Proisotoma communis, Sánchez-garcía & Engel, 2016

Sánchez-garcía, Alba & Engel, Michael S., 2016, Springtails from the Early Cretaceous Amber of Spain (Collembola: Entomobryomorpha), with an Annotated Checklist of Fossil Collembola, American Museum Novitates 2016 (3862), pp. 1-48 : 10-18

Proisotoma communis , new species

Figures 5–6

Micranurida ? sp. Simón-Benito et al., 2002: 85: fig. 1a.

Onychiurus ? sp. Simón-Benito et al., 2002: 85: fig. 1b.

Anurophorus ? sp. Simón-Benito et al., 2002: 85: figs. 1c, d.

Proisotoma (Ballistura) ? sp. Simón-Benito et al., 2002: 85: figs. 1e–g.

Cryptopygus ? sp. Simón-Benito et al., 2002: 87: figs. 2a–d.

DIAGNOSIS: The new species has characteristics of the genus and differs from its congeners by the greatly swollen fourth antennomere; the fourth abdominal segment one and a half times the length of the third; the absence of clavate tibiotarsal setae; the simple ungues and unguiculi; the rather slender and elongate dens that is dorsally tuberculate and ventrally bearing thick setae arranged in pairs; and the small, bidentate mucro.

DESCRIPTION (largely based on the type series): Total body length highly variable; body length of holotype 539 µm; body length of paratypes 514–560 µm. Body slender, not thickened in posterior half, about 4.82× as long as wide, of typical isotomid shape. Dorsal integument smooth; all body setae smooth.

Head length 145 µm, about 0.26× length of body; largely with long, somewhat curved, slender setae; antennae with four antennomeres, with few slender, mostly weakly curved and acuminate, short setae; fourth antennomere swollen, length 46 µm, longer than combined lengths of third and second antennomeres; third and second antennomeres subequal in length, 19 and 19 µm, respectively; first antennomere shortest, usually obscured by head. Eyes poorly visible, with at least four ommatidia. Postantennal organ not visible (perhaps not present).

Thorax length 113 µm, about 0.20× length of body, prothoracic segment indistinct; meso- and metathoracic segments subequal in length; densely covered with short, straight to somewhat curved, slender setae.

Legs usually poorly visible, and often obscured by body; trochanter, femur, and tibiotarsus subequal in length, tibiotarsus with a few short, weakly curved, acuminate setae, apparently not clavate nor truncate; unguis simple, without serration or denticles, rather small, nearly straight and acuminate; unguiculus simple, half length of unguis, nearly straight and acuminate.

Abdomen length 300 µm, about 0.54× length of body, not swollen, only slightly wider than thorax; abdominal segments not fused; fourth abdominal segment 1.55× length of third segment; lengths of abdominal segments III–VI in µm, approximately: III, 53; IV, 82; V, 37; VI, 33. Abdominal segments densely covered with short, straight to somewhat curved, slender setae, as well as some longer setae on distal part of abdomen.

Furcula long and slender; manubrium length 22 µm, with several scattered setae; dens length 79 µm, tapering distally, with numerous small tubercles dorsally, and thick ventral setae arranged in pairs; mucro small, generally poorly visible, bidentate, without lamellae.

HOLOTYPE: MCNA 9273.1 (fig. 5A), in piece with three specimens (2 paratypes, vide infra), the holotype visible in profile and virtually complete, total length 539 µm, and one disarticulated leg of a possible symphypleonan springtail. Preserved in a clear-yellow, turbid piece of amber trimmed to 0.90 × 0.40 × 0.05 cm (set into an epoxy trapezoid of dimensions 2.10 × 1.30 × 0.10 cm), and included with many arthropod and plant remains (e.g., stellate hairs).

PARATYPES: Four paratypes in total. Two paratypes, MCNA 9273.2 – 3 (figs. 5A, 5B) in same piece as holotype, both virtually complete, visible dorsally and ventrally, total lengths 540 ( MCNA 9273.2 , fig. 5A) and 556 ( MCNA 9273.3 , fig. 5B) µm, respectively; other details of piece provided under account of holotype .

