A remarkably aerially extensive (∼2,000 km2) unit of carbonate microbialites occurs in many Triassic–Jurassic boundary interval outcrops of the southwestern United Kingdom and captures petrographic evidence that could link them to the end-Triassic extinction event. The bioherms—known regionally as the Cotham Marble—occur as discrete ∼20-cm-thick, decimeter- to meter-scale mounds, and display at least five growth phases that alternate between laminated and dendritic mesofabrics. Cross sections parallel to bedding through the dendritic phases expose a reticulate dendritic framework separated by polygonal spaces (∼1–3 cm diameter), characteristic of “tubestone” microbialites. Microscopically, the dendrolites contain evenly distributed rod to filamentous putative microfossils (∼2 µm diameter and ∼10 µm in length) in a matrix of micrite and contain higher total organic carbon than the surrounding matrix. Round to ellipsoidal spar-filled regions (∼200 µm in diameter) within the dendrolites (previously interpreted as serpulid worm tubes) likely resulted from the production of gas bubbles within rapidly lithifying mats or are a two-dimensional artifact of evenly spaced three-dimensional branching within the mats. The fill between the dendrolites of the first layer contains abundant phycoma clusters of the green algal prasinophyte Tasmanites, commonly considered a “disaster taxon.” The cyclic phases represent episodic and laterally extensive environmental change within shallow water coastal environments during a marine transgression. Collectively, the presence of microbial micrite in a shallow marine setting, the marked lateral extent of the bioherms, and the abundance of Tasmanites suggest the Cotham Marble microbialites formed during the high pCO2 and relatively warmer conditions associated with the events of the end-Triassic mass extinction.