The Moncada Fm. is represented by a fining-upward sedimentary silicate and calcareous clastic sedimentary sequence, composed of mixtures of different concentrations of lithoclasts and ejecta particles. Previous petrographic observations had revealed the occurrence of abundant shocked quartz grains and whitish vesicular fragments throughout the Moncada Fm. Vesicular fragments preserve the quench texture of clinopyroxene, suggesting their impact melt origin (Tada et al., 2003). Under the optical microscope, the groundmass between the melt fragments is heterogeneous. In Figure 5, various petrographic features are shown at several scales and described at length in captions.

The basal part of the Moncada Fm. (lower part of U1 of Tada et al., 2002) is constituted by ejecta material composed of fine gravel to coarse sand-sized particles. Its composition is mainly represented by melt rock fragments and smectite spherules, surrounded by a matrix of smectite fragments, as well as quartz grains and clay minerals. Clinopyroxene quench texture is common in some smectite-altered vesicular fragments (Fig. 5A, B). Some of the melt grains are 5.5mm in diameter and are composed of smectite-altered glass and inclusions of quartz schist (Fig. 5C). Altered smectite or calcite spherules are 1, 2 and up to 3mm in diameter.

The lower part of the Moncada Fm. (upper part of U1) corresponds to sand-sized ejecta material and is composed of melt rock clasts (Fig. 5D), smectite spherules and smectite shards (Fig. 5E), quartz grains, and scarce micritic limestone fragments. The diameter of the grains fluctuates from 0.6 to 0.9mm, intermediate sizes being more abundant. The matrix is mainly composed of sericite and smectite as the alteration product of glass fragments. Due to subsequent low-grade metamorphic processes muscovite and sericite flakes are abundant in this sample. Bubbles in smectite spherule are filled with spatic calcite. Some quartz grains that seem to be igneous or metamorphic in origin have parallel or undulant extinction, but other grains show impact effects such as Planar Deformation Features (PDFs) and/or features of roasting (Fig. 5F).

FIGURE 4
Lower Danian planktonic foraminiferal species identified in the UpperMost Unit of the Moncada Fm. (UMU) and the Ancón Fm. (Scale bar = 100μm).

A-B) Guembelitria cretacea Cushman, 1933, from the H. holmdelensis subzone in UMU-1 (A) and Pv. sabina subzone (B). C) Guembelitria blowi (Arz, Arenillas and Náñez, 2007), from the E. simplicissima subzone. D-E) Parvularugoglobigerina longiapertura (Blow, 1979), from the Pv. sabina subzone (D). and Pv. longiapertura subzone (E). F) Palaeoglobigerina fodina (Blow, 1979), from the Pv. sabina subzone. G-H) Parvularugoglobigerina sabina (Luterbacher and Premoli Silva, 1964), from the Pv. sabina subzone. I-J) Parvularugoglobigerina eugubina (Luterbacher and Premoli Silva, 1964), from the Pv. sabina subzone. K) Woodringina claytonensis Loeblich and Tappan, 1957, from the Pv. sabina subzone. L) Woodringina hornerstownensis Olsson, 1960, from the Pv. Sabina subzone. M) Eoglobigerina simplicissima (Blow, 1979), from the S. triloculinoides subzone. N) Globanomalina archeocompressa (Blow, 1979), from the E. trivialis subzone. O) Parasubbotina pseudobulloides (Plummer, 1927), from the E. trivialis subzone. P) Praemurica taurica (morozova, 1961), from the E. trivialis subzone. Q) Praemurica inconstans (SuBBotina, 1953), from the E. trivialis subzone. R) Subbotina triloculinoides (Plummer, 1928), from the S. triloculinoides subzone. S) Eoglobigerina trivialis (SuBBotina, 1953), sensu Blow (1979), from the S. triloculinoides subzone.

TABLE 1
Relative abundance of planktonic foraminifera species and genera of the lowermost Danian at Moncada

FIGURE 5
Petrographic images of polished thin sections from the main units of Moncada section.