One paratype, MCNA 9324 : Total length 560 µm, virtually complete, visible dorsally and ventrally, showing details of furcula. Preserved in a clear-yellow, turbid piece of amber trimmed to 0.25 × 0.10 × 0.05 cm (in a microscopic slide preparation), and accompanied by much debris and bubbles; the amber is darkened near the inclusion .

One paratype, MCNA 10070 : Total length 514 µm, virtually complete, visible dorsally and ventrally, showing details of body setae, segmentation, and ommatidia (at least four ommatidia visible). Preserved in a clear-yellow, turbid piece of amber trimmed to 0.25 × 0.15 × 0.05 cm (in a microscopic slide preparation), and accompanied by much debris and bubbles. Syninclusions include two springtails of the same morphotype now in MCNA 10071 , and a symphypleonan springtail (genus and species indeterminate) ( Sánchez-García and Engel, 2016) now in MCNA 10016 .

ADDITIONAL MATERIAL: MCNA 8969.1–2 (among which one was labeled as MCNA 8969a in Simón-Benito et al., 2002): Two specimens, one visible dorsally and ventrally, the other visible ventrally and laterally, total lengths 361 and 530µm, respectively; highly distorted but showing details of furcula and legs. Preserved in a clear-yellow, turbid piece of amber trimmed to 0.20 × 0.20 × 0.05 cm (set into an epoxy trapezoid of dimensions 2.05 × 1.30 × 0.10 cm), and included with fungal hyphae, and many arthropod and plant remains. Further syninclusions include one springtail of the same morphotype now in MCNA 9148.

MCNA 9148: Total length 563 µm, visible dorsally and ventrally, cleared but showing details of furcula (tubercles, setae, and small mucro). Preserved in a clear-yellow, turbid piece of amber trimmed to 0.25 × 0.20 × 0.05 cm (in a microscopic slide preparation). Syninclusions as for MCNA 8969.

MCNA 9162: Total length 739 µm, visible dorsally and ventrally, highly distorted, cleared but showing details of furcula (tubercles and setae). Preserved in a clear-yellow, turbid piece of amber trimmed to 0.40 × 0.15 × 0.05 cm (in a microscopic slide preparation), and included with fungal hyphae, and many arthropod and plant remains.

MCNA 9464.1 – 2 : Two specimens (total length of one specimen 418 µm, visible dorsally and ventrally, showing details of antennae and body setae; the other cleared and not measurable for its length, but showing details of legs and furcula, visible laterally); preserved together with disarticulated remains of a third springtail (disembodied head, antennae, and furcula), fungal hyphae, and many arthropod (e.g., scales) and plant remains (e.g., stellate hairs) in a clearyellow, turbid piece of amber trimmed to 0.55 × 0.20 × 0.05 cm (in a microscopic slide preparation). Further syninclusions include one springtail of the same morphotype now in MCNA 10061 and one fly ( Diptera ) now in MCNA 10062 .

MCNA 9612.1–7 (among which one was labeled as MCNA 9612a in Simón-Benito et al., 2002): Seven specimens of which six are virtually complete (total lengths of each: 265, 298, 331, 348, 391, and 484 µm), and one nearly complete; some specimens are cleared. Preserved together with disarticulared remains of further springtails of the same morphotype (a cleared body and two disembodied heads), plus one disarticulated leg of a possible symphypleonan springtail. Preserved in a clear-yellow, turbid piece of amber trimmed to 0.55 × 0.55 × 0.05 cm (set into an epoxy trapezoid of dimensions 1.05 × 1.10 × 0.10 cm), and included with fungal hyphae, and many arthropod and plant remains. Further syninclusions include a paratype of the bethylid wasp Cretepyris martini Ortega-Blanco and Engel (2013) ( Hymenoptera : Chrysidoidea), now segregated as piece MCNA 9613.

