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A three‐dimensional tubular fabric known as “vermiform microstructure” in Phanerozoic and Neoproterozoic carbonate microbialites has been hypothesized to represent the body fossil of nonspicular keratose demosponges. If correct, this interpretation extends the sponge body fossil record and origin of animals to ~890 Ma. However, the veracity of the keratose sponge interpretation for vermiform microstructure remains in question, and the origin of the tubular fabric is enigmatic. Here we compare exceptionally well‐preserved microbialite textures from the Upper Triassic to channel networks created by modern microbial biofilms. We demonstrate that anastomosing channel networks of similar size and geometries are produced by microbial biofilms in the absence of sponges, suggesting the origin for vermiform microstructure in ancient carbonates is not unique to sponges and perhaps best interpreted conservatively as likely microbial in origin. We present a taphonomic model of early biofilm lithification in seawater with anomalously high carbonate saturation necessary to preserve delicate microbial textures. This work has implications for the understanding of three‐dimensional biofilm architecture that goes beyond the current micro‐scale observations available from living biofilm experiments and suggests that biofilm channel networks have an extensive fossil record.

It is increasingly important to document past records of hydrologic change in areas that are drought‐prone to better predict the region's future vulnerability to recharge and water supply. Holocene spring‐associated carbonate deposits serve as terrestrial records of water balance that can complement other local, high‐resolution proxies that are moisture‐sensitive. Here we examine two carbonate deposits (one inactive perched tufa site and one active fluvial tufa site) that form from ambient‐temperature freshwater springs, as proxies of their depositional conditions. Radiocarbon (¹⁴C) analyses of charcoal fragments from the inactive perched tufa record depositional ages of 6.2 ± 0.06 (2σ) cal kabpand 8.0 ± 0.04 (2σ) cal kabpand agree with the age models from other proxies of past pluvial periods in the region (~16 to 5 ka). The active fluvial tufas date to 853 ± 0.4 calbp,representing conditions similar to modern flow. Geomorphologic and radiocarbon results indicate the perched tufa reflects wetter conditions fed by a higher water table. Stable isotopic analyses of carbonate (δ¹³C, δ¹⁸O) reveal distinct isotopic values between modern and early–mid‐Holocene tufa. This work underscores potential for the analysis of other moisture‐sensitive tufa deposits in coastal central California.