Dinosaur extinction can clarify continental facies shifts on the Cretaceous-Paleogene boundary

New KPB sections immediate a brand new paradigm

It is outstanding that, throughout ~30° latitude (~37°–65° N), ~15° longitude (~104°–117° W) (Fig. 1), and various paleoenvironmental settings^12,22,23, continental KPBs within the Western Interior could be reliably found by concentrating on apparent facies shifts. To check the prevalence of that phenomenon, we sampled for iridium anomalies at formational contacts that had been believable KPB candidates based mostly on paleontological proof (see ‘Methods’), but captured distinct paleolandscape settings²³. These included: Lance-Fort Union formational contacts within the Bighorn Basin, Wyoming (Supplementary Fig. 1; Supplementary Fig. 2), a area bounded by Laramide mountain belts, and the Hell Creek-Fort Union formational contact at Mill Iron, close to Ekalaka, Montana (Supplementary Fig. 3), a area removed from elevated topography (Fig. 1). We found elevated iridium anomalies, which we attribute to the Chicxulub asteroid influence and thus the KPB¹, in any respect 5 of the sampled formational contacts (Supplementary Table 2). These signify the primary KPBs recognized within the Bighorn Basin and the southernmost KPB within the Williston Basin (Fig. 1). Together, they reinforce the sample that continental KPBs invariably coincide with a facies shift throughout disparate paleoenvironments (see Supplementary Discussion).

These observations immediate us to problem the prevailing notion that KPB facies shifts are impartial of the EKME. Instead, we suggest a brand new speculation: dinosaur megafauna had been ecosystem engineers through the Late Cretaceous, and their sudden extinction induced an abrupt shift in continental facies on the KPB that persevered into the early Paleocene (the ‘Dinosaurs as Ecosystem Engineers’ speculation). We display additional assist for this speculation under, focusing particularly on the Williston Basin because it hosts many of the identified continental KPB sections, has very good three-dimensional outcrop exposures that the majority different KPB sections lack, and has featured prominently in debates in regards to the EKME^12,19,22.

KPB facies shifts mirror adjustments in fluvial programs, not widespread ponding

Continental facies shifts on the KPB most clearly mirror two features of fluvial programs: (i) river-meander-belt measurement and stability and (ii) the quantity of clastic sediment (clay, silt, sand) deposited on the distal floodplain. Uppermost Cretaceous meander belts are characteristically slim and unstable, delivering a gradual provide of clastics to the distal floodplain; decrease Paleogene meander belts are characteristically huge and secure, ravenous the distal floodplains of clastics and allowing the buildup of concentrated natural particles. Those variations between pre- and post-KPB fluvial programs are epitomized by the Hell Creek/Lance and Fort Union formations, respectively, of the Williston (Hell Creek) and Powder River (Lance) basins (Fig. 2). The Lance/Hell Creek and Fort Union formations are all meandering-river deposits, evidenced by the preponderance of fine-grained clastics, erosive and asymmetrical channel bases, abandoned-channel deposits (oxbow lakes), and lateral-accretion beds^{15,16,17,18,19} (Fig. 2; Supplementary Fig. 4). The Hell Creek and Lance formations are characterised by (i) predominantly small (~1–3-m thick, ~5–10-m huge), single-storied fluvial-sandstone beds (small, short-lived river channels or crevasse splays), with giant (~5–10-m thick, ~0.1–0.5-km huge), multistoried fluvial sandstone beds being uncommon, interbedded with (ii) large, green-gray mudstone beds that exhibit root traces, slickensides, gleying, and grey–yellow mottling (immature, water-logged soils; i.e., hydromorphic paleosols)^{3,15,16,18,19,20}. The decrease Fort Union Formation is characterised by (i) giant (~8–30-m thick, 0.1–1-km huge), multistoried fluvial sandstone beds (giant, amalgamating, proximal river channels) and ‘variegated beds’ (related level bars; see under) and (iii) coals (swamp-dominated, organic-rich distal floodplains)^3,16,19,24.

