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Jensen et al . ( 1 ) question evidence presented of a chambered heart within placoderms, citing its small size and apparently ventral atrium. However, they fail to note the belly-up orientation of the placoderm within one nodule, and the variability of heart morphology within extant taxa. Thus, we remain confident in our interpretation of the mineralized organ as the heart.

Aims. Mountain ranges in the tropics are characterised by high levels of localised endemism, often-aberrant evolutionary trajectories, and some of the world’s most diverse regional biotas. Here we investigate the evolution of montane endemism, ecology and body size in a clade of direct-developing frogs (Choerophryne, Microhylidae) from New Guinea. Methods. Phylogenetic relationships were estimated from a mitochondrial molecular dataset using Bayesian and maximum likelihood approaches. Ancestral state reconstruction was used to infer the evolution of elevational distribution, ecology (indexed by male calling height), and body size, and phylogenetically corrected regression was employed to examine the relationships between these three traits. Results. We obtained strong support for a monophyletic lineage comprising the majority of taxa sampled. Within this clade we identified one subclade that appears to have diversified primarily in montane habitats of the Central Cordillera (> 1000 m. a.s.l), with subsequent dispersal to isolated North Papuan Mountains. A second subclade (characterised by moderately to very elongated snouts) appears to have diversified primarily in hill forests (< 1000 m a.s.l.), with inferred independent upwards colonisations of isolated montane habitats, especially in isolated North Papuan Mountains. We found no clear relationship between extremely small body size (adult SVL less than 15mm) and elevation, but a stronger relationship with ecology – smaller species tend to be more terrestrial. Conclusions. Orogeny and climatic oscillations have interacted to generate high montane biodiversity in New Guinea via both localised diversification within montane habitats (centric endemism) and periodic dispersal across lowland regions (eccentric endemism). The correlation between extreme miniaturisation and terrestrial habits reflects a general trend in frogs, suggesting that ecological or physiological constraints limit niche usage by miniaturised frogs, even in extremely wet environments such as tropical mountains.

Cooper and Fortey recently argued in TREE[1xCooper, A. and Fortey, R. Trends Ecol. Evol. 1998; 13: 151–156Abstract | Full Text | Full Text PDF | PubMed | Scopus (122)See all References][1]that spectacular evolutionary radiations documented in the fossil record—such as the Cambrian and Tertiary `explosions'—are preceded by long periods of cryptic evolution where major morphological innovations are established in small poorly-preserved ancestors—the `phylogenetic fuse'. Evidence for this came from a variety of molecular studies: for instance, the extensive genetic differences between metazoan phyla imply that they diverged long before their first fossil appearances in early Cambrian strata[2xWray, G.A., Levinton, J.S., and Shapiro, L.M. Science. 1996; 214: 568–573Crossref | Scopus (354)See all References][2]. A reanalysis of the molecular evidence challenges this estimate, and suggests a much more recent divergence time of 600–670 (rather than 1200) million years bp (Ref. [3xAyala, F.J., Rzhetsky, A., and Ayala, F.J. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 606–611Crossref | PubMed | Scopus (242)See all References][3]). Even these revised dates, however, imply a substantial period of cryptic evolution before the Cambrian explosion of 550 million years ago.While it was admitted that `the period of earlier phylogenesis would be overestimated if rates of molecular evolution were considerably accelerated during large radiations', this scenario was largely dismissed[1xCooper, A. and Fortey, R. Trends Ecol. Evol. 1998; 13: 151–156Abstract | Full Text | Full Text PDF | PubMed | Scopus (122)See all References][1]. However, this hypothesis deserves serious consideration. A recent study[4xOmland, K.E. Evolution. 1997; 51: 1381–1393CrossrefSee all References][4]has demonstrated that morphological and molecular rates of evolution are highly correlated, across a variety of groups and a wide range of morphological and genetic traits. Rapid morphological change appears to be accompanied by rapid molecular change. Thus, the unexpectedly large genetic differences between metazoan phyla (or mammalian orders) might document a relatively short period of accelerated genetic evolution in the late Precambrian (or early Tertiary), rather than (implausibly) long intervals of normal genetic change. This rate variability will probably not eliminate altogether the inferred period of cryptic evolution; genetic changes would have to be very greatly accelerated for this to happen[1xCooper, A. and Fortey, R. Trends Ecol. Evol. 1998; 13: 151–156Abstract | Full Text | Full Text PDF | PubMed | Scopus (122)See all References][1]. However, it will help to further shorten this phylogenetic fuse to a length that is acceptable to most palaeontologists.Correlated rates of morphological and molecular evolution would explain why molecular estimates for divergence times are consistently greater than well-supported palaeontological estimates. Indeed, these dating discrepancies constitute strong evidence for Omland's proposals[4xOmland, K.E. Evolution. 1997; 51: 1381–1393CrossrefSee all References][4].