Archaeomalthus synoriakos Yan et al., 2019
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publication ID |
https://doi.org/ 10.1080/08912963.2018.1561672 |
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https://doi.org/10.5281/zenodo.5943535 |
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persistent identifier |
https://treatment.plazi.org/id/038E878E-2249-2211-F3F2-FD35FE90F835 |
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Plazi |
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scientific name |
Archaeomalthus synoriakos Yan et al. |
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gen. et sp. nov. |
† Archaeomalthus synoriakos Yan et al. , gen. et sp. nov.
( Figs. 1 View Figure 1 and 2 View Figure 2 )
Diagnosis
As for the genus.
Derivation of name
The Greek-specific name ‘synoriakos’ – ‘borderline’, refers to the proximity of Babiy Kamen`locality to Permian-Triassic boundary.
Material
Holotype: PIN 4887/102 , single specimen, counter print of the complete body impression with antennae and all legs missing. Repository: Paleontological Institute, Russian Academy of Sciences (curator Prof. Dr A.P. Rasnitsyn).
Occurrence
Babiy Kamen`locality, Tom`river, Kuznetsk Basin, S. Siberian, in the upper part of the Maltsevo Formation .
Description
Head slightly, ca. 0.23 mm, long and about as wide at the ocular region. Compound eyes round, located at anterior third of head, half as long as temporal region. Antennal insertion areas round, with outer margin bordered by fine carina. Antenna with small scapus, larger pedicellus with slightly rounded edges, and stout flagellomeres, apparently moniliform. The arrangement of apical mandibular teeth not recognizable. Prementum rather large, at least as wide as eye.
Anterior angles of pronotum blunt, not protruding forward; pronotal disc distinctly convex; anterior pronotal margin with median subtriangular depression; posterior angles slightly protruding laterally. Propleuron narrow, triangular, obliterating on anterior third of prothorax.
Elytra with fine lateral carina; epipleuron widest near humeral bulge, moderately wide on basal half, strongly narrowed posteriorly.
Mesepimeron oblique, with slightly sinuated anterior and posterior margins. Mesocoxal pits closed laterally by mesepimeron and metanepisternum. Metanotum with distinct alacristae. Metanepisternum parallelogram-shaped. Metaventrite about as long as width between anterolateral edges, posterior edge about 1.5 times as wide.
Procoxae transverse, mesocoxae almost globular, with inconspicuous lateral extension. Metacoxae short, with mesal part posteriorly rounded and extending through approximately two-thirds of first abdominal ventrite, only slightly narrowing laterally.
Sternites II-IV distinctly longer than V-VII, gradually narrowing towards the abdominal apex. Short sternite VII with round median structure with unclear homology (apparently glandular opening). Terminal sternite VIII as long as VI and VII combined, with rounded hind margin. At least posterior abdominal sternites with narrow laterosternites..
