Lineagona, Shear, 2024

Shear, William A., 2024, Lineagona, a new millipede genus (Diplopoda, Chordeumatida, Cleidogonidae) from Texas, USA, and Nuevo León, México, Zootaxa 5397 (2), pp. 283-290 : 285-287

Lineagona , new genus

Type species: Lineagona culmenicola , n. sp., by present designation.

Included species: In addition to the type, Cleidogona chisosi Loomis, 1963 .

Diagnosis: Closely resembling Cleidogona species in nonsexual characters. Differing from that genus in the deep division of the gonopod angiocoxite into two branches, sometimes with the posterior or lateral branch subdivided, and in the reduction of the ninth legpair to four or five short articles, the coxae enlarged and lobed, but the femora not swollen and broadly subtriangular as in Cleidogona . Differing from Pseudotremia and Solaenogona , which it resembles in the divided angiocoxites, in not having the colpocoxites fused basally, and in lacking median processes between the coxites. Differing from Dybasia in lacking a sternal process on the gonopods, and from Cabraca in having telopodites of the male ninth legs.

Etymology: From Latin, linea, borderline, combined with the stem – gona, used to form generic names in this family. The name refers to the occurrence of the genus on both sides of the border between the USA and México. The gender of the name is feminine.

Notes: Upon initial dissection the male holotype of L. culmenicola n. sp. struck me as being nearly a perfect intermediate between species of Cleidogona and Pseudotremia , albeit with the soma of Cleidogona . Closer study revealed that the colpocoxites were not fused as they originally appeared, and that the gonopods resembled a more elaborate version of those of L. chisosi . Nevertheless, the presence of this genus geographically between the Mexican center of diversity for Cleidogona and the more northerly distribution of Pseudotremia is suggestive. I ( Shear 1972, 2003, 2010) emphasized the close relationship between Cleidogonidae and Trichopetalidae , and on that basis postulated that Pseudotremia , which more closely resembles trichopetalids, represented the basic form of North American cleidogonid. This basal stock may have been extinguished in central México by the opening of the Cretaceous seaway, after having invaded south to Central America ( Solaenogona ) and expanding north into North America ( Pseudotremia ). In this model, Cleidogona originated in the Paleogene in Central México and rapidly spread both north and south ( Shear 1972). Now I would add that Lineagona gen. nov. may represent a relict of the pathway from a Pseudotremia -like ancestor to Cleidogona , or perhaps a relict of their common ancestral stock. At some point in cleidogonid studies, it is hoped that mtDNA data amenable to the application of a molecular clock might be obtained. Along with information from nuclear genes, both a phylogeny and time line for evolution of the family might be constructed.