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Middle Jurassic

The Middle Jurassic Epoch represents the central subdivision of the Jurassic Period within the Mesozoic Era, spanning from 174.7 ± 0.8 to 161.5 ± 1.0 million years ago and encompassing approximately 13 million years. It is formally divided into four chronostratigraphic stages: the Aalenian (174.7 ± 0.8 to 170.9 ± 0.8 Ma), Bajocian (170.9 ± 0.8 to 168.2 ± 1.2 Ma), Bathonian (168.2 ± 1.2 to 165.3 ± 1.1 Ma), and Callovian (165.3 ± 1.1 to 161.5 ± 1.0 Ma).^[1] Geologically, the Middle Jurassic was marked by accelerated fragmentation of the supercontinent Pangaea, including the initiation of seafloor spreading in the central Atlantic Ocean and the early formation of the Gulf of Mexico, which contributed to rising sea levels and widespread marine transgressions.^[2] These tectonic processes drove the development of retroarc foreland basins in western North America and extensive rifting in the Tethys region, resulting in diverse sedimentary deposits such as limestones, shales, and sandstones that record both terrestrial and shallow marine environments.^[3] Volcanic activity and orogenic events, such as the Siskiyou Orogeny along the western margin of North America, further shaped continental margins and influenced global sediment distribution.^[4] The paleoclimate of the Middle Jurassic was characterized by greenhouse conditions with elevated atmospheric CO₂ levels (estimated at 1,000–2,000 ppm), fostering warm, humid equatorial zones and extending subtropical climates poleward into mid-latitudes, though punctuated by oscillations including brief arid phases and a notable warming event in the Bathonian.^[5] Zonal belts ranged from arid low-latitude deserts to boreotropical forests and cool-temperate high-latitude regions, with evidence from paleosols indicating mean annual temperatures of 15–25°C and precipitation varying from 500–1,500 mm/year in continental interiors.^[6] Paleobiologically, the Middle Jurassic witnessed the radiation of major dinosaur clades, including primitive sauropods (e.g., Omeisaurus and Mamenchisaurus in Asia), early thyreophorans like stegosaur precursors, and theropods such as megalosaurids (Megalosaurus in Europe), with diverse assemblages documented in formations like the Dashanpu Quarry in China yielding over 15 dinosaur genera alongside crocodylomorphs and early mammals.^[7] Marine ecosystems thrived with abundant ammonites and belemnites serving as index fossils, alongside the diversification of ichthyosaurs and the emergence of pliosaurids as apex predators in epicontinental seas.^[8] Terrestrial floras were dominated by conifers, ferns, and cycads in humid lowlands, supporting herbivorous reptiles and early birds' precursors, while global biodiversity increased amid stable oxygenation levels in oceans.^[9]

Definition and Overview

Geological Timescale Position

The Middle Jurassic Epoch constitutes the central division of the Jurassic Period within the Mesozoic Era of the Phanerozoic Eon, spanning approximately 174.7 ± 0.8 to 161.5 ± 1.0 million years ago according to the 2024 International Chronostratigraphic Chart ratified by the International Commission on Stratigraphy (ICS).^[10] This placement positions it after the Early Jurassic Epoch (201.3–174.7 Ma) and before the Late Jurassic Epoch (161.5–145.0 Ma), representing a key interval in the recovery and diversification of life following the end-Triassic mass extinction event around 201.3 Ma, which had profoundly impacted global ecosystems at the Triassic-Jurassic boundary.^[10] The base of the Middle Jurassic is formally defined at the Global Boundary Stratotype Section and Point (GSSP) for the Aalenian Stage, located at the base of Bed FZ 107 in the Fuentelsaz section of the Iberian Range, Spain, where it coincides with the lowest occurrence of the ammonite species Leioceras oppositum.^[11] This boundary, dated to approximately 174.7 ± 0.8 Ma via calibration to the broader Jurassic timescale, marks the transition from the Late Toarcian ammonite zone and is correlated globally through biostratigraphic markers such as ammonite biozonation and calcareous nannofossil datums.^[10] Radiometric constraints, including U-Pb dating of zircons from interbedded volcanic ash layers in reference sections, support this age assignment, with high-precision analyses yielding estimates around 174.7 ± 0.8 Ma for the lowermost Aalenian strata in correlated European sequences. The top of the Middle Jurassic corresponds to the base of the Oxfordian Stage, provisionally defined by the first appearance of ammonites of the Quenstedtoceras mariae Zone, with candidate GSSP sites under evaluation in sections such as those in the Subalpine Chains of France and the UK.^[12] This boundary falls within the youngest portion of magnetic polarity chron M25n, enabling precise magnetostratigraphic correlation across Tethyan and Boreal realms.^[13] Overall global synchronization of the Middle Jurassic timescale relies on integrated biostratigraphy (primarily ammonite and foraminiferal zones), chemostratigraphy (e.g., carbon isotope excursions), and cyclostratigraphy, supplemented by U-Pb geochronology from ash beds, which collectively refine the epoch's duration to about 13.2 million years.^[10]