Discussion
Taxonomic position of Archaeomalthus
In contrast to specimens of Micromalthus described from Miocene amber,which were not very different from the recent M. debilis Hörnschemeyer et al. 2010 , Permian Archaeomalthus shows distinct differences compared to the Recent species and Cenozoic fossils of the family. Nevertheless, the affinities of extant Micromalthidae are suffi- cient for justified placement in this family. This includes a number of diagnostic features: a small and slender body, the large relative size of the head capsule, and well-developed, round compound eyes. Clear apomorphies shared with fossil and extant Micromalthus are shortened, quadrangular and posteriorly truncated elytra, leaving abdominal tergites V-VIII exposed, and the increased number of abdominal sternites, in contrast to five in ancestral Permian species (Stem group Coleoptera ) and the vast majority of extant beetles ( Ponomarenko 1969; Beutel et al. 2008; Lawrence et al. 2011). Additional potential apomorphies are the fusion of the labrum and clypeus, the long subparallel temporal region, the stout antennal flagellomeres, the semioval scutellar shield ( Figure 1 View Figure 1 (b)), and probably, weak sclerotization of the body. Considering the placement in Archostemata and the ground plan of this suborder (and Coleoptera s.l. ( Beutel 1997; Beutel et al. 2008; Friedrich et al. 2009)), additional apomorphies shared with Micromalthus are the absence of cuticular tubercles and scales, the complete lack of dorsal cephalic protuberances, the absence of a constricted neck region, evenly sclerotized elytra lacking window punctures, the absence of the transverse ridge on the mesoventrite, the concealed metatrochantins ( Beutel et al. 2008; Hörnschemeyer 2009) ( Figures 1 View Figure 1 and 2 View Figure 2 (m,n)) and abdominal glands ( Figures 1 View Figure 1 (b) and 3)
Shortened antennae, wings and legs of a Miocene micromalthid (lately Micromalthus anansi was synonymized with M. debilis by ( Hörnschemeyer et al. 2010)) were considered as a step of progressing morphological simplification of adults ( Perkovsky 2007). However, this term is problematic, as these features have obviously nothing to do with regressive character conditions occurring in beetles (e.g. Rempel and Church 1965, 1969, 1971; Kühne 1972; Kobayashi et al. 2013). It appears likely that small size, shortened elytra, and weakly sclerotized cuticle are related to the short lifespan and sporadic occurrence of adults, and their ‘vestigial’ status in the context of reproduction ( Pollock and Normark 2002; Hörnschemeyer et al. 2010). Relatively short legs are also common in other extant archostematan beetles including Permian forms (e.g. Ponomarenko 1969; see also Yan et al. 2017a) and moniliform antennae characterized as a plesiomorphic character by Perkovsky (2007).
One of several distinct features of Archaeomalthus differing from Micromalthus is the presence of an exposed propleuron separated by distinct sutures. This is clearly a plesiomorphy corresponding to the autapomorphic fusion of all prothoracic sclerites in extant or Cenozoic species (e.g. Beutel et al. 2008; Friedrich et al. 2008). Two other conspicuous features are the presence of a very distinctly delimited groove on the mesoventrite and a corresponding anteromedian process of the metaventrite. Both features resemble conditions commonly found in Adephaga, notably the aquatic groups (excluding Gyrinidae ) and basal grade Carabidae ( Beutel 1992) . However, a placement of Archaeomalthus in these suborders can be clearly excluded based on the short metacoxae without enlarged coxal plates, the lack of a prosternal process, and the increased number of abdominal sternites (e.g. Beutel 1992; Beutel and Haas 2000). The strongly developed process of the metaventrite, which widely separates the mesocoxae, is uncommon in recent Archostemata , even though it occurs in some extinct groups of this suborder, such as Schizophoridae (e.g Tersus Martynov, 1926 ) or Ademosynidae ( Yan et al. 2017b) . It is likely that this condition has evolved several times independently, in scattered groups of Archostemata including Micromalthidae , and also in Adephaga (e.g. Beutel 1986, 1992; Beutel and Haas 2000). In contrast to Archaeomalthus , the thoracic venter of Micromalthus appears distinctly simplified ( Figures 2 View Figure 2 (g–j,m,n)). It is conceivable that this is a reduction linked with the very limited role of the adults in the life cycle of the extant species.
Despite the marked differences between Archaeomalthus and Micromalthus , the former already show a distinct and apparently very stable degree of deviation from the ground plan of Coleoptera s.str. and s.l. (e.g. Beutel 1997). The remarkable series of reductional features shared by the Permian fossil and the extant species include a lightly sclerotized body without any distinct surface sculpture, a head lacking dorsal protuberances, shortened and apparently thin elytra, exposed membranous areas behind the procoxae and between at least two basal visible abdominal sternites, and the reversal of the invagination of the terminal abdominal segments. It is likely that the reduced role of adults of Micromalthidae ( Perotti et al. 2016) was a syndrome of vestigial characters acquired at least 255 million years ago.
