This list of the Mesozoic life of Montana contains the various prehistoric life-forms whose fossilized remains have been reported from within the US state of Montana and are between 252.17 and 66 million years of age.
A
†Achelousaurus – type locality for genus
†Achelousaurus horneri – type locality for species
†Acheroraptor – type locality for genus
†Acheroraptor temertyorum – type locality for species
Acipenser
†Acipenser eruciferus
†Acristavus – type locality for genus
†Acristavus gagslarsoni – type locality for species
†Acrostichopteris
†Acrostichopteris longipennis
†Adiantites
†Adiantites montanensis
†Adocus
†Albanerpeton
†Albanerpeton nexuosus
†Albertaceratops
†Albertaceratops nesmoi
†Albertatherium
†Albertatherium primus
†Albertosaurus
†Aletridelphys
†Aletridelphys clemensi – type locality for species
†Aletridelphys florencae
†Aletridelphys hatcheri
†Allocrioceras
†Allocrioceras annulatum
†Allosaurus
†Alostera
†Alostera saskatchewanensis
†Alphadon
†Alphadon attaragos
†Alphadon halleyi – type locality for species
†Alphadon marshi
†Alphadon wilsoni
†Altacreodus
†Altacreodus magnus
†Alzadasaurus – tentative report
†Alzadites – type locality for genus
†Alzadites alzadensis – type locality for species
†Ambilestes
†Ambilestes cerberoides – or unidentified comparable form
Amia
†Amia fragosa
†Amia uintaensis
†Amphicoelias
†Anatotitan
†Anatotitan copei
†Anchura – tentative report
†Ankylosaurus – type locality for genus
†Ankylosaurus magniventris – type locality for species
†Anomozamites
†Aquiladelphis
†Aquiladelphis incus
†Aquiladelphis minor
†Aquilapollenites
†Aquilapollenites collaris
†Aquilapollenites conatus
†Aquilapollenites marmarthensis
†Aquilapollenites mtchedlishvilii
†Aquilapollenites quadrilobus
†Aquilapollenites turbidus
†Aquilops – type locality for genus
†Aquilops americanus – type locality for species
Aralia – or unidentified comparable form
†Aralia wellingtonia
†Araliaephyllum
†Araliaephyllum westonii
†Araliopsoides
†Araliopsoides cretacea – or unidentified comparable form
†Araucarites
†Araucarites longifolia
†Arcellites
†Archaeolamna
†Archaeolamna kopingensis
†Archaeotriakis – type locality for genus
†Archaeotriakis rochelleae – type locality for species
†Arcticoceras – tentative report
†Arctocephalites
†Arctocephalites gracilis – or unidentified comparable form
†Arctocephalites maculatus – or unidentified comparable form
†Arctocephalites platynotus – or unidentified comparable form
†Arctocephalites sawtoothensis
†Arctocephalites saypoensis
†Argaliatherium – type locality for genus
†Argaliatherium robustum – type locality for species
†Ariadnaesporites
†Ariadnaesporites fluviatilis – type locality for species
†Ascarinites – type locality for genus
†Ascarinites communis – type locality for species
†Aspideretoides
†Aspidophyllum – or unidentified comparable form
†Aspidophyllum trilobatum
†Asplenium
Astarte
†Astarte meeki
†Astarte morion
†Astarte packardi – or unidentified comparable form
†Athrotaxites
†Athrotaxites berryi – or unidentified comparable form