Rapid glaciation and a two-step sea level plunge into the Last Glacial Maximum.

The approximately 10,000-year-long Last Glacial Maximum, before the termination of the last ice age, was the coldest period in Earth's recent climate history1. Relative to the Holocene epoch, atmospheric carbon dioxide was about 100 parts per million lower and tropical sea surface temperatures were about 3 to 5 degrees Celsius lower2,3. The Last Glacial Maximum began when global mean sea level (GMSL) abruptly dropped by about 40 metres around 31,000 years ago4 and was followed by about 10,000 years of rapid deglaciation into the Holocene1. The masses of the melting polar ice sheets and the change in ocean volume, and hence in GMSL, are primary constraints for climate models constructed to describe the transition between the Last Glacial Maximum and the Holocene, and future changes; but the rate, timing and magnitude of this transition remain uncertain. Here we show that sea level at the shelf edge of the Great Barrier Reef dropped by around 20 metres between 21,900 and 20,500 years ago, to -118 metres relative to the modern level. Our findings are based on recovered and radiometrically dated fossil corals and coralline algae assemblages, and represent relative sea level at the Great Barrier Reef, rather than GMSL. Subsequently, relative sea level rose at a rate of about 3.5 millimetres per year for around 4,000 years. The rise is consistent with the warming previously observed at 19,000 years ago1,5, but we now show that it occurred just after the 20-metre drop in relative sea level and the related increase in global ice volumes. The detailed structure of our record is robust because the Great Barrier Reef is remote from former ice sheets and tectonic activity. Relative sea level can be influenced by Earth's response to regional changes in ice and water loadings and may differ greatly from GMSL. Consequently, we used glacio-isostatic models to derive GMSL, and find that the Last Glacial Maximum culminated 20,500 years ago in a GMSL low of about -125 to -130 metres.

Radiation of the coralline red algae (Corallinophycidae, Rhodophyta) crown group as inferred from a multilocus time-calibrated phylogeny.

The subclass Corallinophycidae is the only group of red algae characterized by the presence of calcite crystals in their cell walls. Except for the Rhodogorgonales, the remaining orders - collectively called corallines - are diverse and widely distributed, having calcified cell walls and highly variable morphology. Corallines constitute the group with the richest fossil record among marine algae. In the present study, we investigate the evolutionary history of the subclass Corallinophycidae and provide a time-calibrated phylogeny to date the radiation of the crown group and its main lineages. We use a multi-locus dataset with an extensive taxon sampling and comprehensive collection of fossil records, carefully assigned to corallines, to reconstruct a time-calibrated phylogeny of this subclass. Our molecular clock analyses suggest that the onset of crown group diversification of Corallinophycidae started in the Lower Jurassic and sped up in the Lower Cretaceous. The divergence time of the oldest order Sporolithales is estimated in the Lower Cretaceous followed by the remaining orders. We discuss the long period of more than 300 million years between the early Paleozoic records attributed to the stem group of Corallinophycidae and the radiation of the crown group. Our inferred phylogeny yields three highly-supported suprageneric lineages for the order Corallinales; we confirm the family Mastophoraceae and amend circumscription of the families Corallinaceae and Lithophyllaceae. These three families are distinguished by a combination of vegetative and reproductive features. In light of the phylogeny, we discuss the evolutionary trends of eleven morphological characters. In addition, we also highlight homoplasious characters and selected autapomorphies emerging in particular taxa.

Adeylithon bosencei gen. et sp. nov. (Corallinales, Rhodophyta): a new reef-building genus with anatomical affinities with the fossil Aethesolithon.

Adeylithon gen. nov. with one species, A. bosencei sp. nov., belonging to the subfamily Hydrolithoideae is described from Pacific coral reefs based on psbA sequences and morpho-anatomy. In contrast with Hydrolithon, A. bosencei showed layers of large polygonal "cells," which resulted from extensive lateral fusions of perithallial cells, interspersed among layers of vegetative cells. This anatomical feature is shared with the fossil Aethesolithon, but lacking DNA sequences from the fossils and the fragmentary nature of Aethesolithon type material, we cannot ascertain if Adeylithon and Aethesolithon are congeneric. Morpho-anatomical features of A. bosencei were generally congruent with diagnostic features of the subfamily Hydrolithoideae: (i) outline of cell filaments entirely lost in large portions due to pervasive and extensive cell fusions, (ii) trichocytes not arranged in tightly packed horizontal fields, (iii) basal layer without palisade cells, and (iv) cells lining the canal pore oriented more or less perpendicular to roof surface and not protruding into the canal. However, it showed a predominant monomerous thallus organization and trichocytes were disposed in large pustulate, horizontal fields, although they were not tightly packed and did not become distinctly buried in the thallus. Only mature tetrasporangial conceptacles were observed, therefore the type of conceptacle roof formation remained undetermined. Adeylithon bosencei occurs on shallow coral reefs, in Australia, Papua New Guinea, and South Pacific islands (Fiji, Vanuatu). Fossil Aethesolithon is considered an important component of shallow coral reefs since the Miocene; fossil records showed a broad Indo-Pacific distribution, but a long-term process of range contraction in the last 2.6 million years, resulting in an overlap with the distribution of the extant Adeylithon. While the congeneric nature of extant and fossil taxa remained uncertain, similarities in morpho-anatomy, habitat, and distribution may indicate that both taxa likely shared a common ancestor.