Subdivisions and Duration

The Middle Jurassic Epoch is subdivided into four stages: the Aalenian, Bajocian, Bathonian, and Callovian. These stages collectively span a total duration of approximately 13.2 million years, from 174.7 ± 0.8 Ma to 161.5 ± 1.0 Ma, during which sedimentation rates in epicontinental seas accelerated, leading to thicker deposits in shallow marine environments across Laurasia and Gondwana.^[10]^[14] The Aalenian Stage, the earliest of the four, lasted about 3.8 million years, from 174.7 ± 0.8 Ma to 170.9 ± 0.8 Ma. Its Global Stratotype Section and Point (GSSP) is located at the base of Bed FZ 107 in the Fuentelsaz section, Iberian Range, Spain, defined by the lowest occurrence of the ammonite Leioceras oppositum. This stage is characterized by initial expansions of shallow seas over continental margins, with ammonite faunas providing primary correlation tools across Tethyan and Boreal realms.^[11] The Bajocian Stage followed, enduring roughly 2.7 million years from 170.9 ± 0.8 Ma to 168.2 ± 1.2 Ma. Its GSSP is located at the base of bed AB11 in the Murtinheira section at Cabo Mondego, Portugal, defined by the lowest occurrence of the ammonite Hyperlioceras discites in the Discites Zone. Biostratigraphy relies heavily on these ammonites, supplemented by belemnites and foraminifera, to delineate boundaries in marine sequences worldwide.^[15] The Bathonian Stage endured roughly 2.9 million years from 168.2 ± 1.2 Ma to 165.3 ± 1.1 Ma. The GSSP for this stage is situated at the base of limestone bed RB071 in the Ravin du Bès section, Bas-Auran area, southern Subalpine Chains, France, anchored by the lowest occurrence of the ammonite Gonolkites convergens in the Zigzag Zone. Biostratigraphy relies heavily on these ammonites, supplemented by belemnites and foraminifera, to delineate boundaries in marine sequences worldwide.^[16]^[17] The Callovian Stage concluded the Middle Jurassic, spanning about 3.8 million years from 165.3 ± 1.1 Ma to 161.5 ± 1.0 Ma. Although a formal GSSP has not yet been ratified, the base is provisionally defined by the first appearance of the ammonite genus Kepplerites in the K. keppleri Zone, with candidate sections including the Macrocephalites macrocephalus Zone exposures in Britain. This stage saw peak diversification of macrocephalitid ammonites, serving as critical markers for global correlation amid rising sea levels and expanded epicontinental flooding.^[18]^[14]

Geology and Stratigraphy

Major Formations and Deposits

The Middle Jurassic period is characterized by a diverse array of sedimentary deposits, reflecting varied depositional environments across the supercontinent of Pangaea as it began fragmenting into Laurasia and Gondwana. Principal rock units include limestones, marls, sandstones, evaporites, and shales, formed in settings ranging from shallow marine shelves to restricted basins and terrestrial systems. These formations document the influence of epicontinental seas and emerging rift basins along the Tethys Ocean margins, where carbonate platforms and clastic wedges accumulated in response to fluctuating sea levels.^[19] In England, the Inferior Oolite Group exemplifies oolitic limestones deposited in high-energy, shallow-water environments on a tropical carbonate shelf connected to the Tethys Ocean. Composed primarily of oolitic and shell-fragmental grainstones with minor packstones and wackestones, this group records deposition in very shallow seas above wave base, with thicknesses reaching up to 100 meters in the Cotswolds region. Lithological diversity is further illustrated by evaporite-bearing units such as the Middle Jurassic Gypsum Spring Formation in the western United States, which consists of interbedded gypsum, limestone, and mudstone formed in a restricted salina basin during marine regression. Black shales, indicative of anoxic conditions, appear in formations like the Oxford Clay in the Anglo-Paris Basin, where organic-rich mudstones accumulated in deeper, oxygen-poor shelf settings.^[20]^[21]^[22] In North America, the Sundance Formation represents marine sandstones and limestones deposited in the transgressive Sundance Sea, an epicontinental arm of the Tethys extending across the western interior of Laurasia. This unit, up to 150 meters thick in Wyoming and Montana, includes glauconitic sandstones, shales, and minor carbonates formed in shelf to nearshore environments, with lagoonal facies indicating fluctuating salinity. In Canada, the Fernie Formation (part of the Fernie Group) comprises shales and limestones in the Western Canada Sedimentary Basin, recording deposition in a subsiding foreland basin with marine to deltaic influences, where bituminous shales signal periodic anoxia. Deltaic systems are prominent in the Sichuan Basin of China, as seen in the Shaximiao Formation, a clastic unit of sandstones and mudstones deposited by fluvial-deltaic processes in a foreland lake setting, with thicknesses exceeding 1,000 meters.^[23]^[24] Globally, these deposits highlight a distribution tied to Tethys Ocean margins and intracratonic basins of Laurasia and Gondwana, where shallow epicontinental seas like the Sundance Sea facilitated widespread carbonate and siliciclastic sedimentation. In Gondwanan regions, such as the High Atlas of Morocco, fluvial sandstones in the Guettioua Formation reflect terrestrial depositional environments. These formations underscore the role of eustatic sea-level rises in expanding shallow marine realms, promoting diverse lithologies without invoking structural controls.^[19]^[25]

