Cretaceous-Paleogene Refugia

The Cretaceous-Paleogene boundary at 66 Ma is the archive's second major demographic compression event, distinct from the end-Permian near-reset in mechanism, duration, and consequence. The Chicxulub impactor terminated the Thermosynapsida's Mesozoic peak — by approximately 80 Ma the lineage had reached its highest-water mark across all five principal families and nearly every major landmass and ocean basin — through rapid surface collapse. Impact winter, acid rain, pyroclastic deposit, and prolonged photosynthetic shutdown eliminated the food-web base for surface-active megafauna on a timescale that rendered surface resilience irrelevant. The lineage's survival pathway was subterranean and pelagic.

Terrestrial lineages retreated into geothermal cave refugia whose thermal and chemical buffering insulated inhabitants from the worst of the surface interval. Deep-ocean Thalassodraconidae, already dependent on hydrothermal-vent food webs, experienced the least disruption across the boundary — their relative morphological continuity is the archive's primary evidence for the buffered-habitat model. Coastal and near-surface-marine lineages suffered more severe contraction as shallow marine productivity collapsed.

The net result was a severe bottleneck but not a near-reset. The K-Pg impact winter was catastrophic on a decades-to-centuries scale, considerably shorter than the Permian recovery interval; the lineage experienced a severe contraction rather than the longer near-reset modeled for the end-Permian. Post-K-Pg body sizes were systematically reduced relative to late Cretaceous maxima — a demographic filter, not directed genetic change: the bottleneck disproportionately removed the largest, highest-caloric-demand individuals.

The Paleocene-Eocene recovery added a second selective layer. Warmer cave temperatures biased TSD-competent populations toward female-heavy cohorts, driving modifier-allele selection that restored sex-ratio balance. Volucridraconidae, surviving at reduced body size, found warm Paleocene atmospheric conditions favorable for sustained flight. The recovery was therefore not simple population rebound but a morphological and demographic reshaping driven by the interaction of bottleneck legacy and new Paleocene conditions — reshaping that ultimately produced the distribution the archive tracks through the Cenozoic into the hominin-encounter window at 2.5 Ma.

Claims

c0001 — The K-Pg bottleneck is a true demographic compression, not a lineage-ending event

The archive treats the K-Pg event as a severe but survivable bottleneck rather than a lineage termination. The distinction from the end-Permian near-reset is duration: Chicxulub impact winter was catastrophic on a decades-to-centuries scale, whereas Permian recovery stretched across millions of years. Geothermal refugia insulated torpor-cycle lineages from the acute surface interval, allowing populations at depth to outlast the surface collapse and emerge into an early Paleocene world that was depleted but not uninhabitable.

c0002 — Deep-substrate and deep-ocean habitats provide the primary survival pathway

Survival across the K-Pg boundary follows the same architectural principle the archive identifies at the end-Permian: geothermally buffered caves for terrestrial lineages, hydrothermal-vent systems for marine lineages. Deep-ocean Thalassodraconidae are the primary evidence for this model — their relative morphological continuity across the boundary reflects their insulation from the surface food-web collapse that hit coastal and terrestrial populations directly. The archive treats buffered-habitat survival as the necessary condition for any post-K-Pg recovery.

c0003 — Post-bottleneck body-size reduction reflects survival bias toward smaller individuals

The restricted fossil record shows systematic body-size reduction in post-K-Pg material relative to late Cretaceous maxima. The archive's explanation is selective rather than genetic: the bottleneck interval disproportionately removed the largest, highest-caloric-demand individuals, leaving smaller animals to found the Paleocene recovery populations. This is treated as a demographic filter rather than a directed evolutionary change.

c0004 — Thalassodraconidae continuity across the boundary is the primary K-Pg refugium evidence

Among the five principal families, Thalassodraconidae shows the least disruption in K-Pg boundary material. The archive attributes this to reinforcing factors: dependence on hydrothermal-vent food webs that remained productive when surface photosynthesis collapsed, and deep-ocean habitat insulation from ejecta and atmospheric effects that drove surface and shallow-marine mortality. Thalassodraconidae continuity is therefore the best-preserved evidence for the claim that buffered-habitat survival was the lineage's K-Pg mechanism.

c0005 — Paleocene-Eocene recovery reshapes sex-ratio and flight-morphology on the bottleneck founder population

The archive models the Paleocene-Eocene warming as a second selective layer operating on an already bottlenecked founder population. Warmer cave environments bias TSD-competent lineages toward female-heavy cohorts, selecting for modifier alleles that restore sex-ratio balance over subsequent generations. Volucridraconidae, surviving at reduced body size, find those reduced masses and warm atmospheric conditions favorable for improved sustained flight efficiency. The recovery is therefore not simple population rebound but a morphological and demographic reshaping driven by the interaction of bottleneck legacy and new Paleocene conditions.