MCNA 10061: Total length 477 µm, visible dorsally and ventrally, highly distorted. Preserved in a clear-yellow, turbid piece of amber trimmed to 0.30 × 0.15 × 0.05 cm (in a microscopic slide preparation), and included with fungal hyphae, and many arthropod and plant remains. Syninclusions as for MCNA 9464.

MCNA 10040.28, 30–35, 37–49: Up to 20 specimens (total length ranging from 246–530 µm), virtually complete. Preserved in a thick, dark-orange, turbid piece of amber trimmed to 1.80 × 0.85 × 0.20 cm (set into an epoxy trapezoid of dimensions 1.80 × 0.85 × 0.45 cm), and included with fungal hyphae, and many arthropod and plant remains. Further syninclusions include 20 flies ( Diptera ), one wasp ( Hymenoptera ), two mites (Acari), one partial roach (Blattaria), and two jumping bristletails ( Archaeognatha ).

MCNA 10071.1 – 2 : Two specimens (total length of one specimen 408 µm, visible dorsally and ventrally; the other not measurable in length), showing details of body setae. Preserved in a clearyellow, turbid piece of amber trimmed to 0.15 × 0.15 × 0.05 cm (in a microscopic slide preparation), and accompanied by much debris and bubbles. Syninclusions as for MCNA 10070 .

MCNA 10744.2: Total length 418 µm, visible dorsally and ventrally. Preserved in a dark-orange, turbid piece of amber trimmed to 0.90 × 0.60 × 0.10 cm (set into an epoxy trapezoid of dimensions 2.10 × 1.50 × 0.20 cm), and accompanied by much debris, fungal hyphae, and plant remains (e.g., stellate hairs). Further syninclusions include the paratype of the scelionid wasp Amissascelio temporarius Ortega-Blanco et al. (2014) ( Hymenoptera : Platygastroidea: Scelionidae ).

MCNA 11231.2 – 46 : Up to 45 specimens (total length ranging from 186–597 µm, likely representing varied instars) among which 42 are virtually complete, and three are nearly complete; preserved together with disarticulated remains of several further springtails of the same morphotype (at least three disembodied heads, one furcula, and two partially preserved abdomens), a symphypleonan springtail (genus and species indeterminate) ( Sánchez-García and Engel, 2016), and much debris, fungal hyphae and plant remains (e.g., pollen) in a thick, darkorange, turbid piece of amber trimmed to 1.20 × 0.90 × 0.30 cm (set into an epoxy trapezoid of dimensions 2.10 × 1.40 × 0.30 cm).

MCNA 12609: Total length 371 µm, visible dorsally and ventrally, highly distorted. Preserved in a clear-yellow, turbid piece of amber trimmed to 0.10 × 0.10 × 0.05 cm (in a microscopic slide preparation), and accompanied by much debris and bubbles.

MCNA 12674.1: Total length 491 µm, visible in profile. Preserved in a dark-orange, turbid piece of amber trimmed to 0.90 × 0.70 × 0.15 cm (set into an epoxy trapezoid of dimensions 2.05 × 1.35 × 0.25 cm), and included with fungal hyphae, many arthropod and plant remains (e.g., stellate hairs), and one disarticulated roach (Blattaria). Further syninclusions include one fly ( Diptera ) now in MCNA 12675.

OCCURRENCE: Peñacerrada I amber site (Peñacerrada I = Moraza), Utrillas Group, eastern area of the Basque-Cantabrian Basin, Burgos, northern Spain; Early Cretaceous (Late Albian).

ETYMOLOGY: The specific epithet is taken from the Latin communis , meaning “common” or “universal,” and refers to the abundance of this species in Spanish amber.