Tectonic and Volcanic Activity

During the Middle Jurassic, the continuation of Pangaea's breakup involved the widening of the Central Atlantic, with rifting propagating northward between Morocco and Nova Scotia. This process featured a transition from syn-rift extension to early drift, influenced by inherited Variscan structures that segmented the margin into magma-rich southern zones and magma-poor northern zones, occurring broadly between 195 and 175 Ma.^[26] Concurrently, rifting initiated in the Gulf of Mexico extended into the Middle Jurassic as part of the broader Pangea fragmentation, with active continental extension and crustal thinning persisting after Late Triassic-Early Jurassic onset, leading toward seafloor spreading by the Late Jurassic.^[27] In the Tethys Ocean, subduction along southern margins drove dynamic plate interactions, characterized by suprasubduction zone (SSZ) settings with slab rollback and mantle flow that facilitated arc splitting and back-arc basin formation. This subduction regime contributed to the emplacement of ophiolites, representing fragments of oceanic lithosphere formed in proto-arc to fore-arc environments during Jurassic-Cretaceous closure phases of the Neo-Tethys.^[28] Orogenic events marked significant compressional phases, including the early Cimmerian Orogeny in Central Asia, where late Middle Jurassic collision of the Karakoram-Lhasa block with Eurasia induced crustal shortening, folding, and uplift across northwest China and the Tibet-Pamir region around 170-155 Ma. Along South America's western margin, Andean-type subduction persisted, with Middle Jurassic magmatic arcs showing enriched sources linked to slab tears and rollback, as evidenced by andesitic to dacitic volcanism in southern central Chile.^[29]^[30] Volcanic activity included tholeiitic basalts tied to extensional tectonics in back-arc settings, though major large igneous provinces like the Ferrar were primarily active in the late Early Jurassic around 183 Ma, with possible minor extensions into the Aalenian.^[31]^[32]

Paleoenvironment

Continental Configurations

During the Middle Jurassic epoch (approximately 174–163 million years ago), the supercontinent Pangaea underwent significant fragmentation, with the northern landmass of Laurasia—consisting of North America, Greenland, Europe, and parts of Asia—progressively separating from the southern Gondwana supercontinent, which included South America, Africa, India, Antarctica, and Australia. This rifting process, initiated in the Early Jurassic, continued to widen the nascent Central Atlantic Ocean between eastern North America and northwestern Africa; by the Callovian stage (late Middle Jurassic, around 165 million years ago), the ocean basin had expanded to roughly 600–700 km in width, facilitating initial oceanic circulation while still maintaining a relatively narrow seaway connected to the Tethys Ocean.^[33]^[34] Key tectonic movements characterized this period, including the onset of continental separation between South America and Africa around 150–155 million years ago in the Late Jurassic, driven by extensional forces that would eventually form the South Atlantic Ocean. Simultaneously, the Indian craton within Gondwana began its northward drift toward the southern margin of Asia, traversing paleolatitudes from approximately 20°S in the Early-Middle Jurassic, as evidenced by paleomagnetic data from Tethyan Himalayan strata. These motions were part of broader plate kinematics associated with the dispersal of Pangaea, influenced by mantle convection and subduction along the Tethyan margins.^[35]^[36] Paleomagnetic reconstructions from this interval reveal that major landmasses of Laurasia occupied mid-to-high northern paleolatitudes, generally between 30° and 50°N, with North America positioned such that its western margins experienced variable latitudes influenced by regional rotations. In contrast, Gondwana's core regions, including parts of Africa and South America, lay at symmetric southern paleolatitudes of 30° to 50°S, creating a latitudinal spread that shaped global climate gradients and biogeographic barriers. These positions are derived from apparent polar wander paths compiled from volcanic and sedimentary rocks across both hemispheres, confirming a relatively stable configuration before accelerated Late Jurassic breakup.^[37]^[38] In North America, the expansion of epicontinental seas marked early precursors to the later Cretaceous Western Interior Seaway, with shallow marine incursions like the Bajocian-Bathonian Sundance Sea flooding portions of the western interior basin from the Arctic to the Gulf of Mexico. These inland waters, up to several hundred kilometers wide, resulted from epeirogenic subsidence and global sea-level fluctuations tied to the ongoing rifting, depositing evaporites and marine sediments across the Cordilleran foreland.^[23]^[39]