Continental facies typical of the uppermost Cretaceous and lowermost Paleogene, epitomized by the Hell Creek Formation (A–D) and Tullock Member of the Fort Union Formation (E–J) within the Hell Creek area of northeastern Montana (Fig. 1). The higher (A), center (B), and decrease (C) parts of the Hell Creek Formation are all dominated by fine-grained clastic sediments, interpreted as hydromorphic paleosols, and small fluvial channels or crevasse splays (which typically scour into these paleosols, as in D). The Tullock Member of the Fort Union Formation is characterised by laterally persistent coals, ‘variegated beds’, and multistoried proximal fluvial-channel deposits (E). We interpret the ‘variegated beds’—packages of thinly interbedded coarse-fine clastic sediments—as lateral-accretion or point-bar deposits related to giant meandering channels as a result of they (i) dip at a 5–25° angle and coarsen down-dip and advantageous upwards (F), (ii) exhibit paleoflow indicators which might be perpendicular to the dip path (G), and incessantly rework and cross-cut one another (H). The KPB claystone mattress is incessantly overlain by a coal after which a sequence of these cross-cutting lateral-accretion beds (I, J). Triangles in B and F point out fining-upwards sequences; notice the distinction between these within the Hell Creek versus Fort Union. Arrows in E and F spotlight depositional dip instructions. Carb. carbonaceous, LA lateral-accretion set, MS multistoried proximal fluvial-channel deposit.

The sudden, geographically widespread look of coals is essentially the most conspicuous characteristic of decrease Paleogene facies within the Western Interior and defines the bottom of the Fort Union Formation^3,12,16,24. Even within the absence of coals, nevertheless, the inorganic sedimentology of the Fort Union Formation is instantly distinguished from the underlying Hell Creek or Lance formations, largely because of the ‘variegated beds’—brightly coloured, thinly interbedded claystone, siltstone, and fine-grained sandstone beds which might be typically under- and overlain by coals and had been thus possible deposited laterally and coevally to coal deposits elsewhere^16,17. These ‘variegated beds’ have traditionally been interpreted as pond or lake deposits^16,18,19, and Fort Union deposition has thus been attributed to numerous mechanisms to extend ‘wetness’ within the early Paleocene, leading to principally ponded–marshy accumulation of clastic and natural sediments^{12,15,16,17,18,19,20}. We collectively time period these proposed mechanisms, which embrace elevated precipitation^16,17,20 and/or an increase in water desk^16,18,19,20, the ‘Wet Paleocene’ speculation.

New observations from our work within the Hell Creek area of northeastern Montana, at the side of earlier analysis^24,25,26, solid doubt on the ‘Wet Paleocene’ speculation: (i) Hell Creek Formation paleosols denote the prior existence of a really excessive water desk, and (ii) most ‘variegated beds’ of the Fort Union Formation usually are not pond deposits however relatively lateral-accretion deposits related to giant, broadly meandering rivers²⁷ (Fig. 2; Supplementary Fig. 4). There is substantial assist for the primary level^19,20,24—most overbank deposits within the Hell Creek Formation signify ponds or hydromorphic paleosols. The second level, to our information, has not been totally appreciated. In the handfuls of outcrops studied throughout the jap Montana portion of the Williston Basin, we discovered that the ‘variegated beds’ encompass alternating fine- and coarse-grained sediments that dip at ~5–25°, coarsen down-dip, incessantly grade into proximal channel-sandstone deposits, and exhibit paleoflow indicators (e.g., ripple marks) which might be perpendicular to dip path (Fig. 2)^24,26. Further, though paleoflow instructions stay roughly east–northeast throughout the KPB in our research space (Hell Creek imply = 109°, n = 166; Fort Union imply = 63.5°, n = 310), the usual deviations from these imply paleoflow instructions are considerably larger within the Fort Union Formation (SD = 69.3°) in comparison with the Hell Creek Formation (SD = 52.9°), suggestive of a extra pronounced meandering system within the early Paleocene. Thus, as an alternative of representing pond or lake deposits, the ‘variegated beds’ signify point-bar deposits (and probably counterpoint bar deposits, see Supplementary Discussion) related to the lateral migration of fluvial-channel meanders²⁷. This interpretation is in step with the out there paleoclimatic knowledge, which signifies neither an abrupt nor persistent change in precipitation depth throughout the KPB²³ (see Supplementary Discussion).

Abiotic drivers are inadequate to clarify KPB facies shifts

Transgression of the Cannonball Sea (Fig. 1) is mostly invoked to clarify the ‘Wet Paleocene’ interpretation of KPB facies change within the Williston Basin^12,15,18,19 (the ‘Marine Transgression’ speculation), largely based mostly on cases of minor (cm–m scale) stratigraphic offset between purported KPBs and the Hell Creek-Fort Union formational contact (i.e., the facies change is time-transgressive, not synchronous with the KPB). However, most of these KPBs are recognized utilizing non-impact-related instruments or proxies like palynology or carbon-isotope stratigraphy¹², that are delicate to remodeling²⁸ and lithology²⁵ and thus insufficiently exact to position the KPB at ≤ 2 m scales (this has been famous beforehand^19,22). Palynology continues to be an vital instrument for constraining the approximate stratigraphic place of the KPB on the outcrop scale, and pollen is usually the perfect instrument out there in sure places^12,19, regardless of its propensity for remobilization by fluvial motion^19,22,28 (see Supplementary Discussion for a fuller dialogue in regards to the deserves and shortcomings of utilizing pollen or carbon isotopes to position the KPB).

