Beetle larvae with unusually large terminal ends and a fossil that beats them all (Scraptiidae, Coleoptera) (pdf)

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Beetle larvae with unusually large terminal ends and a fossil that beats them all (Scraptiidae, Coleoptera)

Beetle larvae with unusually large terminal ends and a fossil that beats them all (Scraptiidae, Coleoptera) Joachim T. Haug1,2 and Carolin Haug1,2 1 Department of Biology II, Ludwig-Maximilians-Universität München, Planegg-Martinsried, Germany 2 GeoBio-Center at LMU, München, Germany ABSTRACT Submitted 15 May 2019 Accepted 11 September 2019 Published 14 October 2019 Corresponding author Joachim T. Haug, Academic editor Joseph Gillespie Additional Information and Declarations can be found on page 19 DOI 10.7717/peerj.7871 Copyright 2019 Haug and Haug Distributed under Creative Commons CC-BY 4.0 Larvae, and especially fossil larvae, are challenging to deal with from a purely taxonomic view. Often one cannot determine which species the larvae belong to. Yet, larvae can still contribute to various scientiﬁc questions. Especially morphological traits of a fossil larva can be highly informative for reconstructing character evolution. Also the occurrence of speciﬁc larval types and larval characters in time and the disappearance of such forms can well be reconstructed also without being able to narrow down the phylogenetic relationship of a larva very far. Here, we report two new beetle larvae preserved in Baltic amber which are identiﬁed as representatives of Scraptiidae, based on an enlarged terminal end (‘9th abdomen segment’); this is only the third record of such larvae. In comparison to modern forms, the terminal ends of the two new fossil larvae is even larger in relation to the remaining body than in any known larva. Unfortunately, our knowledge of such larvae in the modern fauna is very limited. Still, one of the two already known fossil larvae of Scraptiidae also has a very long terminal end, but not as long as those of the two new fossils. These three fossil larvae therefore seem to possess a speciﬁc morphology not known from the modern fauna. This might either mean that they (1) represent a now extinct larval morphology, a phenomenon well known in other euarthropodan lineages, or that (2) these forms represent a part of the larval phase not known from modern day species as they have not been described yet; such cases occur in closely related lineages. In any case, the fossils expand the known diversity of larval morphologies. Subjects Entomology, Evolutionary Studies, Paleontology, Zoology Keywords Larval diversity, Baltic amber, False ﬂower beetle, Combinatorial morphospace, Fossil larva INTRODUCTION Zoological research is in general heavily centred around adult individuals. There are, of course, exceptions to this, e.g. embryology or evo-devo, but for many sub-ﬁelds of zoology this is deﬁnitely the case (Minelli et al., 2006). This may be coupled to the fact that in many cases zoological thinking is focussing on taxonomic units, mostly species. Adults can much easier be identiﬁed to species level than non-adults as even in the age of DNA barcoding morphological characters are still used as the major tool for identifying specimens. How to cite this article Haug JT, Haug C. 2019. Beetle larvae with unusually large terminal ends and a fossil that beats them all (Scraptiidae, Coleoptera). PeerJ 7:e7871 DOI 10.7717/peerj.7871 This situation is unfortunate as immatures, especially larvae (see Haug, 2018 for challenges of this term), represent an important part of the life time of an organism and often fulﬁl a different ecological role than the adult. As zoological research is adult centred we often lack such information on larvae. This does not only account for modern day metazoans, but also for fossil representatives. Though morphology is still the prime method for identifying extant and fossil species, it can also be used for other aspects of an animal. It is possible to recognise a larva as something special, even without being able to narrow down its taxonomic identity very far. Especially for representatives of Insecta and their relatives there are numerous examples of larvae that cannot be easily taxonomically treated, but still provide important information for various zoological questions: 1. A larva may possess an unusual, so far unknown or unrecognised overall morphology (Williamson, 1960; Henry, 1978; Gamô, 1979; Martin & Ormsby, 1991; Chen et al., 2014; Haug & Haug, 2014; Haug et al., 2016a; Rudolf, Haug & Haug, 2016). 2. A larva may possess a combination of characters so far unknown or unrecognised for a speciﬁc larval stage (Villamar & Brusca, 1988; Lindley et al., 2002; Haug et al., 2016b). 3. Larvae may possess much more variability among each other than expected based on the variability of the adults (De Beer, 1958; Lang, Krapp & Melzer, 2007; Hübner et al., 2017). 4. Fossil larvae may provide minimum ages for speciﬁc larval types known from the modern fauna (Maisey & De Carvalho, 1995; Kadej & Háva, 2011; Waloszek & Dunlop, 2002; Briggs et al., 2005; Pohl, 2009; Wang & Zhang, 2010; Zhang et al., 2010; Makarkin, Wedmann & Weiterschan, 2012; Haug, Wiethase & Haug, 2015; Haug, Martin & Haug, 2015; Serrano-Sánchez et al., 2016; Néraudeau et al., 2017; Haug, Müller & Haug, 2018; Pérez-De La Fuente et al., 2018). This may well also represent a case of the oldest representative of a lineage, i.e. provide a taxonomic or phylogenetic signal as well. 5. Fossil larvae may possess more plesiomorphic characters no longer represented in the modern fauna, providing important clues for reconstructing character evolution (Müller & Walossek, 1986a, 1986b; Walossek & Müller, 1990; Wang, Ponomarenko & Zhang, 2009; Godunko, Staniczek & Bechly, 2011; Haug & Haug, 2013, 2015, 2016; Haug et al., 2013a, 2015; Badano et al., 2018). 6. Fossil larvae of such more ancestral forms (under point 5) may persist longer in time than at ﬁrst expected (‘morpho-type survival’ in the sense of Haug et al., 2012) extending the range of such larval morphologies (Kukalova-Peck, 1978; Godunko, Staniczek & Bechly, 2011; Haug, Haug & Garwood, 2016). 7. Fossil larvae may possess characters today only known from different modern lineages, but not occurring together in the same specimen, i.e. represent a speciﬁc combination unknown today (Badano et al., 2018; Haug et al., 2019; Haug, Müller & Haug, 2019). 8. Larvae may possess unusually sized body parts. Unlike most of the cases above, which are easily recognisable based on qualitative character, this refers to quantitative differences (Haug, Müller & Haug, 2019, in review a, in review b). Haug and Haug (2019), PeerJ, DOI 10.7717/peerj.7871 2/25 Here, we report two unusual appearing beetle larvae preserved in Baltic amber, representing a case 8. The terminal end is unusually large compared to that in modern day forms. We use a quantitative approach to evaluate the exceptionality of this ﬁnd. MATERIALS AND METHODS Material Two specimens preserved in amber were bought from an amber trader from Vilnius, Lithuania (ambertreasure4u.com). Amber pieces were already prepared to a high quality when bought. Specimens are now deposited in the (...truncated)