Climate and Sea Levels

The Middle Jurassic epoch was characterized by a predominantly warm and humid global climate, with mean sea surface temperatures approximately 5–10°C higher than modern averages. Oxygen isotope analyses (δ¹⁸O) of belemnite rostra, particularly when combined with clumped isotope thermometry, indicate that equatorial seas maintained temperatures of 25–30°C, reflecting a greenhouse state without polar ice caps. These proxies reveal calcification temperatures for Middle Jurassic belemnites ranging from 20.0°C to 29.5°C in mid-latitude settings, underscoring the era's elevated thermal regime compared to today's oceans. Such warmth supported expansive tropical and subtropical conditions extending toward higher latitudes.^[40]^[41] Regional climatic variations were pronounced across the supercontinent Pangaea, with arid conditions dominating the interiors of western Pangaea in low- to mid-latitudes. Evaporite deposits, such as those in the Paris Basin, signal hyperarid environments driven by the continental configuration's limited moisture transport. In contrast, the Pangaean tropics experienced monsoonal patterns, where seasonal winds facilitated heavy rainfall and humid conditions in eastern and southern margins. Clay mineral assemblages further corroborate widespread humidity, as abundant kaolinite—formed through intense chemical weathering—dominates sediments from the northern South China Plate, indicating prolonged warm and wet phases during the Early to Middle Jurassic. Sea levels during the Middle Jurassic exhibited dynamic fluctuations, with notable highstands reaching approximately 200 m above present-day levels at the Bathonian-Callovian boundary. These elevations resulted from increased mid-ocean ridge volumes, which enhanced oceanic crustal production and displaced water onto continental shelves. Transgressive-regressive cycles punctuated the period, including a significant Bajocian regression marked by a sea-level drop exceeding 75 m, followed by overall eustatic rise trends from the late Aalenian onward. Carbon isotope ratios (δ¹³C) from fossil wood provide evidence of elevated atmospheric CO₂ levels, estimated at 1000–2000 ppm, which contributed to the greenhouse forcing behind these climatic and eustatic patterns.

Biodiversity

Marine Invertebrates and Microfossils

During the Middle Jurassic, ammonites exhibited significant diversity and played a crucial role as index fossils for biostratigraphy, with over 100 ammonite zones established across the entire Jurassic period, many of which fall within the Bajocian, Bathonian, and Callovian stages.^[42] In the Bajocian, genera such as Graphoceras dominated certain assemblages, particularly in the Graphoceras concavum zone, where species like G. concavum and G. crickmayi are characteristic of early Bajocian (Aalenian-Bajocian transition) deposits in regions including eastern Oregon and Europe.^[42] By the Bathonian and Callovian, Cadoceras became prominent, with species such as C. milaschewici marking the Middle Callovian Jason Zone in areas like northwestern Turkmenistan and the Northern Caucasus, reflecting a shift toward boreal influences in ammonite faunas.^[43] Bivalves, including members of the Gryphaeidae family, formed dense blooms in shallow marine environments, contributing to shell beds that indicate high productivity in epicontinental seas.^[44] These oysters, adapted to soft substrates in low-energy settings, are represented by species persisting from the Early Jurassic into the Middle, thriving in nutrient-rich, shallow waters across Laurasian platforms.^[45] Brachiopods, particularly terebratulids, were abundant in Middle Jurassic assemblages, with genera such as Ptyctothyris, Sphriganaria, and Kutchithyris recorded in Bathonian-Callovian formations of the southern Tethys, including the Jordan Valley and Sinai.^[46] These terebratulides often dominated in stable, inner-shelf environments on carbonate ramps, exhibiting high endemism and reflecting protected, low-energy conditions rather than exclusively deeper waters.^[47] Microfossils provided essential tools for reconstructing paleobathymetry and ocean conditions, with benthic foraminifera like Lenticulina serving as key indicators of mid-shelf depths.^[48] In Bathonian-Callovian assemblages, Lenticulina species (e.g., 51 recorded in Bathonian, 24 in Callovian) showed biogeographic variations between Laurasia and Gondwanaland, influenced by eustatic sea-level changes, with higher diversity in deeper, open-marine settings during Callovian highstands.^[48] Ostracods complemented these analyses, enabling paleodepth reconstructions through assemblage clustering, as seen in Callovian deposits of the East European Platform where they delineated shifts from shallow to deeper basinal environments.^[49] Calcareous nannofossils, dominated by Watznaueria (e.g., W. britannica), underwent diversification in the Early Bajocian, boosting pelagic productivity by a factor of five and increasing nannofossil fluxes from 10⁹ to 10¹¹ per m² per year, though their carbon export remained modest at 1.1–1.4 × 10¹³ g C/yr.^[50] Reef-building communities in the Tethys Ocean featured early associations of corals and sponges, forming patch reefs in mesophotic zones (20–30 m depth) during the Bajocian.^[51] Genera such as Periseris, Isastrea, Thamnasteria, and Dendraraea constructed low-diversity but abundant frameworks on clay-rich substrates along the northern Tethys margin, signaling a global pulse of coral recovery around 30°N and 30°S latitudes.^[51] These structures supported diverse sclerobionts and highlighted ecological shifts in warm, oligotrophic waters.^[51]