A) Altered-devitrified glass fragment with quench texture (photomicrograph is 600µm across, plane-polarized light), basal part of the Moncada Fm. (lower Unit 1). B) Detail of A, illustrating the quench texture of the altered glass fragment (photomicrograph is 160µm across, plane-polarized light), basal part of the Moncada Fm. (lower Unit 1). C) Schist clast included in melt rock fragment (photomicrograph is 2.4mm across, crossed-polarized light), basal part of the Moncada Fm. (lower Unit 1). D) Flow-textured ejecta with quartz, sericite and muscovite (photomicrograph is 600µm across, crossed-polarized light, lower part of the Moncada Fm. (upper Unit 1). E) Smectite shards and calcite filled bubble from a broken spherule (photomicrograph is 600µm across, plane-polarized light), lower part of the Moncada Fm. (upper Unit 1). F) Shocked quartz grain with at least two Planar Deformation Features (PDFs) (photomicrograph is 160µm across, crossed-polarized light), lower part of the Moncada Fm. (upper Unit 1). G) Broken smectite altered spherule with radiating pyroxene crystals (photomicrograph is 600µm across, plane-polarized light), middle part of the Moncada Fm. (Unit 3). H) Shocked quartz grain showing at least two set of PDFs (photomicrograph is 155µm across, plane-polarized light), upper part of the Moncada Fm. (Unit 4). I) Lime mudstone showing burrowing-like structures and dark clay lamination (photomicrograph is 2.4mm across, crossed-polarized light), dark clayey mudstone bed or “K/T boundary clay” (UMU-1, H. holmdelensis subzone). J) Contact between medium-sand sized ejecta below and clayed micrite above (photomicrograph is 600µm across, crossed-polarized light), contact between uppermost part of the Moncada Fm. (Unit 5) and UMU1. K) Lime mudstone showing numerous clay lamination (photomicrograph is 600µm across, plane-polarized light), dark clayey mudstone bed or “K/T boundary clay” (UMU-1). L) Shocked quartz grain. Detail from reworked shocked quartz grain in clayey micritic limestone to show PDFs, (photomicrograph is 160µm across, plane-polarized light), basal part of Ancón Fm. (Pv. longiapertura subzone).

The middle part of the Moncada Fm. (U2) corresponds to medium sand-sized ejecta material with a flow-like banded texture. It is composed of melt rock fragments, calcite and smectite spherules and shards (Fig. 5G), and quartz, bound together by a matrix consisting of calcite, sericite, smectite, and minor amounts of chlorite, produced by the alteration of glass. Like other samples from the lower part, the ejecta grain size varies from 0.1 to 1mm.

The upper part of the Moncada Fm. (U4) also corresponds to fine to medium sand-sized (0.1 to 0.8mm) ejecta with no orientation of the grains. In equal proportions, the matrix is composed of recrystallized micrite and smectite spherule fragments and shards. The alteration minerals are mainly smectite, calcite and minor chlorite. The matrix encloses altered melt rock, glass, shards, quartz and minor amounts of pyroxene and biotite fragments. The quartz (0.1 to 0.2mm) from the upper part preserves planar microstructures (Fig. 5H).

The 2cm thick clayey lime mudstone in the uppermost part of the Moncada Fm. (UMU-1) is characterized by the presence of several very thin clay laminations and some burrow structures (Fig. 5I). The contact between the upper part of the ejecta-rich CCU and the overlying clayey UMU-1 is marked by a tiny, 30-50µm thick layer of dark clay. The ejecta material below this dark clay (Fig. 5J), which corresponds to the CCU (uppermost part of U5), is constituted by melt rock fragments, broken calcite/ smectite spherules, quartz grains and dispersed smectite and calcite crystals. The 1cm thick UMU-2 is characterized by medium to fine sand-sized ejecta (0.1 to 0.7mm) with a roughly flow-like texture. Its matrix is composed of smectite resulting from the alteration of glass fragments and shards, and abundant calcite. This smectite-calcite matrix encloses altered melt rock, glass, shards and quartz fragments.

Tada et al. (2002) found a high Ir concentration not only in UMU-1, but also in the 1cm thick yellowish marly limestone at the base of the Ancón Fm. The Ir concentration remains relatively high in the first 5 centimeters of the Ancón Fm., i.e., throughout the entire G. cretacea zone. This stratigraphic interval corresponds to lime mudstone with several quartz grains, microfossils and widespread micron-sized dark clay laminations (Fig. 5K). The majority of the quartz grains lack impact features, but others show planar microstructures probably related to shock metamorphism (Fig. 5L).

