The story begins with a puzzle that has lingered since 1995, when the renowned arachnologist Pablo Goloboff described Acanthogonatus centralis from the mountainous systems of central Argentina. The species was recorded from a strikingly broad swath of territory, spanning southern Buenos Aires province, western Córdoba, central San Luis, and eastern San Juan. For a sedentary, burrow-dwelling spider with poor dispersal abilities, that distribution seemed suspiciously expansive. Mygalomorph spiders, the ancient lineage that includes tarantulas and trapdoor spiders, are famous for extreme site fidelity: females spend their entire lives in or near a single silk-lined burrow, and populations separated by unsuitable habitat rarely, if ever, exchange genes.

An earlier phylogeographic study had already hinted that something was amiss. Work published in 2022 found significant genetic structuring between populations in the Ventania mountain range of Buenos Aires province and those in the Pampean ranges of Córdoba, with the two groups estimated to have diverged roughly three to five million years ago during the Pliocene. But that study relied on a short fragment of the mitochondrial 16S rRNA gene, a marker that the new study’s authors caution is imperfect for species delimitation when used in small fragments. To settle the question, the team turned to the cytochrome c oxidase subunit I gene, or COI, the workhorse of animal DNA barcoding, which has repeatedly proven superior for identifying species boundaries.

The researchers examined specimens accumulated since 2009 in the arachnological collection of CERZOS-CONICET in Bahía Blanca, along with type material held at the Museo Argentino de Ciencias Naturales Bernardino Rivadavia in Buenos Aires. DNA was extracted from leg tissues of six individuals representing both mountain systems and amplified using the standard Folmer primer pair. The resulting sequences were aligned and analyzed under both Bayesian Inference and Maximum Likelihood frameworks, with the optimal evolutionary model determined computationally and nodal support assessed through bootstrap resampling and posterior probabilities. Related taxa, including theraphosid tarantulas and the European genus Nemesia, served as outgroups to root the tree.

The phylogeny delivered a clear verdict. Within a well-supported Pycnothelidae clade, the sequences formed two independent lineages: one from the Ventania range near Funke in Buenos Aires province, and another from the Córdoba localities of Dean Funes and Villa Berna. Acanthogonatus campanae, a Chilean species, emerged as the sister to this complex. Two complementary species delimitation methods then weighed in. The Bayesian Poisson Tree Processes approach, which models how mutations accumulate along a phylogenetic tree, split the complex into exactly two lineages. The distance-based Automatic Barcode Gap Discovery method suggested three, further dividing the Córdoba populations into a central group around Villa Berna and a northern group at Dean Funes. The authors interpret this third split cautiously, noting that ABGD is known to overestimate diversity in strongly structured, sedentary species like these spiders, where geographic isolation produces deep genetic structure even within a single species.

Morphology ultimately provided the decisive, independently verifiable evidence. Although the two lineages look nearly identical in overall appearance, with both sexes reaching roughly twelve millimeters in body length and wearing similar yellowish-brown coloration with mottled abdomens, their genitalia tell a different story. Males of the new species possess a pyriform palpal bulb with a long, slender embolus bearing eight to ten subapical prolateral teeth, whereas true A. centralis males have a bulb with exactly seven such teeth and a differently shaped, less tapered embolus. Females are separated by the architecture of the spermathecae, the sperm-storage organs: the new species has an elongated duct leading to a sub-spherical receptacle with a finger-like internal projection nearly as long as the receptacle itself, while A. centralis females have an oval receptacle with a blunt apex and a shorter internal projection. In taxonomy, these fine-scale genitalic differences are treated as reliable markers of reproductive isolation, the biological essence of separate species.