Marine Vertebrates

During the Middle Jurassic, ichthyosaurs continued to thrive in marine environments, with several taxa achieving notable diversity, particularly in the Bajocian stage. Species such as Stenopterygius aaleniensis and related forms reached lengths of 2–3 meters and exemplified the group's viviparous reproduction, where embryos were found preserved in adult specimens, indicating live birth as a key adaptation for fully aquatic life.^[52] This period marked a transition toward ophthalmosaurid dominance, with Bajocian records including Mollesaurus periallus and Chacaicosaurus cayi, contributing to a peak in ichthyosaurian morphological and ecological variety before a later decline. In 2025, a new ichthyosaur species was described from Middle Jurassic deposits in Germany, further illustrating the group's diversity during this epoch.^[53]^[52] Plesiosaurs, particularly early members of the cryptoclidid clade, emerged prominently in Middle Jurassic seas, adapting long necks for efficient fish predation. Forms like the oldest-known cryptoclidid from the early Bajocian (Humphriesianum Zone) in Luxembourg displayed elongated cervical vertebrae suited for ambush hunting in open waters.^[54] Although transitional plesiosauroids such as Microcleidus are better documented from the late Early Jurassic, Bathonian deposits reveal similar long-necked morphologies in cryptoclidids, with necks comprising over half the body length to facilitate rapid strikes on schooling fish.^[54] These adaptations underscored the clade's radiation into diverse pelagic niches during the Bathonian. Fish assemblages in Middle Jurassic oceans showed the initial rise of modern teleost groups alongside persistent archaic forms. Crown-group teleosts began diversifying in this interval, with otolith and statolith records indicating increased abundance and morphological innovation starting in the Bajocian, laying the foundation for their later dominance.^[55] Pycnodonts, specialized durophagous teleosts with robust, crushing dentition, underwent high origination rates, exemplified by genera like Gyrodus that targeted shelled prey in shallow marine settings.^[56] Hybodont sharks remained the dominant chondrichthyans, with diverse species such as Hybodus and Planohybodus inhabiting coastal and lagoonal habitats, their varied tooth morphologies reflecting adaptations to piscivory and durophagy.^[57] Marine crocodylomorphs, primarily teleosaurids, occupied brackish and coastal lagoons, preying on fish and smaller reptiles. Steneosaurus, a widespread genus reaching up to 4 meters in length, featured elongate snouts with conical teeth ideal for grasping slippery prey in shallow, restricted environments like those of the Bathonian and Callovian stages.^[58] These semiaquatic predators, including species like S. edwardsi, thrived in lagoonal ecosystems across Europe, their body plans optimized for ambush hunting in low-salinity waters.^[59]