Timing of the evolutionary history of Corallinaceae (Corallinales, Rhodophyta).

The temporal dimension of the most recent Corallinaceae (order Corallinales) phylogeny was presented here, based on first occurrence time estimates from the fossil record. Calibration of the molecular clock of the genetic marker SSU entailed a separation of Corallinales from Hapalidiales in the Albian (Early Cretaceous ~105 mya). Neither the calibration nor the fossil record resolved the succession of appearance of the first three emerging subfamilies: Mastophoroideae, Corallinoideae, and Neogoniolithoideae. The development of the tetra/bisporangial conceptacle roofs by filaments surrounding and interspersed among the sporangial initials was an evolutionary novelty emerging at the Cretaceous-Paleogene boundary (~66 mya). This novelty was shared by the subfamilies Hydrolithoideae, Metagoniolithoideae, and Lithophylloideae, which diverged in the early Paleogene. Subclades within the Metagoniolithoideae and Lithophylloideae diversified in the late Oligocene-middle Miocene (~28-12 mya). The most common reef corallinaceans (Hydrolithon, Porolithon, Harveylithon, "Pneophyllum" conicum, and subclades within Lithophylloideae) appeared in this interval in the Indo-Australian Archipelago.

Phylogenetic relationships of corallinaceae (Corallinales, Rhodophyta): taxonomic implications for reef-building corallines.

A new, more complete, five-marker (SSU, LSU, psbA, COI, 23S) molecular phylogeny of the family Corallinaceae, order Corallinales, shows a paraphyletic grouping of seven well-supported monophyletic clades. The taxonomic implications included the amendment of two subfamilies, Neogoniolithoideae and Metagoniolithoideae, and the rejection of Porolithoideae as an independent subfamily. Metagoniolithoideae contained Harveylithon gen. nov., with H. rupestre comb. nov. as the generitype, and H. canariense stat. nov., H. munitum comb. nov., and H. samoënse comb. nov. Spongites and Pneophyllum belonged to separate clades. The subfamily Neogoniolithoideae included the generitype of Spongites, S. fruticulosus, for which an epitype was designated. Pneophyllum requires reassesment. The generitype of Hydrolithon, H. reinboldii, was a younger heterotypic synonym of H. boergesenii. The evolutionary novelty of the subfamilies Hydrolithoideae, Metagoniolithoideae, and Lithophylloideae was the development of tetra/bisporangial conceptacle roofs by filaments surrounding and interspersed among the sporangial initials.

Bryozoans are Major Modern Builders of South Atlantic Oddly Shaped Reefs.

In major modern reef regions, either in the Indo-Pacific or the Caribbean, scleractinian corals are described as the main reef framework builders, often associated with crustose coralline algae. We used underwater cores to investigate Late Holocene reef growth and characterise the main framework builders in the Abrolhos Shelf, the largest and richest modern tropical reef complex in the South Western Atlantic, a scientifically underexplored reef province. Rather than a typical coralgal reef, our results show a complex framework building system dominated by bryozoans. Bryozoans were major components in all cores and age intervals (2,000 yrs BP), accounting for up to 44% of the reef framework, while crustose coralline algae and coral accounted for less than 28 and 23%, respectively. Reef accretion rates varied from 2.7 to 0.9 mm yr-1, which are similar to typical coralgal reefs. Bryozoan functional groups encompassed 20 taxa and Celleporaria atlantica (Busk, 1884) dominated the framework at all cores. While the prevalent mesotrophic conditions may have driven suspension-feeders' dominance over photoautotrophs and mixotrophs, we propose that a combination of historical factors with the low storm-disturbance regime of the tropical South Atlantic also contributed to the region's low diversity, and underlies the unique mushroom shape of the Abrolhos pinnacles.