Evolutionary stability of Micromalthidae
There are two main competing hypotheses to explain clades that accumulate phenotypic variation at a slow rate: the first one invokes genetic and developmental constraints that restrict the production of phenotypic variation ( Smith 1981; Raff 1996; Gould 2002). The second explanation is that stasis is a result of stabilizing selection ( Charlesworth et al. 1982; Kirkpatrick 1982; Estes and Arnold 2007). Exceptional evolutionary stability, bradytely, or ‘arrested evolution’ ( Simpson 1944) of Micromalthidae , resulted in very slow morphological change over time, which characterizes ‘living fossils’. On principle, this could be due to very long periods of stable environmental conditions ( Hörnschemeyer et al. 2010, see historical analysis of bradytely in Clarke and Chatzimanolis 2009). In the case of Micromalthidae this is the subcortical space of fungus-infested wood, which may have been one factor that buffered micromalthids from strong selection for morphological change ( BuslovClarke and Chatzimanolis 2009). Almost any change, but particularly a major change, of the phenotype in such a well-balanced system will be deleterious. Although minor gene substitution may be frequent, the well-buffered system of developmental canalizations shields the phenotype from major changes. There is an opportunity for speciation, but a major alteration of the morphotype is impossible as long as the epigenotype is intact ( Mayr 1970).
Association with prokaryotic endosymbionts or parasites can result in genome reduction and also an overcomplicated lifecycle including hypermetamorphosis and parthenogenesis. A long co-evolution with Wolbachia Hertig, 1936 is assumed in the case of M. debilis ( Grimaldi and Engel 2005; Perkovsky 2012). One of the results is apparently the ‘ghost sex life’, linked with a marginalization and vestigialization of the ‘ghost adults’ ( Perotti et al. 2016). It is likely that the distinctly simplified adult morphology of Micromalthus is closely linked with the obsolete role in the life cycle, and that this applies also to the lesser but already distinct reduction in Archaeomalthus . The apparent stability of morphological simplification in the adult stage suggests a very old origin of a specialized mode of reproduction and possibly an equally old association with endosymbiotic organisms. Selective pressure on mechanical protection and a highly efficient locomotor apparatus in ‘ghost adults’ is likely minimal. Apparently, the resulting structural simplifications are irreversible once accomplished, even though the sex-life of adults can be revived as shown by Perotti et al. (2016). In the case of Micromalthidae , the increasing degree of reduction from the Permian to the Miocene is likely a matter of reduced investment in adult structures, and therefore a result of ‘evolutionary economy’.
A side effect of an association with symbionts can be a reduced ability to survive overheating ( Dunbar et al. 2007; Perkovsky 2012). It is likely that the immediate ancestors of extant micromalthid species were also confined to extratropical areas, while the climate of Western Europe in the Ypresian (Early Eocene) was undoubtedly macrothermal, which is confirmed by, among other things, the absence of any Holarctic elements in the amber, in particular, in the myrmecofauna of Oise ( Aria et al. 2011; Perkovsky 2016).
There are no specific paleoclimatic studies on Babiy Kamen`locality. However, some conclusions about climate during sediment accumulation could be made on the basis of paleofloristic analyses. The Kuznetsk Basin is mostly coal bearing, comprising industrial reserves of hard and brown coal. The process of coal-forming occurred on the former territory of a Carboniferous shallow marine bay, which was replaced by rather swampy lowland plains during the Permian. Alteration of coal and paleosols indicate sediment accumulation in rather humid environments, necessary for the increased accumulation of coal-forming biomass. For the upper part of the section, in which the fossil insects have been found (Dr E. Karasev PIN RAS, 2018 pers. comm. to DV), it is possible to assume alteration of humid and arid climatic conditions. Temperatures were not yet reconstructed for Babiy Kamen`locality. However, significant abundance of conifers (e.g. Quadrocladus, Elatocladus, Voltzia ) is reported from the upper part of the Maltsevo Formation (including insect-bearing layers) ( Betechtina et al. 1986). It was shown for Permian-Triassic deposits of the Severodvinian Basin that climatic cooling was also characterized by an increased proportion of conifers in Babiy Kamen`locality ( Krassilov and Karasev 2009).
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