Although some impact-identified KPBs are positioned just a few cm under, inside, or atop a coal (Fig. 1; Supplementary Table 2), these putative examples of time-transgressive KPB facies change assume that coals are the only sedimentological indicator of that change¹². Yet, a preponderance of coals in decrease Paleocene strata is merely one symptom of the systematic shift within the nature of fluvial programs on the KPB (Fig. 2). Indeed, coal-bearing models do happen within the Late Cretaceous^21,29,30, together with sometimes within the Hell Creek Formation^3,14; that some KPBs could be preserved in Cretaceous coal swamps that persevered into the early Paleocene ought to due to this fact be anticipated (Supplementary Fig. 5; see Supplementary Discussion).

Further, magnetostratigraphy signifies that Cannonball Sea transgression started no sooner than chron C29n and occurred most considerably throughout chron C28r^31,32, thus postdating the KPB (in the course of chron C29r) by ca. 300–900 ka¹⁴ (Supplementary Fig. 5). Marine transgression must also lead to high-accommodation options within the continental report, similar to hydromorphic paleosols, low channel:floodplain ratios, and single-story channels^29,30; a sample that extra carefully matches the Hell Creek Formation (which does interfinger with marine deposits to the east) than the Fort Union Formation^15,19 (Fig. 2; Supplementary Fig. 5). Lastly, the ‘Marine Transgression’ speculation fails to clarify why facies adjustments analogous to these within the Williston Basin coincide exactly with the KPB in geographically distant areas just like the Alberta and Powder River basins^12,17,22, relatively than monitoring the advance of Cannonball Sea¹⁵ (Supplementary Fig. 5), or why facies adjustments happen on the KPB in areas just like the Bighorn Basin that had been separated from the remainder of the Western Interior by Laramide arches (Fig. 1). Thus, the preponderance of proof casts doubt on the ‘Marine Transgression’ speculation; the KPB and decrease Fort Union extra possible coincided with a marine regression or lowstand, each domestically³¹ and globally³³.

The Laramide Orogeny has additionally been proposed to clarify KPB facies shifts²¹, however that course of was spatially diachronous and insufficiently abrupt to generate the noticed stratigraphic patterns throughout such a big geographic vary (Fig. 1)—Laramide deformation superior from the southwest to the northeast of the Western Interior over the course of the Late Cretaceous by means of early Paleogene, and there’s no proof of a continent-wide pulse of basement-cored block uplifts coincident with the KPB³⁴. Landscape denudation through Chicxulub-associated wildfires, in distinction, could possibly be each abrupt and widespread^19,35, however such a mechanism can not clarify the persistence of these facies adjustments all through the decrease Fort Union Formation for a temporal period exceeding 1 million years^14,24,26. Thus, no earlier hypotheses adequately clarify the noticed facies shifts throughout the KPB (see Supplementary Discussion).

Dinosaur megafauna as ecosystem engineers

We suggest that the sedimentological and stratigraphic options that outline Upper Cretaceous versus decrease Paleogene continental strata are the results of ecosystem engineering by dinosaur megafauna within the Cretaceous and their disappearance within the Paleogene (Fig. 3). The ‘Dinosaurs as Ecosystem Engineers’ speculation higher explains the synchroneity, type, and persistence of continental facies change and is linked to a causal mechanism that’s well-documented in Quaternary terrestrial ecosystems^7,8,9,10,11. Mammalian megafauna profoundly have an effect on landscapes and vegetation^7,8, however dinosaurian megafauna had been many instances bigger, and the spatial extent of ecosystem engineering scales positively with physique measurement³⁶. Even smaller herbivorous dinosaurs possible reached physique lots of ~1000 kg, and a few (e.g., Triceratops horridus) could have approached 15,000 kg³⁷, so their influence on terrestrial vegetation construction and fluvial dynamics was possible profound.

