Rust Flies, Psilidae, are small-to-medium sized Dipteran ('True') Flies with elongate bodies and rounded heads, found largely in temperate regions of the Northern Hemisphere. Their larvae are parasitic on Plants, and some species, such as the Carrot Fly, Chamaepsila rosae, are considered to be significant agricultural pests. The Family Psilidae is divided into three subfamilies, the Chylizinae and Psilinae, which are considered to be sister groups, and the Belobackenbardiinae, which forms a sister to the other two groups combined. The oldest known fossil Rust Fly is Electrochyliza succini from Baltic Amber, which is probably Late Eocene in age. Electrochyliza succini is not placed within any extant subfamily, being considered to be a sister taxon to Chylizinae and Psilinae closer than Belobackenbardiinae, which suggests it appeared after the ancestor of these groups split from the Belobackenbardiinae, but before they had themselves diverged.
In a paper published in the journal Insect Systematics and Diversity on 13 November 2024, Andrew Ross of the Department of Natural Sciences at National Museums Scotland, Jiale Zhou of the Key Laboratory of Zoological Systematics and Evolution at the Institute of Zoology of the Chinese Academy of Sciences and the Department of Entomology at the China Agricultural University, Christel Hoffeins of Hamburg in Germany, and Bill Crighton, also of the Department of Natural Sciences at National Museums Scotland, describe a new species of Chylizine Rust Fly from the Late Eocene Insect Limestone Bed of the Bembridge Marls of the Isle of Wight, England.
The Insect Limestone is a single bed of fine-grained micrite found within the Bembridge Marls member of the Bouldnor Formation on the Isle of Wight. This bed is noted for the production of exquisitely preserved Insects, which often show three-dimensional preservation, for which reason it is sometimes known as 'opaque amber'. This bed has been precisely dated to 34.3 million years before the present.
The new species is described upon the basis of a single specimen collected by George William Colenutt, a prominant Isle of Wight solicitor and amateur geologist, and donated to the Oxford University Museum of Natural History in 1931. The species is placed in the genus Chyliza, the sole genus in the subfamily Chylizinae, on the basis of comparison of its wing venation to that of living members of the genus, and named Chyliza colenutti in honour of the collector.
The specimen is preserved in a split block as part and counterpart, and has a preserved body length of 3.9 mm and a maximum abdomen width of 1 mm. The forewing is 3.6 mm in length and 1.4 mm wide. All of the wings appear coloured towards their tips.
Prior to the description of Chyliza colenutti, the oldest described Chylizine Rust Fly was a Middle Miocene specimen identified as Chyliza sp. from the Middle Miocene Upper Freshwater Molasse of southwestern Germany, which is thought to be between 15 and 12 million years old.
The dating of Baltic Amber can be problematic, as it ranges from Middle Eocene to Middle Oligocene in age (i.e. is between 48 and 27 million years old), and tends to reach collections via amber dealers, often trading hands several times before coming to the attention of a scientist, with any data on the location and age of the specimens often being lost. However, the majority of Baltic Amber comes from the Upper Blue Earth Member of the Prussian Formation, which has been dated to between 36.5 and 33.5 million years old.
This gives a gap of around 20 million years between Electrochyliza succini and Chyliza sp., which led many palaeoentomologists to conclude that there was a long gap between the divergence of the Belobackenbardiinae from the common ancestor of the Chylizinae and Psilinae, and the splitting of these two groups, particularly as no specimens assigned to either of these groups have been found in Insect-rich deposits such as the Dominican and Mexican ambers. However, if Ross et al.'s interpretation of Chyliza colenutti as a Chylizine is correct, then the group was present 34.2 million years ago, making it unlikely that it is more than two million years younger than Electrochyliza succini.
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