Timing of Morphological Change

The change in suture shape over time is illustrated in more detail in Fig. 8.3. PC1 describes ~83% of the total variance in the data set and therefore is a good proxy for the general pattern of suture evolution during the Paleozoic. PC1 scores for all sampled genera (light gray dots) are plotted for each one myr time bin. Data points are connected vertically (light gray lines) to emphasize the range of morphospace occupied during each time interval. The mean trend in morphospace occupation through time is emphasized by the solid black line.

Average PC1 scores remained fairly conserved throughout the early Devonian, but increased significantly by the middle of the period. The morphologies causing that shift were lost at the Frasnian-Famennian extinction, returning the morphospace to a more typical early Devonian state (Fig. 8.3). This changed again during the initial rebound after the end-Devonian extinction and by the late Tournaisian (353 Ma), the mean PC1 value had shifted significantly when compared to the Devonian values (Wilcoxon rank sum: Z = -7.55, p < 0.0001), and remained fairly conserved until the Moscovian-Kasimovian boundary (305 Ma). The interval following that boundary marked the onset of a morphologic diversification, later supplemented by taxonomic radiation, reflected in the steady increase in mean PC1 values through the end of the Permian.

8 Understanding Ammonoid Sutures 35

25 J

Cluster A

Mean PC1 (for all sutures

8 Understanding Ammonoid Sutures 35

Cluster A

Mean PC1 (for all sutures

25 J

Devonian

F-F Hangenberg

MCB JL

Mean PC1 (without cluster A) P-T

Carboniferous

—i—'—i—i—i—i—i—i—i—" —■—i—>—i—i—i—>—i—i—i—■—i—»>—i—i—r—i—i—■—r

410 400 390 380 370 360 350 340 330 320 310 300 290 280 270 260 250

Devonian

F-F Hangenberg

MCB JL

Mean PC1 (without cluster A) P-T

Carboniferous x:

—i—'—i—i—i—i—i—i—i—" —■—i—>—i—i—i—>—i—i—i—■—i—»>—i—i—r—i—i—■—r

410 400 390 380 370 360 350 340 330 320 310 300 290 280 270 260 250

Fig. 8.3 Detailed view of the change in suture morphology through time, using PC1 scores as a proxy. Data are binned into 168 1-myr time intervals. Points correspond to the score for each genus and are connected vertically to emphasize total range of values per interval. Black points highlight a statistically distinct cluster of morphologies (A) generated mainly during the Upper Carboniferous-Permian radiation. Comprising taxa belong almost exclusively to three subclades (Goniatitina, Prolecanitida, Ceratitida), with the middle Devonian anarcestid Beloceras [4] being the only notable exception. The solid black line tracks the mean evolutionary trajectory for all sampled taxa. Note how the average runs through unoccupied space during the Permian due to the effect of the outlying Cluster A taxa. The dotted black line tracks the trend after removal of those taxa, emphasizing the morphological stasis that persisted until the P-T extinction.

The late Pennsylvanian to early Permian morphological radiation led to a bifurcation in the distribution of taxa within morphospace (Figs. 8.1D, 8.3). To assess the significance of this division a k-means cluster analysis was performed on the original spectral data for all taxa. This revealed two statistically distinct morphological groups within the ammonoids, interestingly segregated by apparent complexity: highly multilobate forms, or those with serrated or ammonitic lobes ("complex" morphologies; Cluster A) and those with sutures containing ~7 or fewer undifferentiated lobes (true "goniatitic" morphologies; Cluster B). The two clusters are highlighted in the plot of PC1 through time (Fig. 8.3), where Cluster A is emphasized in black. A recalculated mean trend in suture morphology that disregards the statistically distinct morphologies (A) is indicated by the dashed line. This reanalysis reveals a striking pattern in the trajectory of mean suture morphology over time.

When calculated over all taxa, the average fails to accurately trace the evolution of Permian suture morphology; it falls within a region of morphospace that was unoccupied until the Triassic! After excluding the extreme morphologies, the average PC1 value drops back down to Carboniferous levels, emphasizing the conservation of Carboniferous suture geometries until the end-Permian extinction.

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