Terrestrial Vertebrates

During the Middle Jurassic epoch, terrestrial vertebrate faunas on the supercontinents of Laurasia and Gondwana were dominated by early radiations of dinosaurs, alongside pterosaurs adapted to continental environments. These animals inhabited a range of habitats, including riverine floodplains, forested lowlands, and semi-arid basins, where they interacted with evolving plant communities such as conifers and ferns that served as primary food sources.^[60] Fossils from formations like the Shaximiao in China and the Cañadón Asfalto in Argentina reveal a transition toward greater body sizes and ecological specialization among these groups.^[61] Sauropod dinosaurs, particularly the mamenchisaurids, were prominent herbivores in Asian continental interiors during this period. Mamenchisaurus, known from the Middle Jurassic Shaximiao Formation in Sichuan Province, China, exemplifies this group with its exceptionally elongated neck, which in some species reached lengths of up to 9 meters, allowing access to high vegetation in forested environments. These sauropods belonged to a clade of derived eusauropods that shared features like fused dorsal vertebrae, indicating adaptations for supporting massive bodies in stable, resource-rich habitats.^[60] Precursors to later diplodocids, such as early diplodocoids, began appearing in Middle Jurassic assemblages, showing nascent traits like elongated tails and necks that foreshadowed the gigantism of Late Jurassic forms like Diplodocus, though they remained less specialized.^[62] Theropod dinosaurs, the carnivorous and omnivorous relatives of birds, exhibited diversity in predatory roles across Middle Jurassic landmasses. Megalosauroids, a basal group of large theropods, were widespread in Laurasia, with Poekilopleuron bucklandii from the Bathonian-stage deposits of Normandy, France, representing a key example; this species reached an estimated length of 7 meters and featured robust limbs suited for hunting large prey in coastal plain ecosystems.^[63] Classified within Spinosauroidea, Poekilopleuron shared synapomorphies like elongated premaxillae with other early megalosauroids, highlighting a faunal turnover toward more specialized predators.^[64] Meanwhile, early coelophysoids, small agile theropods dominant in the Early Jurassic, showed signs of decline by the Middle Jurassic, with their global diversity waning as megalosauroids and other tetanurans rose to prominence in terrestrial niches.^[65] Ornithischian dinosaurs, the armored and beaked herbivores, began diversifying in the Middle Jurassic, particularly in southern continents. Stegosaurids emerged as a notable group, with Huayangosaurus taibaii from the Callovian Lower Shaximiao Formation in Sichuan, China, marking one of the earliest definitive members; this basal stegosaur measured about 4.5 meters long and possessed primitive features like a broader skull and smaller plates compared to later forms, adapted for browsing in subtropical woodlands.^[61] In Gondwana, heterodontosaurids—small, bipedal herbivores with specialized dentition for grinding plants—were represented by Manidens condorensis from the Middle Jurassic Cañadón Asfalto Formation in Patagonia, Argentina, which had a body length of 60–75 cm and featured a jugal boss, indicating close relations to African lineages and a Gondwanan distribution.^[66] Pterosaurs, flying reptiles that inhabited continental margins and interiors, were primarily non-pterodactyloid forms during the Middle Jurassic. Rhamphorhynchids, characterized by long tails and toothed jaws, dominated these assemblages, with genera like Angustinaripterus from the Middle Jurassic Dashanpu Formation in Sichuan, China, exemplifying the group; these pterosaurs had wingspans reaching up to 1.5 meters, enabling aerial foraging over floodplains and lakes in warm, humid climates.^[67] Their lightweight skeletons and elongated fourth finger supported membranous wings suited to the period's diverse terrestrial landscapes.^[67]

Invertebrates and Early Mammals

During the Middle Jurassic, terrestrial and freshwater ecosystems supported a modest diversity of invertebrates, reflecting ongoing recovery from the end-Triassic extinction. Insects underwent early diversification, with notable records of Diptera and Coleoptera. Crane flies of the family Tipulidae, such as species from the Daohugou Beds (Callovian stage), exhibit elongated bodies and wing venation adapted for flight in humid forest environments, indicating an expansion of nematoceran flies during this period.^[68] Similarly, beetles (Coleoptera) showed increasing variety, including cleroid lineages from the Jiulongshan Formation (Bathonian-Callovian boundary), where specimens preserve elytra and thoracic structures suggestive of predatory or detritivorous habits in riparian zones.^[69] Well-preserved compression fossils from these lagerstätten, akin to amber-like fidelity in volcanic ash deposits, reveal details of antennal morphology and leg segmentation, highlighting the role of stable, vegetated landscapes in insect evolution.^[70] Other terrestrial invertebrates included scorpions and millipedes, which occupied detrital and soil niches in floodplain and woodland settings. Scorpions, represented by isolated pedipalps and metasoma from Middle Jurassic deposits in Europe and Asia, displayed pectinal combs for sensory detection in leaf litter, consistent with arachnid adaptations persisting from the Paleozoic.^[71] Millipedes, though less commonly preserved, are evidenced by fragmentary body rings from lacustrine shales, suggesting myriapod scavenging in decaying plant matter amid rising angiosperm precursors. In freshwater systems, unionid-like bivalves (mussels) thrived in riverine and lake environments, as seen in small-shelled specimens from the Jurassic Atacama Desert lakes (Chile), where thin, equivalved shells (5-12 mm) indicate filter-feeding in low-energy, vegetated waterways with periodic ashfall influences.^[72] These mussels formed dense shell beds, underscoring their ecological importance in nutrient cycling within braided river systems.^[73] Early Mammaliaformes emerged as rare but ecologically specialized components of Middle Jurassic faunas, comprising less than 1% of vertebrate diversity amid post-Triassic recovery. Docodonts from Middle Jurassic deposits, such as those from the Bathonian of Europe, featured robust molars for crushing tough invertebrates, with postcranial skeletons indicating semi-fossorial habits in humid undergrowth.^[74] Eutriconodonts, including triconodontid forms from the Forest Marble Formation (Bathonian) in Britain, spread globally across Laurasia and Gondwana, evidenced by multicusped teeth suited for shearing prey and isolated dentaries suggesting agile, cursorial locomotion in forested margins.^[75] These mammals adapted primarily to nocturnal insectivory, leveraging enhanced olfaction and audition—traits inferred from endocranial casts—to exploit night-active arthropods, thereby carving niches in dinosaur-dominated landscapes without direct competition.^[76] This adaptive radiation, peaking in the mid-Jurassic, marked a shift toward dietary and locomotor diversification, setting the stage for later Mesozoic expansions.^[77]

