Fossilized microorganisms (microfossils) preserved within Precambrian sedimentary rocks represent a vital source of evidence for early life on Earth, and studies of such microfossils and the organic matter (kerogen) of which they are typically composed have helped guide the modern search for past life on Mars. Investigations into the morphology, molecular structure, and stable isotopic composition of such cellular kerogenous microfossils have revealed significant insights into the preservation of these ancient biosignatures, including the reconstruction of early microbial metabolisms (e.g., carbon-fixation pathways). However, significant questions remain unresolved, such as the potential consistency of carbon isotope values (d13Corg) preserved between similar morphological fossils from different Precambrian units, the effects of increasing thermal alteration on the d13Corg of fossil kerogen, and the expected observations for detections of putative kerogen biosignatures on Mars by the Perseverance rover. To address these issues, the research presented here utilizes a combination of optical microscopy, Raman spectroscopy using visible and deep-UV laser wavelengths, and secondary ion mass spectrometry (SIMS) in an effort to characterize the morphological, molecular-structural, and carbon isotopic composition of numerous Precambrian microfossils and associated particulate (detrital) kerogen from a collection of shallow-marine cherts ranging in thermal maturity from unmetamorphosed to greenschist facies, and spanning more than ~1 billion years of Earth history (~400 Ma to ~2,100 Ma). Raman geothermometry calculations were performed on measured Raman spectra of kerogenous microfossils using a relative numerical metric, the Raman Index of Preservation, with values ranging values from ~9.0 to ~1.0, and correlated temperature estimates from ~200–400 �C. SIMS carbon isotope measurements of individual microfossils yielded d13Corg values ranging from approximately –33‰ to –17‰ between the multiple geologic units investigated, and exhibit a similar trend with higher (i.e., less negative) d13Corg values associated with the more thermally altered units, though relatively low values were also observed at high temperatures (>350 �C), suggesting that carbon isotopic biosignatures may be potentially retained within cherts that have experienced greenschist facies metamorphism. In addition, deep-UV Raman spectra of such Precambrian microfossils have revealed notable Raman resonance features reflecting changes in the thermal alteration of biogenic kerogen, which may help guide the detection of well-preserved organic matter on the surface of Mars.