Thursday 29 December 2022

Yprezethinus grimaldii: A Pselaphine Rove Beetle from Early Eocene Cambay Amber.

The Indian Plate separated from Madagascar about 88 million years ago, and collided with Eurasia between 60 and 45 million years ago. Since this time, there has been significant floral and faunal exchange between India and Eurasia, but during the period of isolation, the wildlife of India should, at least in theory, have been closer to that of Madagascar and Africa. However, fossils of African affinities from India during this interval are actually quite rare, while many appear to be more closely related to groups from Europe.

Cambay Amber, from Gujarat State, India, has been dated to about 54.5 million years ago, making it Ypressian (Early Eocene) in age, i.e. dating from the period during which India was colliding with Eurasia. This deposit has yielded a rich assemblage of Insects and other small Arthropods, although the majority of these show Laurasian affinities, and the overall Insect assemblage resembling that of the slightly younger Baltic Amber of northern Europe. Groups with affinities to Eurasian faunas include Electrapine and Melikertine Bees, Lygistorrhinid Flies, and a variety of Termites, while Africa-linked groups include Scelembid Webspinners, and a Paleoamblypygid Whipspider.

In a paper published in the journal Palaeoentomology on 22 September 2022, Joseph Parker of the Division of Biology and Biological Engineering at the California Institute of Technology, and the Division of Invertebrate Zoology at the American Museum of Natural History, describes a new species of Pselaphine Rove Beetle from Cambay Amber, with clear affinities to species from tropical Africa.

The Beetle is assigned to a new species and genus, and given the name Yprezethinus grimaldii, where 'Yprezethinus' derives from 'Ypressian' plus 'Zethinus', a modern genus of Pselaphine Rove Beetle which it resembles, and 'grimaldii' honours the American palaeoentomologist David Grimaldi on the advent of his 65th birthday.

Yprezethinus grimaldii Holotype AMNH Tad-130. (A) Habitus with visible abdominal sternites labelled. (B) Head and pronotum with segments of maxillary palpomeres numbered MP2–MP4. Bythinoplectina-type lateral excavation, eye, apical  pseudosegment of maxillary palp, APs, and mandible, Md, indicated. (C) Right antenna with antennomeres numbered. Note the  club formed from tightly appressed, hemisphere-shaped antennomeres 10 and 11. D, Close-up view of apically globose tubercles  of maxillary palpomeres 3 and 4. (E) Proximal leg articulation, showing conically projecting metacoxa, MC, short metatrochanter, MT, and femur, F. (F) Right metatarsus, F with bythinoplectine-type enlarged third tarsomere spanning distance between arrowheads, and minute first visible tarsomere. Parker (2022).

Yprezethinus grimaldii is about 0.8 mm in length with a flattened body and a glossy black carapace. It is assigned by Parker to the subtribe Bythinoplectina, which is found today in leaf-litter in tropical forests, and considered to be closely related to the modern Zethinus, which is found in the tropical forests of West-Central Africa, from Côte d’Ivoire south to Angola. 

Interestingly, many modern Pselaphine Rove Beetles are considered to be Ant-associated, and while examples of them actually living in Ant's nests are rare, they certainly appear to do better in environments where the leaf-litter community is dominated by Ants. The oldest abundant Ant fossils come from Middle Cretaceous Amber from Kachin State, Myanmar ('Burmese Amber'), but these Ants are quite unlike modern Ants, and no members of any (modern) Ant-associated group are known from Kachin Amber. Cambay Amber represents one of the earliest known ecosystems dominated by this modern Ants, making the discovery of a potentially Ant-related Beetle here significant.

Many of the flying Insects from Cambay Amber appear to be more closely related to Eurasian than African groups. Pselaphine Rove Beetles are, in contrast, poor fliers, as are Webspinners (a group in which, in modern species at least, only the males can fly); Whipspiders are wingless Arachnids and do not fly at all. Thus the flying Insect assemblage at Cambay appears to have been replaced by species from Eurasia, while more terrestrial groups still show African affinities. 

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