Plant Life

During the Middle Jurassic, terrestrial ecosystems were dominated by gymnosperms, which formed the backbone of vegetation across various paleolatitudes, with ferns and horsetails thriving in more humid, lowland settings. Bennettitales, such as Williamsonia, were prominent in understory layers, featuring complex flower-like reproductive structures on stocky, cycad-like stems up to 2 meters tall, as evidenced by fossils from formations like the Oxford Clay in the UK and Rajmahal Hills in India.^[78]^[79] Cycads, including anatomically preserved seeds like Oxfordiana motturii from the Callovian-Oxfordian Oxford Clay (~163.5 Ma), contributed to these understories with their pinnate leaves and multi-layered integuments, indicating a stem-group lineage akin to modern Cycas.^[79] In higher elevations or drier uplands, conifers of the Araucariaceae family, such as Araucaria mirabilis, formed dense forests, achieving peak diversity during this period with global distribution from Laurasia to Gondwana.^[80] Ferns and horsetails were abundant in wetland environments, particularly in coal-forming swamps of the Bathonian stage, where fronds of Cladophlebis species, such as C. australis, preserved in French and Australian Bathonian coals, indicate riparian or floodplain communities tolerant of periodic flooding.^[81] Horsetails (Equisetales) co-occurred in these settings, as seen in the Ordos Basin flora of China, comprising up to 10% of assemblages alongside ferns in moist, low-lying areas.^[82] No true angiosperms existed, but gnetophyte-like fossils, such as the mid-Jurassic Daohugoucladus sinensis from China's Daohugou Formation (~165 Ma), featured opposite phyllotaxy, linear leaves, and pedicled chlamydosperms—primitive reproductive traits that foreshadowed angiosperm evolution through transitional flower-like organs.^[83] In Laurasia, ecosystems centered on conifer-dominated forests with Araucariaceae and other gymnosperms in humid to temperate zones, while Gondwana's interiors supported seasonal floras adapted to arid conditions, with charcoal layers and pollen records from wood genera like Agathoxylon (over 50% of assemblages) indicating fire-prone, winter-wet or warm-temperate communities less diverse than their Laurasian counterparts.^[84] These distributions reflect latitudinal gradients, with pollen spectra showing dominance of conifer and bennettitalean spores in coastal lowlands.^[84]