To help with the micropaleontological identification and dating of the CCU in the Gulf of Mexico and the Caribbean, Arenillas et al. (2011) described several planktonic foraminiferal markers of the K/Pg boundary based on high-resolution biostratigraphic studies of the most continuous Tethyan sections such as El Kef (see Arenillas et al., 2004; Molina et al., 2006, 2009). i) Identification of the P. hantkeninoides zone in the uppermost part of Cretaceous deposits, since this is the highest biozone of the Maastrichtian in tropical-subtropical latitudes, with an estimated duration of ~300kyr, and its top coincides with the K/Pg boundary mass extinction of planktonic foraminifera. ii) Identification of reworked specimens of P. hantkeninoides and/or other end-Cretaceous species (e.g., Abathomphalus mayaroensis, Pseudoguembelina hariaensis, or Racemiguembelina fructicosa) within the CCU, corroborating the hypothesis that eroded uppermost Maastrichtian sediments accumulated in the local CCU. iii) Identification of zone P0 or H. holmdelensis subzone, since this biozone spans approximately the first 6kyr of the Danian, and its identification above the CCU confirms that the latter is related to the K/Pg boundary. iv) Identification of PFAS 1, since this is the first planktonic foraminiferal acme-stage recorded in the Danian above the K/Pg boundary, with an estimated duration of 10 to 12kyr, and it is a particularly interesting K/Pg marker because it minimizes potential taxonomic problems in the species assignments.

Biostratigraphic markers 1, 3 and 4 allow direct identification of the K/Pg boundary, while marker 2 allows only indirect identification, because it is based on reworked specimens. The identification of markers 1, 3 and 4 does not necessarily imply the absence of stratigraphic hiatuses at the top of the Cretaceous and/or at the base of the Paleogene. However, their identification implies that such hiatuses, if there are any, affect only small intervals of time on a geological scale.

In the upper part of the Cretaceous deposits of the Pons Fm., only Favusella washitensis, Heterohelix moremani? and Hedbergella sigali have been identified in the disaggregated samples analyzed (Fig. 2). According to Caron (1985), the presence of these species suggests that the top of the Pons Fm. at Moncada is Albian in age, but it is not possible to determine to which biozone it belongs. It suggests a significant erosional hiatus between the Pons Fm. and the Moncada Fm. affecting all the Upper Cretaceous rocks. Similar erosional unconformity related to the deposition of the CCU has been identified recently in several wells from the north-central Gulf of Mexico (Scott et al., 2014), making it nearly impossible to use marker 1 in this region. The amount of time represented by these hiatuses in different sections is variable, but on average it increases in magnitude with proximity to the impact site, i.e., the Chicxulub crater (Arenillas et al., 2011).

FIGURE 6
Relative abundance of Planktonic Foraminiferal genera and Acme Stages (PFAS) in the lowermost Danian of the Moncada section.

*Guembelitria cretacea zone, **Hedbergella holmdelensis subzone, ***Parvularugoglobigerina longiapertura subzone.

The clastic nature and sedimentary structures of the Moncada Fm. indicate that it only contain reworked and mixed foraminiferal assemblages, so this stratigraphical interval cannot be assigned to any biozone and is included in a barren interzone (Fig. 2). Nevertheless, reworked specimens have allowed us to analyze the age of the material eroded, transported and redeposited in the Moncada Fm. In this formation, we have only been able to recognize poorly preserved planktonic foraminiferal specimens of Pseudotextularia nuttalli, Racemiguembellina fructicosa, Gansserina gansseri, Globotruncanita fareedi and Globotruncana arca. In particular, the presence of R. fructicosa suggests that part of the carbonate material deposited in the Moncada Fm. is Maastrichtian in age (marker 2).

At Moncada, the six lowermost Danian subzones of the Arenillas et al. (2004) zonation were identified above the CCU, i.e., in the UMU and the Ancón Fm. (Figs. 2; 3; 6). This complete planktonic foraminiferal biostratigraphic sequence as well as the identification of the H. holmdelensis subzone or zone P0 (marker 3) corroborate that the underlying CCU, i.e., the Moncada Fm., is K/Pg in age. The H. holmdelensis subzone contains oligotaxic planktonic foraminiferal assemblages composed only of Guembelitria species. We highlight the lack of other species deriving from the Cretaceous, such as Heterohelix globulosa, Hedbergella holmdelensis and H. monmouthensis, generally considered survivors of the K/Pg mass extinction and usually present in zone P0 worldwide (see Olsson et al., 1999; Molina et al., 2006, 2009; and references quoted there). Except for Guembelitria species, there is also no Cretaceous species in the Pv. longiapertura subzone.

