Ground Beetles Rewrite the Map of Gondwana’s Breakup

Few debates in historical biogeography have raged as long as the question of why the southern continents share so many similar plants and animals. As far back as 1867, Joseph Hooker documented striking overlaps among the floras of New Zealand, Australia, and South America, counting 252 of his 303 New Zealand plant genera in Australia and 174 in South America. Were these shared lineages ancient residents that rode the breakup of Gondwana to their present homes, or intrepid travelers that crossed vast oceans long after the continents separated? A new phylogenetic analysis of a charismatic group of flightless ground beetles, published in the Deutsche Entomologische Zeitschrift, brings fresh molecular-free but character-rich evidence to this century-old argument, and the results lean decisively toward the ancient-continents side of the story.

The beetles in question belong to the subfamily Migadopinae, a group of carabid ground beetles whose members cluster at the southern ends of South America, Australia, and New Zealand, with outliers on subantarctic islands and even a high-elevation species on Ecuador’s Chimborazo. As early as 1938, René Jeannel proposed that migadopines originated in the Cretaceous on Gondwanan terranes that later drifted apart, while Philip Darlington countered in the 1960s that all faunas are derived through dispersal, arguing that winged ancestors must have flown across the Southern Ocean. The new study, built on an expanded matrix of 75 morphological characters scored for 42 taxa, subjects these rival hypotheses to rigorous cladistic testing using both maximum parsimony and Bayesian inference.

The analysis delivers several headline results. First, the tribe Amarotypini, erected in 1985 for the New Zealand genus Amarotypus, is newly synonymized under Migadopini, because keeping it separate would render Migadopini paraphyletic, an unacceptable condition in classification. The distinctive elongate unguitractor plate at the tip of the fifth tarsomere that defined Amarotypini turns out to mark only a subsidiary clade, now dubbed the Amarotypus clade, comprising Amarotypus, Amaroxenus, Amarophilus, and the Tasmanian genus Migadopiella. Second, the remarkable Andean genus Aquilex, discovered on Chimborazo at over 4000 meters, is confirmed as sister to all other migadopines, warranting recognition of the tribe Aquilicini and filling the biogeographic gap between the southern beetles and their Holarctic relatives in Elaphrinae and Loricerinae.

The methodological machinery behind these conclusions is worth appreciating. Specimens were borrowed from ten institutions across the United States, New Zealand, Australia, the United Kingdom, Denmark, and Germany, then dissected, cleared in potassium hydroxide, and measured under magnifications up to 100 times. Body proportions were quantified through standardized ratios of head, pronotal, and elytral dimensions, while genitalic structures, the workhorses of carabid taxonomy, provided a wealth of phylogenetically informative characters. Parsimony searches in TNT yielded 16 shortest trees of 276 steps, and Bayesian analysis in MrBayes ran six million generations until split frequencies converged below 0.01 and all effective sample sizes exceeded 630, ensuring the tree estimates were statistically stable.

Converting the resulting cladogram into a taxon-area cladogram allowed the authors to reconstruct ancestral distributions using the RASP software and its Bayesian Binary MCMC algorithm over one million cycles. The deepest node within the subfamily optimizes to South America, with the sister group of Migadopinae residing in the Holarctic at a probability of 0.93. This pattern implies that the lineage leading to modern migadopines was isolated when Laurasia and Gondwana sundered, and that the subfamily’s earliest radiations unfolded across the southern continent before any of today’s ocean basins divided it.

The sequence of divergences that follows reads like a geological timetable. Early branches of Migadopini occupy southern South America and the Falkland Islands. Subsequent nodes implicate Australia, New Zealand, and the Campbell Plateau, the largely submerged continental fragment carrying the Auckland and Antipodes Islands. Strikingly, the node subtending the Campbell Plateau fauna is ambiguously optimized among South America, Australia, and the plateau itself, a result the authors interpret as most consistent with the fragmentation of these Gondwanan terranes rather than any single dispersal corridor. The Campbell Plateau, they argue, has an independent biogeographic history from mainland New Zealand west of the Alpine Fault, echoing geological hypotheses about mid-Cretaceous rifting along the Campbell Rift and the later opening of the Tasman Sea.