Large-bodied dinosaurs possible promoted an open vegetation construction that permitted frequent fluvial avulsion and clastic sediment enter to distal floodplains. After the end-Cretaceous mass extinction, dense, closed-canopy forests may take root, stabilizing meander belts and ravenous the distal floodplain of clastic sediment, selling the buildup of organic-rich strata.

Not solely had been people of those species large, however taphonomic research point out most ceratopsians and hadrosaurs had been herd animals³⁸. These herds, together with rarer large-bodied predators (e.g., Tyrannosaurus rex, ~8000 kg³⁷), possible maintained open or patchily forested habitats through the Late Cretaceous through trampling and uprooting³⁹. Consequently, fluvial programs would flood and avulse incessantly⁴⁰, leading to: (i) short-lived meander belts and (ii) a frequent provide of clastic sediment to the distal floodplain, in step with the (i) small, single-storied fluvial channels, skinny crevasse-splay sandstone beds, and (ii) hydromorphic paleosols that characterize Upper Cretaceous strata. When these megafauna went extinct, so too did their affect on vegetation cowl, permitting dense forests to take root, leading to secure and long-lived river-meander belts that will have: (i) allowed extended transforming of channel meanders and (ii) starved the distal floodplain of clastics thus selling natural accumulation, in step with the (i) giant, multistoried sandstone channels with considerable adjoining lateral-accretion bedding and (ii) laterally steady coals that characterize decrease Paleocene strata (Fig. 3).

Whether or not non-avian dinosaurs had been in decline previous to the EKME stays debated^41,42. Nevertheless, within the Western Interior, large-bodied dinosaurs, particularly ceratopsians, persevered as much as the KPB and their disappearance through the EKME was geologically abrupt⁴³. Indeed, in almost the entire KPB localities mentioned right here, dinosaurian megafauna are well-known from under the boundary and disappear on the KPB. Further, proposals of dinosaur decline particularly seek advice from taxonomic richness, not relative abundance—the previous is germane to evolutionary interpretations, however the latter is most related to ecological impacts, and there’s no proof that large-bodied dinosaurs had been numerically depauperate previous to the EKME. Lastly, though mammals underwent adaptive radiations within the Late Cretaceous²³ and elevated in physique measurement rapidly within the earliest Paleocene⁴⁴, mammals remained comparatively small for a lot of the early Paleogene⁴⁵; as such, mammals didn’t possible considerably influence the construction of standing vegetation till the Eocene or Oligocene⁷.

Importantly, the quick penalties of the Chicxulub asteroid influence on standing vegetation cowl had been comparatively short-lived and solely evident on the cm scale within the rock report^12,22. Although plant taxonomic restoration from the EKME possible took 1000’s to hundreds of thousands of years^44,46,47, decrease taxonomic richness doesn’t preclude forest density or cover construction³⁹. Indeed, the nice and cozy, monsoonal climates of the earliest Paleocene of the Western Interior²³ would have allowed the institution of dense forest vegetation in a matter of many years to centuries post-KPB⁴⁴. The ‘Dinosaurs as Ecosystem Engineers’ speculation can due to this fact clarify the tempo, type, and persistence of the continental KPB facies shift and is in step with the vertebrate^3,4,5,19,26 and plant^{12,22,39,44,46,47} fossil information (see Supplementary Discussion).

Regional variation within the KPB facies shift

The facies shifts described above mirror landscape-scale adjustments in fluvial geomorphology which might be possible synchronous with the KPB^18,19; but, KPB-coincident facies adjustments usually are not an identical in mountain-proximal versus mountain-distal parts of the Western Interior (Fig. 4). Those within the Williston Basin are the perfect studied, they usually broadly match the KPB facies shifts noticed in different mountain-distal basins, such because the Powder River, Alberta, and Brackett basins^12,17 (Fig. 4; however see Supplementary Discussion). The KPB within the Willow Creek Formation of the Alberta Foothills was deposited off the flank of the Sevier thrust belt (Fig. 1) and thus displays a singular facies change reflective of domestically well-drained, semiarid situations⁴⁸. KPBs within the Raton Formation of the Raton Basin (Fig. 1) mark the contact between the underlying ‘lower coal zone’ and overlying ‘barren series’ and superjacent ‘upper coal zone’^21,49; the previous is characterised by single-storied channels and hydromorphic paleosols, whereas the ‘barren series’ is dominated by multistoried, laterally aggraded, channel-sandstone beds^21,49. KPB facies shifts within the Bighorn Basin carefully match these within the Raton Basin (Fig. 4; see Supplementary Discussion), differing primarily in an absence of coals within the underlying Lance Formation. KPBs from the Denver Basin could exhibit a facies change like these within the Raton and Bighorn basins; nevertheless, the one impact-identified Denver Basin KPBs come from a drill core and a poorly uncovered outcrop within the central and jap basin⁴⁴, respectively, precluding exact evaluation of the KPB facies change presently (however see Supplementary Discussion). We contend that these similarities between the Raton, Bighorn, and probably Denver basins mirror their proximity to rising Laramide mountain belts (Figs. 1 and  4).