The sequence of PFAS of Arenillas et al. (2006) is also recognizable in the lower Danian of the Moncada section (Fig. 6). PFAS-1 or the Guembelitria acme includes the UMU and the first 1.5 centimeters of the Ancón Fm. Reworking and infiltration processes due to the bioturbation in the Ancón Fm., mainly in its basal part, may have altered the relative abundances of the planktonic foraminifera taxa, but the PFAS sequence also confirms the stratigraphic continuity of the lower Danian between the Moncada and Ancón formations. Although minor stratigraphic hiatuses may exist between the different units (CCU, UMU-1, UMU-2 and Ancón Fm.), no significant hiatus is biostratigraphically recognizable in the lower Danian of the Moncada outcrop.

The Global boundary Stratotype Section and Point (GSSP) of the base of the Danian, or the K/Pg boundary, was defined at the base of the bed informally known as the “K/T Boundary Clay” in the El Kef section, Tunisia, specifically at the base of a 2-5mm thick Ir-rich layer that contains a peak of Ni-rich spinels and coincides with the catastrophic mass extinction of planktonic foraminifera (Molina et al., 2006). This definition implies that the impact-generated sediments in the K/Pg boundary belong chronostratigraphically to the Danian (Arz et al., 2004; Molina et al., 2009; Schulte et al., 2010). The “K/T Boundary Clay” in continuous sections of the Tethyan region is composed of two beds (Arenillas et al., 2011). i) A millimeter-thick ejecta-rich airfall layer, usually reddish, with high concentrations of Ir (the wellknown Ir anomaly); Arenillas et al. (2006) showed that the Chicxulub-related CCU in the Gulf of Mexico and Caribbean sections is synchronous with this airfall layer and the K/Pg catastrophic mass extinction horizon in the Tethyan sections. ii) A dark clay bed with low values in δ¹³C, spanning the H. holmdelensis subzone (i.e., zone P0) and the lowermost part of the Pv. longiapertura subzone (i.e., lowermost part of zone Pα).

According to this description, we can draw two relevant conclusions at Moncada. i) Mineralogical and petrographic evidence indicates that U1 to U5 of the Moncada Fm. are isochronous with the K/Pg millimeter-thick ejecta-rich airfall layer, and the K/Pg boundary should be placed at the base of this formation. Since the deposition of the Moncada Fm. is linked to the Chicxulub impact event, the latter is also K/Pg in age. ii) Micropaleontological evidence indicates that UMU-1 represents the “K/T Boundary Clay” at Moncada, specifically the dark clay bed. The planktonic foraminiferal assemblages are composed exclusively of Guembelitria species, therefore belonging to zone P0 and the acme-stage PFAS-1 (markers 3 and 4 respectively). This clayey subunit is truncated by the sandy UMU-2, which is a thin turbiditic layer whose deposition is temporarily disconnected from that of the CCU.

It is important to notice that these biostratigraphic results are similar to those reported from the Bochil and Guayal sections, southwestern Mexico, where the “K/T Boundary Clay” is 5 and 3 centimeters thick respectively (Arenillas et al., 2006). As at Moncada, the lowermost Danian zone P0 or the H. holmdelensis subzone and PFAS-1 were also identified in a dark clay bed just above the CCU, proving that such a bed is chronostratigraphically equivalent to the “K/T Boundary Clay” of the El Kef section (Fig. 7). In the southwestern Mexican sections, the CCU exhibits four subunits (Grajales-Nishimura et al., 2003). i) Subunit 1 consists of very coarse-grained carbonate breccia, with clasts composed predominantly of facies with shallowwater taxa (e.g., rudist fragments); the breccia matrix contains reworked Maastrichtian foraminifera specimens (such as Pseudoguembelina hariaensis). ii) Subunit 2 consists of coarse-grained sandstone with shocked quartz and spherules. iii) Subunit 3 consists of fine-grained sandstone and siltstone with shocked materials, spherules and accretionary lapilli. iv) Subunit 4 consists of a thin yellow-red shaly layer and shows a distinctive Ir anomaly, 1.5ppb at Bochil and 0.8ppb at Guayal at the top of their CCU, and it represents the finest ejecta (Montanari et al., 1994; Tagle and Claeys, 2001).