Ecological niche modeling added a third, independent line of evidence. Using the MAXENT algorithm with carefully screened bioclimatic variables, the team generated distribution models for each lineage under both present-day and Pliocene climatic conditions. The models performed with high accuracy, achieving area-under-the-curve values between 0.937 and 0.989. For the Ventania lineage, temperature seasonality and isothermality dominated the prediction, jointly accounting for over 91 percent of the model, and suitable habitat was confined to western Buenos Aires province and adjacent areas. For the Córdoba lineage, now formally A. monicae, temperature seasonality combined with precipitation variables of the coldest quarter and precipitation seasonality explained 83.3 percent of the prediction, and suitable areas extended across northeastern Córdoba, northwestern San Luis, central Santiago del Estero, and western Chaco, associated with the dry Chaco ecoregion. Crucially, the Pliocene models showed that the two lineages occupied distinct climatic refugia millions of years ago, with no overlap between their high-probability areas, consistent with the estimated three-to-five-million-year divergence and the persistence of an environmental barrier to gene flow ever since.

The formal description of Acanthogonatus monicae sp. nov. is anchored by a male holotype collected at Alpa Corral in Córdoba in February 2017 and a female paratype from Ascochinga. The species name is a matronym honoring Mónica, the mother of the first author, in recognition of her support throughout his academic career. The new species belongs to the family Pycnothelidae, a group revalidated at family rank by recent molecular work that dismantled the old, sprawling concept of Nemesiidae. Pycnothelids are nocturnal spiders found across Africa, Australasia, and South America, recognizable by traits such as an intercheliceral tumescence in males, flexible tarsi, and the absence of claw tufts. They construct open silk tubes with a single entrance and hunt passively, ambushing prey that wanders too close to the burrow mouth. The genus Acanthogonatus, the most diverse Neotropical member of the family, now counts 31 described species, and the authors provide an updated identification key covering all Argentine species, a practical tool for future surveys.

Beyond the novelty itself, the study underscores why mygalomorph spiders have become model organisms for cryptic species research. Their low morphological variability, evolutionary stasis, and limited dispersal conspire to hide diversity from morphology-based inventories alone, a pattern documented repeatedly in related lineages across the world. Similar cases in the trapdoor spider genera Bertmainius and Calisoga revealed deeply structured populations concealed by external uniformity. Each hidden species discovered in these ancient lineages reshapes estimates of regional biodiversity and, importantly, of conservation priorities: a species restricted to a single mountain system faces very different risks than one spread across half a country. The authors argue that integrative studies like this one help close both the Linnean shortfall, the gap in our knowledge of what species exist, and the Wallacean shortfall, the gap in our knowledge of where they live.

For now, Acanthogonatus centralis remains the proper name for the spiders of the Ventania range in Buenos Aires province, its distribution considerably narrowed by the revision, while the Pampean ranges of Córdoba harbor the newly recognized A. monicae. Whether the northern and central Córdoba populations flagged by the ABGD analysis represent further species awaiting description remains an open question, one that additional nuclear loci and denser sampling may eventually answer. What is already clear is that the granite hills and grasslands of central Argentina still guard evolutionary secrets, and that the quiet spiders spinning silk tubes beneath their stones have been diversifying, unseen, for millions of years longer than science has been watching.

Subject of Research: Integrative taxonomy and species delimitation in the mygalomorph spider genus Acanthogonatus in central Argentina

Article Title: Integrative taxonomy unveils a new lineage within Acanthogonatus centralis Goloboff, 1995 ( Araneae , Mygalomorphae , Pycnothelidae ): description of Acanthogonatus monicae sp. nov. from central Argentina

Article References: Signorotto, F., & Ferretti, N. (2026). Integrative taxonomy unveils a new lineage within Acanthogonatus centralis Goloboff, 1995 (Araneae, Mygalomorphae, Pycnothelidae): description of Acanthogonatus monicae sp. nov. from central Argentina. Evolutionary Systematics, 10(1), 31-46. https://doi.org/10.3897/evolsyst.10.172168

Image Credits: AI Generated

DOI: 10.3897/evolsyst.10.172168

Keywords: Acanthogonatus monicae, Acanthogonatus centralis, Pycnothelidae, Mygalomorphae, integrative taxonomy, species delimitation, COI barcoding, ecological niche modeling, Argentina, Córdoba, cryptic species, new species