Diagram illustrating the abrupt continental facies adjustments that happen at impact-identified KPBs (pink line) throughout the Western Interior (see Fig. 1), with approximate latitude given beneath every space to emphasise the geographic extent of these facies adjustments (notice that these are trendy latitudes, and diagrams are illustrative, to not exact stratigraphic scale). Question marks on completely different facies symbols signify uncertainties or ambiguities about these facies; the query mark on the KPB line within the Denver Basin represents uncertainty in regards to the exact location of the KPB within the mountain-proximal parts of that basin. Note that these are generalized schematics that don’t seize all sedimentological or stratigraphic particulars of a given space—for a extra detailed dialogue of those completely different localities, see the Supplementary Discussion. AB Alberta, CAN Canada, CR Castle River space of southern Alberta, CV Coal Valley space of western Alberta, Dist. mountain-distal parts of the Denver Basin, ID’d recognized, lat. lateral, MS multistoried, Prox. mountain-proximal parts of Denver Basin, RDV Red Deer Valley Alberta, SS single-storied, UCZ ‘upper coal zone’, USA United States of America.

That continental KPB facies shifts in mountain-proximal settings usually are not an identical to these in mountain-distal settings is anticipated provided that (i) the consequences of vegetation and ecosystem engineering on fluvial geomorphology^10,50, and (ii) the drivers of river avulsion⁵¹, each of that are central to the ‘Dinosaurs as Ecosystem Engineers’ speculation, differ markedly in mountain-distal versus mountain-proximal settings (Fig. 3). Those variations may clarify the abrupt look of laterally expansive and vertically thick proximal channel deposits overlying the KPB in locations just like the Raton^21,49 and Bighorn basins (see Supplementary Discussion).

Implications for the evolution of landscapes and biota throughout the KPB

The ‘Dinosaurs as Ecosystem Engineers’ speculation explains each the geographically widespread coincidence between the KPB and continental facies shifts and gives a mechanism for these adjustments. Nonetheless, extra empirical knowledge are wanted to robustly consider its legitimacy; specifically, extra quantitative assessments of fluvial geomorphology and plant-canopy construction throughout the KPB all through the Western Interior. Our speculation is falsifiable and testable in quite a few methods, similar to: (i) Continental, impact-identified KPBs ought to at all times be carefully related to a sedimentary facies change; (ii) Forest cover construction, which is usually inferred through light-sensitive proxies⁵², must be comparatively open within the newest Cretaceous and comparatively closed within the earliest Paleocene³⁹; and (iii) Fluvial deposits overlying the KPB ought to mirror extra broadly meandering and temporally long-lived rivers relative to these under the KPB, at the least in mountain-distal settings.

If true, the ‘Dinosaurs as Ecosystem Engineers’ speculation has vital implications for the evolution of terrestrial ecosystems. For instance, early flowering crops had been weedy opportunists that thrived in incessantly disturbed environments⁵³, and avulsion-prone fluvial programs of the Cretaceous maintained by dinosaur megafauna could have promoted their early ecological proliferation. The closing of forest canopies predicted by the ‘Dinosaurs as Ecosystem Engineers’ speculation can be in step with noticed will increase in angiosperm-seed measurement after the KPB, attributed to the emergence of dense forests following the extinction of dinosaur megafauna^54,55,56. Lastly, most Mesozoic mammals had been possible terrestrial or semifossorial⁵⁷, matching expectations for an open relatively than densely forested habitat⁵⁸. Widespread emergence of dense forest cowl within the Paleocene could have due to this fact prompted the post-KPB rise of arboreal and frugivorous mammals⁵⁸, similar to primates.

In sum, this work highlights the necessity for continued analysis on the Cretaceous–Paleogene transition in continental settings. It is more and more obvious that abiotic drivers like local weather change and tectonics have formed the distribution and evolution of life²³; but, as demonstrated right here, life itself can even sculpt our planet. By bearing these relationships in thoughts, we could discover that Earth’s local weather, landscapes, and biota have been tightly entwined all through geologic historical past.