Despite being relatively near the Chicxulub impact site, the Moncada section, unlike Bochil and Guayal sections, contains no breccia subunit (Subunit 1) in the lower part of its CCU. This absence suggests that the landslides and probably the action of megatsunamis triggered by the Chicxulub impact prevented the deposition of breccia in the uppermost slope of the eastern Yucatan peninsula (see Fig. 1), where Moncada was located (Tada et al., 2003). The uppermost subunits (2 to 4) of the CCU in the southwestern Mexican sections are equivalent to those found in the Moncada Fm. (Fig. 7). All this evidence supports the theory that the Chicxulub impact occurred just at the K/Pg boundary, and triggered the catastrophic mass extinction event as well as severe short and longterm paleoenvironmental effects (Schulte et al., 2010). This conclusion agrees with Renne et al. (2013), who have precisely determined the age of the Chicxulub impact by performing ⁴⁰Ar/³⁹Ar dating on Haitian tektites and on sanidine from bentonites clearly associated with the K/Pg boundary in Montana, USA. These ⁴⁰Ar/³⁹Ar data show the synchrony between the K/Pg boundary extinction event and the Chicxulub impact to within 32Kyr, countering the hypothesis that the latter predated the K/Pg boundary by ~300kyr.

FIGURE 7
Comparison of relative abundance of Planktonic Foraminiferal Groups and Acme Stages (PFAS) in the lowermost part of the Danian at El Kef, Moncada and Bochil.

The high Ir concentration throughout the UMU and the first centimeters of the Ancón Fm. at Moncada, i.e., across the G. cretacea zone, seems to contradict the event sequence identified in the Tethyan sections, where the Ir peak is mainly concentrated in the millimeter-thick airfall layer equivalent to the CCU in the Gulf of Mexico and the Caribbean region. However, it is well known that iridium is easily mobilized by diffusion through the sediments (Colodner et al., 1992; Lee et al., 2003; Morgan et al., 2006), which could partly explain its high concentration in the lowermost Danian of Moncada. Bioturbation processes, recognizable in the UMU and the basal part of the Ancón Fm., could have also contributed to its dispersion by reworking across the G. cretacea zone. Furthermore, the high Ir concentration in this biozone could also be the result of the progressive contribution of ejecta material from shallower environments for thousands of years after the K/ Pg impact, as suggested by the presence of the ejecta-rich clay laminations in the lowermost part of the Ancón Fm.

Quartz grains with planar microstructures, micronsized dark clay laminations, intimate clay-lime mud associations, and burrow structures observed in the lime mud of the first 5 centimeters of the Ancón Fm. (Fig. 5I, J, K) suggest two sedimentary processes during the earliest Danian of Moncada after the Chicxulub impact. i) A sedimentary mixing process by bioturbation. ii) Erosion and destabilization of the CCU in higher topographic levels (e.g., on land and/or shallow shelves), probably by severe storms and/or earthquakes, giving rise to normal turbidite flows as well as the re-suspension of Ir-rich, clayey ejecta material, and its re-sedimentation in deeper locations such as Moncada. Both these sedimentary processes mixed the autochthonous marine lime mud and the reworked/resuspended ejecta that fell into the basin in the Moncada area. The greatest proportions of re-suspended ejecta occur in the UMU-1 claystone and the Ir-rich yellowish marly limestone of the basal part of the Ancón Fm. reported by Tada et al. (2002). Smit and Romein (1985) proposed a similar scenario at Deep Sea Drilling Project (DSDP) Site 577 (Shatsky Rise, North Pacific), in which enhanced Ir levels in G. cretacea and Pv. eugubina zones are probably either due to bioturbation/reworking or to erosion and redeposition from other eroded fall-out deposits, e.g., on land. Higher concentrations of iridium in the lowermost Danian have also been identified in other Gulf of Mexico localities, such as El Mimbral, northeastern Mexico (Smit et al., 1996).

Pollution by re-suspended particles and contaminant elements probably contributed to stressful ecological conditions worldwide for thousands of years after the K/ Pg boundary and influenced the evolution of planktonic foraminifera in the earliest Danian. Poisoning by heavy metals, ejected from the impact site (i.e., Chicxulub), may have provided long-term mechanisms of environmental stress for plankton. Calculations of trace element fluxes to the ocean associated with a hypothetical 10kmdiameter Chicxulub impactor indicate that the mass of heavy metals contained in the bolide, such as Co, Ni, Cu and Hg, are comparable to or larger than the world ocean burden (Erickson and Dickson, 1987). According to these authors, heavy metals with relatively inefficient removal mechanisms, such as Cu and Ni with greater than 1000yrs steady-state oceanic residence time, would have mixed throughout the entire volume of the ocean. Later, the worldwide sediments contaminated with heavy metals, including Ir and other platinoids, could have been eroded, removed and re-suspended in oceans for a long time (Smit and Romein, 1985; Smit et al., 1996). Enhanced levels of toxic heavy metals (e.g., Ni, Zn, Cr, or Cu) have been identified in G. cretacea and Pv. eugubina zones (mainly in the “K/T Boundary Clay”, i.e., the first 10kyr after the K/Pg boundary), even in distant localities from Chicxulub as Aïn Settara, Tunisia (Dupuis et al., 2001), and Stevns Klint, Denmark (Premovic et al., 2008).

