The Curiosity rover has run across some intriguing small objects on Mars. One astrobiologist has observed that they look very like the trace fossils from the Ordovician Era he has studied. The rover had gone past the site, but was brought back to it again, when the peculiar objects were noticed in the images secured there. These mineralized worm-like traces will be studied further using the Chemcam and Alpha Particle and X ray spectrometers on the rover. Given what we believe we know about the conditions on Mars in the remote past, worm-like fossils do not seem out of the question. Please find a link below to an article with further information, and images of the possible fossils.

https://www.space.com/39294-mars-rov...tructures.html