PMC7596322_01
Completeness of the early tetrapod record. (A) Geographic distribution of tetrapod fossil localities across the late Palaeozoic. (B) Number of tetrapod-bearing localities recorded in the Paleobiology Database (http://paleobioDB.org) by stage, showing heterogeneity in regional and temporal sampling.
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In particular, the early tetrapod record is itself relatively heterogeneous in both space and time (Figure 4). At the time, this represented a single continental landmass restricted to within 10 degrees of the equator (Figure 4A). A few localities exist outside of this narrow equatorial band in the Devonian, Carboniferous, and Permian, but taxonomic diversity and worker effort remain far lower in these regions than in Euramerica, particularly in Gondwana, which had not fully joined the Pangaean supercontinent until the late Carboniferous to early Permian. A robust record outside of Euramerica does not appear until the latter part of the Middle Permian (Figures 4A,B), at which point concurrent well-sampled records appear in the Karoo Basin of South Africa, the Parana Basin of southern Brazil, and a series of basins across Russia and China. Furthermore, the early tetrapod record is punctuated by several key intervals of minimal sampling of the tetrapod record (Figure 4B). Further intervals are also substantially undersampled in addition to these historical "gaps." This extends throughout the Lower Carboniferous (Figure 4B), where sampling effort is not only very poor but is extremely geographically restricted (Figure 4A).