveling amount, and largest interdigitation size, values are means ± SD. Sinuousity (index 5) values were measured using illustrations in the literature: Eusthenopteron (30), Acanthostega (31), and Phonerpeton (24).
†
Interdigitation index = Total length of suture in cross-section ÷ distance between the endo- and ectocranial ends.††
Sinuousity = Total length of suture on the surface of the skull ÷ distance between the ends of the sutural trace.φ
Abbreviations: SMNH = Swedish Museum of Natural History (Stockholm); MGUH = Museum Geologicum Universitatis Hafniensis (Geological Museum, University of Copenhagen); MCZ = Museum of Comparative Zoology (Harvard University, Cambridge, MA).°Eusthenopteron specimen SMNH p. 222 was used by E. Jarvik to create grinding series 2 for his definitive analysis of that species (see ref. 3). Measurements were made from drawings based on photographs taken of the specimen during the grinding process (see Materials and Methods).
‡
Measurements of Acanthostega sutures made using thick sections of specimens MGUH field number 236 and MGUH field number 1305.*Measurements of Phonerpeton sutures made using high-resolution x-ray CT scan of specimen conducted at the University of Texas, Austin (for M.J.M.).