Economic and Scientific Significance

Resource Deposits

The Middle Jurassic epoch is associated with several economically significant resource deposits, particularly hydrocarbons, coal, and certain metallic ores, formed in diverse sedimentary environments ranging from marine basins to continental settings. Hydrocarbon accumulations are prominent in the North Sea region, where Middle Jurassic strata serve both as reservoirs and, to a lesser extent, source rocks. The Brent Group, comprising deltaic and shallow marine sandstones deposited during the Bathonian to Callovian, forms the primary reservoir for numerous oil and gas fields in the UK and Norwegian sectors. These sandstones host a substantial portion of the North Sea's recoverable reserves, with the Brent Delta system a major contributor to the North Sea's oil and gas production, hosting over 50 fields through examples like Statfjord and Brent. Coaly Middle Jurassic source rocks, such as the Bryne Formation in the Danish Central Graben, exhibit high organic content (up to 70% TOC in coal seams) and have generated petroleum that migrates into overlying reservoirs, supporting plays in the broader North Sea petroleum system. Globally, Middle Jurassic strata contribute meaningfully to Jurassic-sourced hydrocarbons, though exact proportions vary by basin; in the North Sea alone, they underpin a mature province that has yielded tens of billions of barrels of oil equivalent. In North America, Middle Jurassic strata in the Sverdrup Basin host significant natural gas reserves.^[3] Coal deposits from the Middle Jurassic are well-documented in continental basins of eastern Asia, where humid paleoclimates favored peat accumulation in fluvial-deltaic systems. In China's Ordos Basin, the Yan'an Formation (Bathonian in age) contains multiple minable coal seams, with thicknesses up to 10 meters and total resources estimated in the billions of tons. These seams, interbedded with sandstones and shales, formed in swampy lowland environments and are actively mined for thermal coal, supporting regional energy needs. Similar Bathonian coal measures occur in the Qaidam Basin's Dameigou Formation, where organic-rich layers indicate periodic wildfire activity and high preservation potential, contributing to China's significant Jurassic coal endowment. In Gondwana, coal measures in the Early to Middle Jurassic Karoo Supergroup of South Africa represent important regional resources.^[5] Metallic resources from Middle Jurassic deposits include iron ores and localized bauxites, often linked to specific depositional and weathering processes. Oolitic ironstones, rich in goethite and chamosite, accumulated in shallow marine settings within the Cleveland Basin of northeast England and the adjacent Paris Basin in France, where Bathonian-Aalenian limestones host economically viable beds with iron contents exceeding 30%. These ores, historically mined in the Cleveland Ironstone Formation's upper extensions and the Minette oolitic deposits of Lorraine, provided raw materials for 19th-20th century steel production.

Key Discoveries and Research

One of the most iconic fossil sites for Middle Jurassic vertebrates is the Solnhofen Limestone in southern Germany, renowned for preserving exceptionally delicate structures, including early evidence of pterosaur embryos that provide insights into reproductive biology and early ontogeny of these flying reptiles.^[85] Although primarily Late Jurassic, its lagerstätte conditions echo those in Middle Jurassic deposits and have informed interpretations of contemporaneous pterosaur evolution. Complementing this, the Bighorn Basin in northern Wyoming, USA, hosts rare Middle Jurassic (Bajocian-Bathonian) dinosaur megatracksites in the Gypsum Spring and Sundance Formations, featuring thousands of bipedal theropod tracks and potential early sauropod impressions that reveal migratory behaviors and paleoenvironments along ancient coastlines approximately 170-167 million years ago.^[39] In the 2020s, advanced imaging techniques have revolutionized the study of Middle Jurassic soft tissues, exemplified by correlative neutron and X-ray tomography applied to a 165-million-year-old ammonite (Sigaloceras enodatum) from the Kellaways Formation in Gloucestershire, UK, which unveiled unprecedented 3D details of internal muscles, including a hyponome for jet-propelled swimming distinct from modern nautiloids. This non-destructive method has since been adapted to other Middle Jurassic invertebrates, enhancing understanding of cephalopod locomotion and anatomy. Similarly, recent analyses of docodont teeth from Middle Jurassic deposits, such as those in Siberia and the UK, have employed high-resolution micro-CT and phylogenetic modeling to elucidate dental complexity and ecological diversification among early mammaliaforms, with a 2023 study refining the evolutionary timeline of docodontan occlusion through comparative morphology of specimens like Itatodon tatarinovi.^[86] Despite these advances, significant research gaps persist, particularly in Gondwanan faunas, where Middle Jurassic vertebrate records remain sparse compared to Laurasian sites, limiting global biogeographic reconstructions; for instance, new discoveries in India's Jaisalmer Basin in 2025 highlight untapped potential in southern continents, but systematic surveys are needed to address biases toward northern hemisphere exposures.^[87] Climate modeling efforts integrating δ¹³C excursions, such as the prominent positive carbon isotope event in the middle Callovian (Calloviense/Jason Zones), suggest enhanced organic burial and subsequent cooling without major glaciation, yet models struggle with sparse Southern Hemisphere data and require better calibration to polar amplification effects observed in belemnite and bivalve records from England.^[88] Methodological progress in geochronology has sharpened Middle Jurassic stage boundaries through refined U-Pb dating of zircon and tuff layers, achieving resolutions better than 0.1 million years; for example, CA-TIMS (chemical abrasion-thermal ionization mass spectrometry) applied to eastern Australian nonmarine strata has recalibrated the Bajocian-Bathonian transition to ~168.1 Ma, enabling precise correlations with marine sequences and volcanic events.^[89] These improvements, combined with integrated stratigraphic frameworks, have reduced uncertainties in Jurassic timescales by up to 50% since the early 2000s, facilitating accurate placement of fossil events.

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