One biogeographic event does emerge unambiguously as dispersal: the origin of the Tasmanian genus Migadopiella. Because Migadopiella nests within a paraphyletic assemblage of New Zealand genera, the analysis points to an east-to-west crossing of the Tasman Sea, from New Zealand’s South Island to Tasmania. That conclusion is remarkable because the beetles involved are almost entirely wingless. Only Amarotypus edwardsii retains reduced, brachypterous wings, and even those reach just 65 to 75 percent of elytral length. A flightless beetle somehow made a transoceanic journey, likely by rafting or other passive means, underscoring that even the most sedentary animals can occasionally leap geographic barriers.

This flightlessness evidence directly undercuts Darlington’s dispersalist scenario. He had proposed that winged taxa such as the South American Antarctonomus complanatus and the Australian Dendromigadops represented the ancestral stock that flew between continents. The new phylogeny shows instead that macroptery is restricted to two highly subordinate lineages, and that the earliest-diverging migadopines are apterous. Repeated evolution of flightlessness is a well-documented phenomenon in Carabidae, particularly in montane and island habitats, so the presence of wings in a few derived species says nothing reliable about the ancestral condition. The fossil record adds temporal anchors: a mid-Cretaceous larva, Cretomigadops bidentatus, preserved in 99-million-year-old Kachin amber from Myanmar, pushes the subfamily’s origin deep into the Cretaceous, consistent with a Gondwanan vicariance scenario, while the Miocene Antarctic fossil carabid Antarctotrechus balli from the Transantarctic Mountains demonstrates that beetles once thrived on a warmer southern continent and hints that fossil migadopines may yet be found there.

The study also delivers taxonomic novelty alongside its biogeographic synthesis. Two new species of Amaroxenus are described from New Zealand’s Southern Alps, both discovered in extreme alpine settings. Amaroxenus embersoni, named for the late Lincoln University entomologist Rowan Emberson, was collected under rocks at the edge of receding snowfields near 1750 meters on Mount Aicken in Arthur’s Pass National Park. Amaroxenus marrisi, honoring John Marris, came from fractured graywacke blocks at 1695 meters on a ridge above Mount Cedric in Nelson Lakes National Park, sharing its crevice habitat with the lichen-feeding silvanid beetle Protodendrophagus antipodes. These discoveries illustrate how much remains unknown about high-elevation invertebrate life, even in well-studied temperate countries.

What makes this work resonate beyond beetle taxonomy is its contribution to a grand scientific argument that Hooker and Darwin themselves framed. By combining a densely sampled morphological matrix with explicit ancestral-area modeling and careful attention to fossil evidence, the study shows that the deep structure of southern biotic distributions is best explained by the fragmentation of Gondwana, with dispersal reserved for the occasional, exceptional event. The authors predict that future fossil discoveries in Antarctica, biogeographically homologous to the living South American, Australian, and New Zealand fauna, will corroborate the vicariance hypothesis further. If a fossil Antarctic migadopine ever surfaces, the character framework laid out here stands ready to test its place in the family tree, closing another chapter in the story of how a shattered supercontinent shaped the life we see today.

Subject of Research: Phylogenetics and biogeography of the circum-Antarctic ground beetle subfamily Migadopinae

Article Title: Phylogenetic analysis of the circum-Antarctic Subfamily Migadopinae ( Coleoptera , Carabidae ) and assessment of the trans-Tasman Amarotypus clade

Article References: Liebherr, J. K., Roig-Juñent, S., & Will, K. W. (2024). Phylogenetic analysis of the circum-Antarctic Subfamily Migadopinae (Coleoptera, Carabidae) and assessment of the trans-Tasman Amarotypus clade. Deutsche Entomologische Zeitschrift, 71(2), 319-338. https://doi.org/10.3897/dez.71.134268

Image Credits: AI Generated

DOI: 10.3897/dez.71.134268

Keywords: Migadopinae, Carabidae, Gondwana, vicariance, biogeography, ground beetles, New Zealand, Tasmania, phylogenetics, Amarotypus, Campbell Plateau, flightlessness