Mesozoic Diversity Research Articles

The female reproductive unit of Ephedra (Gnetales): comparative morphology and evolutionary perspectives

Morphological variation in Ephedra (Gnetales) is limited and confusing from an evolutionary perspective, with parallelisms and intraspecific variation. However, recent analyses of molecular data provide a phylogenetic framework for investigations of morphological traits, albeit with few informative characters in the investigated gene regions. We document morphological, anatomical and histological variation patterns in the female reproductive unit and test the hypothesis that some Early Cretaceous fossils, which share synapomorphies with Ephedra, are members of the extant clade. Results indicate that some morphological features are evolutionarily informative although intraspecific variation is evident. Histology and anatomy of cone bracts and seed envelopes show clade-specific variation patterns. There is little evidence for an inclusion of the Cretaceous fossils in the extant clade. Rather, a hypothesized general pattern of reduction of the vasculature in the ephedran seed envelope, probably from four vascular bundles in the fossils, to ancestrally three in the living clade, and later to two, is consistent with phylogenetic and temporal analyses, which indicate that extant diversity evolved after the Cretaceous-Tertiary boundary. Notwithstanding striking similarities between living and Cretaceous Ephedra, available data indicate that the Mesozoic diversity went almost entirely extinct in the late Cretaceous causing a bottleneck effect in Ephedra, still reflected today by an extraordinarily low level of genetic and structural diversity.

When molecules and morphology concur: the ‘Gondwanan’ midges (Diptera: Chironomidae)

A phylogeny of the Chironomidae subfamily Podonominae, significant in the history of phylogenetic biogeography, is estimated from an analysis of four genes. Fragments of two ribosomal genes (18S and 28S), one nuclear protein-coding gene (CAD), and one mitochondrial protein-coding gene (COI) were sequenced from specimens representing 13 of 15 genera, and analysed using mixed model Bayesian and maximum likelihood inference methods. Podonominae is monophyletic and sister to Tanypodinae – the shared development of the larval ligula is synapomorphic and diagnostic. Tribe Podonomini is monophyletic with the inclusion of Trichotanypus; tribe Boreochlini is a grade. Monophyly is confirmed for the genera Podonomus Philippi, Podonomopsis Brundin, Podochlus Brundin, Archaeochlus Brundin and Austrochlus Cranston, Edward & Cook: Parochlus Enderlein becomes monophyletic through the inclusion of Zelandochlus Brundin (n.syn.) with its type species, P. latipalpis (Brundin) n.comb. The ‘mandibulate’Archaeochlus plus Austrochlus is monophyletic with nonmandibulate Afrochlus weakly supported as a member of, or sister to, the African Archaeochlus. Subtending this group is Lasiodiamesa, although it associates in some analyses with the sister group Tanypodinae. Generic relationships coincide with those proposed based on morphology, particularly as understood via all life history stages of some problematic (autapomorphic, adult-based) taxa. Divergence time analysis (beast) allows inference of Mesozoic diversification of higher taxa in Podonominae, of appropriate timing for fragmentation of Gondwana, post-African divergence, to have caused vicariance. Shallower nodes (within genera) imply both younger vicariance involving Antarctica and some recent dispersal, including southern to northern hemisphere movement in the New World. New Zealand taxa test controversial biogeographical relationships and show proximity to southern South America without direct Australian sister taxon pairs: dating implies persistence of midges through the ‘Oligocene’ bottleneck.

Evolutionary paleoecology of dense ophiuroid populations

Issues of scale are becoming increasingly important to paleobiological interpretations of the fossil record. Nevertheless, a number of biological processes display scale-independent behaviors. The effects of predation on the distribution of dense populations of epifaunal, suspension-feeding ophiuroids are scale-independent, at scales ranging from the microecological to the macroevolutionary. On a microecological scale (meters to kilometers, hours to days), dense ophiuroid populations are limited in shallow-water environments by predatory fishes and crabs. On a larger, ecological scale (tens to hundreds of kilometers, decades to centuries), circumstantial evidence indicates that oceanographically driven, multidecadal cycles of predator abundance determine the abundance of ophiuroids throughout the western English Channel. On a macroevolutionary scale (millions to tens of millions of years, global spatial scale), dense, autochthonous assemblages of ophiuroids declined in conjunction with the Mesozoic diversification of modern shell-crushing predators: teleostean fishes, decapod crustaceans, and neoselachian sharks. The sporadic reappearance of dense ophiuroid populations in a late Eocene, shallow marine deposit in Antarctica suggests that predator-prey relationships were disrupted as temperatures declined in the region at that time. Scale-independence is a useful model for explaining and predicting patterns of distribution of dense ophiuroid populations in time and space.