Table 1 Description, images, and ecology of nannoplankton taxa that are found as fossils in the Upper Cretaceous and survive the K/Pg mass extinction event.

Coccosphere morphology is based on observation of preserved coccospheres except for Calciosolenia, which is based on the shape of the coccosphere in its living species. Neocrepidolithus, Zeugrhabdotus, Lapideacassis, and Octolithus are extinct taxa that have never been observed as intact coccospheres, but modern coccospheres of taxa with the same coccolith morphology (muroliths) as Neocrepidolithus and Zeugrhabdotus tend to have very high lith numbers and are typically found in more coastal areas. Information about living relatives can be found summarized in (27). Survival mechanism listed here is based on whether we consider that their ecology and/or coccosphere morphology points to potential mixotrophy. Markalius is the most ambiguous as they form typical placolith-morphology coccospheres that are spherical and have low Cn (number of coccoliths per coccosphere) and no openings. Source of SEM images: (27, 38, 73). The coccosphere next to Octolithus is a representative modern holococcolith coccosphere (27). SEM images are not to the same scale.

Survivor taxon and familyCoccosphere
morphology
Paleobiogeography,
living relative ecology
Survival mechanism(s)Rationale
Braarudosphaera,
Braarudosphaeraceae
Embedded ImageImperforate dodecahedron
in calcified phase (seen as
fossils)
Coastal, coastalMixotrophy in dominant,
noncalcified motile phase,
possible resting cyst (the
dodecahedron coccosphere),
endosymbiotic
cyanobacteria.
*Coastal distribution, mixotrophy,
and dominance of motile phase
have been documented (68, 69),
and calcified resting cyst stage is
highly likely (Hagino pers. comm.).
In addition, extant forms have
endosymbiotic cyanobacteria (70).
Cyclagelosphaera,
Watznaueriaceae
Embedded ImageImperforate sphereCoastal, coastalCoastal mixotrophy, existed
in dominant motile
(noncalcified) phase?
†Coccolith-bearing phase shows
no evidence for a flagellar
opening, but modern dominant
phase (noncalcified) is likely
motile, and coastal ecology points
to mixotrophy (68).
Biscutum,
Biscutaceae
Embedded ImageEllipsoidal to cylindrical
with possible flagellar
opening
Coastal, coastalCoastal mixotrophy.Ellipsoidal coccosphere with
potential flagellar openings plus
coastal/near-shore ecology (19, 71)
points to mixotrophic capacity.
Markalius,
Incertae sedis
Embedded ImageImperforate sphereCoastal, extinctCoastal mixotrophy?No evidence for a flagellar
opening but coastal ecology
could point to mixotrophic
capacity. Similar to
Cyclagelosphaera?
Neocrepidolithus,
Chiastozygaceae
Embedded ImageUnknown, similar to
modern murolith spheres
with high Cn?
Coastal, extinctMixotrophy?Coastal ecology and potentially
high Cn could point to
mixotrophic capacity. Many
modern coastal murolith species
are flagellate [see (27)].
Zeugrhabdotus
Chiastozygaceae
Embedded ImageUnknown, similar to
modern murolith spheres
with high Cn?
Coastal, extinctMixotrophy?High latitude and coastal ecology
(19), plus potentially high Cn could
point to mixotrophic capacity.
Many modern coastal murolith
species are flagellate [see (27)].
Lapideacassis,
Lapideacassaceae
Embedded ImageUnknownCoastal, extinctMixotrophy?Unusual coccolith morphology
and atypically restricted coastal
ecology point to mixotrophic
capacity.
Calciosolenia,
Calciosoleniaceae
Embedded ImageFusiform with flagellar
opening
Coastal, coastalMixotrophyStrongly fusiform coccosphere
with flagellar openings plus
coastal ecology (72) points to
mixotrophic capacity.
Goniolithus,
Goniolithaceae
Embedded ImageImperforate dodecahedronCoastal, extinctResting cyst and/or
mixotrophy?
Similar morphology to
Braarudosphaera could suggest
similar ecology, but atypical
restricted coastal ecology also
points to mixotrophic capacity.
Octolithus,
holococcolith
Embedded ImageHolococcolith, unknown
sphere
Coastal, extinctMixotrophyLiving holococcolith-phase
coccolithophores are typically
flagellate‡, and coastal ecology
also points to mixotrophic
capacity

*Modern Braarudosphaera is highly anomalous among coccolithophores in having a possible resting phase and endosymbionts.

Cyclagelosphaera, while exhibiting a typical imperforate coccosphere in its calcifying phase, again appears highly anomalous as modern observations indicate that it is coastal and only exists for a short time in this life-cycle phase (68).

‡Holococcoliths are specific to the haploid phase of coccolithophores that is characteristically motile (25).