The key word is favorable: collagen preservation depends strongly on temperature, pH, moisture, and microbial activity.

Cold, dry, neutral pH conditions preserve collagen well; warm, wet, acidic conditions degrade it rapidly.

California island sediments are not an ideal collagen-preservation environment — the temperatures are moderate and the sediments are well-watered — but the specific conditions at Arlington Springs apparently allowed some collagen to survive.

Osteocalcin is a different protein — a smaller molecule, specific to bone and dentin, involved in the mineral metabolism of living bone.

Because of its small size and specific chemistry, osteocalcin tends to be more susceptible to degradation than collagen.