The fossil record is inherently biased, but few studies have examined the effects of these biases on our understanding of non-marine vertebrate ectotherms through time. Here, I investigate the taxon richness of non-marine lepidosaurs and turtles from the Triassic-Paleogene (252-23 Ma) using subsampling methods and completeness metrics to correct for/highlight sampling and preservation biases. Additionally, I used generalized least-squares regressions to study the relationships between taxon richness and other potential explanatory variables (e.g. temperature, sampling proxies). The lepidosaur record is more poorly sampled than the turtle record, but for both clades the Southern Hemisphere is clearly undersampled compared to the Northern Hemisphere. Consequently, the majority of sequential richness signal is observable in Northern Hemisphere continents only. Lepidosaur and turtle richness patterns differed prior to the Eocene, but were very similar afterwards. Across the K-Pg boundary, North American lepidosaurs decreased in richness while turtles increased. Both clades exhibited high richness in the Late Cretaceous and in the early and late Eocene, often corresponding to times of very warm global climates. During these times richness was greater at higher latitudes than at lower latitudes, which is the opposite of the modern latitudinal biodiversity gradient. Richness decreased at the Eocene-Oligocene boundary in the Northern Hemisphere, coincident with a steep drop in global temperatures, but remained relatively high compared to the Mesozoic. It is unclear whether taxa with lower cold tolerance migrated south at these times or went extinct, due to poor sampling in the tropics. Ectotherm taxon richness through time is strongly influenced by sampling biases, including the number of collections available and the prevalence of Lagerstätten (localities of exceptional preservation). Temperature might have influenced lepidosaur richness, but other factors not tested for might have also been important for ectotherm richness, such as precipitation (particularly for freshwater turtles).