The action of these pollutants must have been very intense just after the Chicxulub impact (Erickson and Dickson, 1987) and was probably one of the paleoenvironmental factors that caused the K/Pg boundary mass extinction of planktonic foraminifera, along with the short-lived global cooling during the “impact winter” (see Vellekoop et al., 2014), the breakdown of food chains and the productivity collapse (see Aberhan et al., 2007), and the acidification of the ocean surface (see Alegret et al., 2011). The real aftermath of the contaminants of meteoritic origin and other paleoenvironmental factors on the planktonic foraminiferal assemblages remains to be evaluated, but the Moncada section may provide some hints. Except for Guembelitria species, the absence of cosmopolitan, generalist Cretaceous species, such as Heterohelix globulosa, Hedbergella holmdelensis and Hedbergella monmouthensis (Petrizzo, 2002), in the UMU and the basal part of the Ancón Fm. is remarkable. The low abundance of planktonic foraminifera in this stratigraphic interval and the poor preservation prevents a better test in Moncada, but the lack of these Cretaceous species cannot be attributed only to taphonomical/diagenetic factors because, although poorly preserved, planktonic foraminifera species with small and thin tests have been identified in the same levels. By contrast, Cretaceous species are frequent in the lowermost part of the Danian, mainly in zone P0, in most continuous K/Pg sections of the Gulf of Mexico and the Tethys (see Molina et al., 2006, 2009). Given the new evidence from the Moncada section, it would be reasonable to argue that, except for guembelitrids, the presence of Cretaceous species within the lowermost Danian at most K/Pg localities may be the result of normal reworking/ bioturbation processes worldwide (see Huber 1996; Kaiho and Lamolda, 1999; Rodríguez-Tovar et al., 2010).

The Cretaceous rocks from the main source area of the clastic material of the Moncada and Ancón formations. could have contained a low abundance in planktonic foraminifera, explaining partly why reworked Cretaceous specimens are scarce and/or absent in both formations. The lowermost Danian planktonic foraminiferal assemblages of Moncada thus suggest that only Guembelitria species seem to have survived the K/Pg boundary mass extinction as originally suggested by Smit (1982).

The large amount of CO₂ gas devolatilized in the meteorite impact could have initiated an intense global warming by a greenhouse effect after the “impact winter” phase (see Kawaragi et al., 2009), persisting for thousands of years. In addition, the pollution of the oceans by contaminants of meteoritic origin could have persisted for at least 10 to 20kyr after the K/Pg boundary (a time interval equivalent to the G. cretacea zone), as suggested in Moncada and other sections worldwide. These stressed conditions in the surface pelagic environment may explain the peak in Guembelitria (PFAS-1) for approximately 10kyr after the K/Pg boundary (a time interval equivalent to the deposition of the “K/T Boundary Clay”), and the subsequent evolutionary radiation and acme of parvularugoglobigerinids (PFAS-2). Guembelitria comprises opportunist, r-strategy species that mainly inhabited nutrient-rich shallow, neritic waters (Kroon and Nederbragt, 1990; Olsson et al., 1999), so it may have been the only Cretaceous planktonic foraminifera taxon that could withstand the short- and long-term environmental stress conditions generated after the Chicxulub impact event, dominating the PFAS-1 for the first 10kyr after the K/Pg boundary. Parvularugoglobigerinids, which dominated the PFAS-2 from 10 to 50kyr after the K/Pg boundary, were also likely r-strategy opportunists, which rapidly colonized and speciated at all latitudes after the re-establishment of the pelagic environmental conditions in terms of temperature, nutrients, etc. (Li et al., 1995). The evolutionary radiation of planktonic foraminifera in the earliest Danian (Arenillas et al., 2000) was due to the progressive environmental recovery after the short-term “impact winter” phase and the subsequent repopulation of vacated niches during the long-term greenhouse phase (Molina, 2015). We suggest that the rate of evolution could have been increased by the effect of the long-term pollution of the oceans by contaminants of meteoritic origin, which could have caused the heightening of mutation rates for a time interval of more than 10kyr after the K/Pg boundary.