For long stretches of the Paleozoic Era, roughly 340 to 270 million years ago, the Tucson area sat near the equator under shallow seas. Layer after layer of lime mud and shell debris piled up on those sea floors and hardened into the thick gray limestones you can now trace along the flanks of the Catalinas, Rincons, and the ranges just outside Tucson.

The cast of characters preserved in those Tucson-area limestones is consistent and, once you learn it, easy to spot:

• Crinoids — "sea lilies," animals on stalks built from stacked disc segments. Weathered-out stem pieces look exactly like stone Cheerios, and they are far and away the most common fossil around Tucson.
• Brachiopods — ribbed, clam-like shells, often preserved whole in the rock face.
• Horn corals — solitary corals shaped like curved ice cream cones, showing internal ribbing in cross-section.
• Bryozoans — colonial animals leaving delicate fan and mesh patterns in the stone.

Much later, in the Cretaceous, rivers and floodplains crossed southern Arizona, and a few formations from that world have produced rare dinosaur material. Rare is the operative word — the everyday fossils around Tucson are marine, and they're wonderful.

• Vertebrate fossils on federal land are permit-only, always. Bones, teeth, tracks — if it had a backbone, it stays put unless you hold a research permit. Federal law is explicit about this.
• Common invertebrate and plant fossils — crinoids, brachiopods, corals — may be casually collected in reasonable personal-use quantities on most BLM and national forest land around Tucson, without heavy tools, and never for sale.
• Saguaro National Park and other monuments: nothing leaves, fossil or otherwise. Camera only.
• Arizona state trust land requires a permit for access, and collecting is not part of casual recreation there.
• Private land: the landowner's fossils, the landowner's rules. Permission first, always.

The rule that keeps you right everywhere near Tucson: backbone? Leave it and report it. A vertebrate find in place is scientifically precious precisely because of where it sits — photograph it, mark the location, and contact the land manager or the University of Arizona. That's how finds get named after the finder instead of confiscated from them.

• Alfie Norville Gem & Mineral Museum (downtown Tucson) — the University of Arizona's world-class collection in the historic courthouse, minerals up front and earth history throughout. The best rainy-day geology in Tucson.
• Arizona-Sonora Desert Museum (west Tucson) — famous for living desert, but its earth sciences section and cave exhibit quietly teach the regional geology story very well, and kids love it.
• University of Arizona campus — beyond the gem museum, the campus is the research heart of Arizona geology and paleontology, and a good place to point a fossil question.
• The Tucson gem show, every February — for two weeks, effectively a museum you can buy from, right in town.

Tucson Fossils is written and maintained by Joel Bennett, a Tucson-based geologist who thinks the phrase "stone Cheerios" has recruited more young Tucson geologists than any textbook. This site exists to answer the trailside question — "wait, is that a fossil?" — and the follow-up nobody expects: "can I keep it?"

It's part of a network of Tucson field guide sites, linked in the sidebar.