Novel Insect Leaf-Mining after the End-Cretaceous Extinction and the Demise of Cretaceous Leaf Miners, Great Plains, USA

Abstract

Plant and associated insect-damage diversity in the western U.S.A. decreased significantly at the Cretaceous-Paleogene (K-Pg) boundary and remained low until the late Paleocene. However, the Mexican Hat locality (ca. 65 Ma) in southeastern Montana, with a typical, low-diversity flora, uniquely exhibits high damage diversity on nearly all its host plants, when compared to all known local and regional early Paleocene sites. The same plant species show minimal damage elsewhere during the early Paleocene. We asked whether the high insect damage diversity at Mexican Hat was more likely related to the survival of Cretaceous insects from refugia or to an influx of novel Paleocene taxa. We compared damage on 1073 leaf fossils from Mexican Hat to over 9000 terminal Cretaceous leaf fossils from the Hell Creek Formation of nearby southwestern North Dakota and to over 9000 Paleocene leaf fossils from the Fort Union Formation in North Dakota, Montana, and Wyoming. We described the entire insect-feeding ichnofauna at Mexican Hat and focused our analysis on leaf mines because they are typically host-specialized and preserve a number of diagnostic morphological characters. Nine mine damage types attributable to three of the four orders of leaf-mining insects are found at Mexican Hat, six of them so far unique to the site. We found no evidence linking any of the diverse Hell Creek mines with those found at Mexican Hat, nor for the survival of any Cretaceous leaf miners over the K-Pg boundary regionally, even on well-sampled, surviving plant families. Overall, our results strongly relate the high damage diversity on the depauperate Mexican Hat flora to an influx of novel insect herbivores during the early Paleocene, possibly caused by a transient warming event and range expansion, and indicate drastic extinction rather than survivorship of Cretaceous insect taxa from refugia.

A revised checklist of Nepticulidae fossils (Lepidoptera) indicates an Early Cretaceous origin

“…• # 29 Nepticulidae: incertae sedis Juglandiphyllites DT91, DT105 Insect-feeding damage; Donovan et al 2014: fig. 3j, fig …”

“…• # 33 Nepticulidae: incertae sedis Browniea DT91 Insect damage; Donovan et al 2014 The host family Cornaceae (including Nyssaceae, to which Browniea was originally assigned) is used as a host by some Ectoedemia in North-America (Doorenweerd et al 2015) and undescribed Acalyptris species in Asia (data EJvN). However, the mines of this record are different from the mines of extant Cornaceae-feeding species.…”

“…• # 34 Nepticulidae: incertae sedis Dicot morphotype 1 DT36, DT91 Insect damage; Donovan et al 2014: fig. 15a- figure 15 shows what seems to be an exquisitely preserved egg-capsule.…”

“…• # 35 Nepticulidae: incertae sedis Cercidiphyllum DT41 Leaf mines; Donovan et al 2014: fig. 10h- Crane and Jarzembowski (1980) stated that this mine is similar to an unidentified species of Stigmella on Quercus cerris L..…”

“…Fossils of adult Lepidoptera are rare, but recently fossils of leaf mines frequently have been encountered Sohn et al 2012;Donovan et al 2014). However, identifying leaf-mining taxa from traces, or larval mediated damage on fossilized leaves requires an alternative approach, when compared to the identification of extant species.…”