REMARKS: The historical concept of the genus Proisotoma , as conceived by Gisin (1960), Fjellberg (1980), and others, has recently undergone significant revision, with many species reallocated to other genera ( Potapov, 2001; Potapov et al., 2006, 2009). The group has been subdivided at times into different subgenera, which are often raised to generic status ( Christiansen and Nascimbene, 2006; Potapov et al., 2006). Following Potapov (2001), the genus Proisotoma has generally been defined as containing all species of Isotomidae with: 1, a normal or slender habitus, ranging from rather small to large; 2, a sometimes weakly reticulate or wrinkled integument lacking secondary granulation; 3, ommatidia present; 4, antennae bearing a postantennal organ, but lacking an apical bulb; 5, empodia present; 6, clavate tibiotarsal setae present or absent; 7, , abdominal segments IV, V, and VI separate, and lacking anal spines; 8, a fully developed furcula, with the mucro separated from the dens; 9, a manubrium with a few setae on the anterior surface; 10, a usually stout dens, sometimes rather slender, crenulate, and continuously narrowed; 11, a mucro bi- or tridentate, without seta, and sometimes with lamellae; 12, setae usually short, macrosetae differentiated at least on the apicalmost abdominal segments; and, 13, ventromedial setae of the thorax present or absent.

The genus Proisotoma makes up 96% approximately of all the entomobryomorphans, and 87% of the whole collembolan record in Spanish amber. It is remarkable that, in spite of the great number of specimens, not a single individual displayed all the characters of the genus and species clearly, likely owing to the darkness and frequent debris in Spanish amber as well as the often dessicated nature of many specimens. Overall, the new species is distinguished by the morphology of the antennae and dens (the latter with characteristic chaetotaxy and tubercles), the small and bidentate mucro, simple ungues and unguiculi, and the relative proportions of the third and fourth abdominal segments (fig. 6). The number of ommatidia cannot be established exactly although at least four have been observed in some specimens. Intraspecific variability in these characters is low, and that variation observed mainly concerns the relative size of specimens.

Simón-Benito et al. (2002) described some of the specimens of P. communis , n. sp., as belonging to five genera in three families (see table 1): Onychiurus Gervais ( Poduromorpha : Onychiuridae ), Micranurida Börner ( Poduromorpha : Neanuridae ), and Anurophorus , Cryptopygus Willem , and Proisotoma ( Entomobryomorpha : Isotomidae ). However, after repreparing all the amber samples and examining further specimens of P. communis (not examined by Simón-Benito et al., 2002), it was revealed that they correspond to the same morphotype. Moreover, after suitable preparation it is clear that some structures of this species were not observed or were misinterpreted by Simón-Benito et al. (2002), accounting for their broad misidentifications. Their assignment of some specimens to the Poduromorpha ( D’Haese, 2003b) (genera Onychiurus and Micranurida ) clearly was unsupported based on numerous traits, most notably: 1, the greatly reduced prothorax that never bears setae (instead of well developed and bearing dorsal setae in the Poduromorpha ); and, 2, the fourth abdominal segment generally longer than the third segment (instead of subequal in size in the Poduromorpha ). Specimen MCNA 9162, classified as “ Onychiurus sp. ” by Simón-Benito et al. (2002), was briefly described as lacking a furcula when in fact a well-developed furcula showing details of setae and tubercles is actually present. In other cases, such as the three specimens classified as “ Anurophorus sp. ” by Simón-Benito et al. (2002) (one specimen in MCNA 10070, and two specimens in 10071), the inability to discern a furcula is due to the position of the specimens as fossilized rather than a real absence. While the lateral profile, observable in some specimens, appears to be ideal for seeing the furcula, even when it is appressed to the body, the structure is often not visible in some of the dorsoventrally exposed individuals. Apart from the absence of the furcula, the extant genus Anurophorus is diagnosed by the presence of an apical bulb on antennomere IV ( Potapov, 2001), which is absent in P. communis . The free abdominal segments V and VI (rather than fused) prevent attribution of the two specimens in MCNA 8969, three specimens in MCNA 9273, one specimen in MCNA 9324, and five specimens in MCNA 9612 to Cryptopygus . Remarkably, the separation of Cryptopygus and Proisotoma is far from clear, and Cryptopygus are mainly differentiated from the latter only by the fusion of abdominal segments V and VI ( Linnaniemi, 1912; Gisin, 1944; Palissa, 1964), and the genus as a whole is assuredly composed of various unrelated lineages (e.g., Rusek, 2002; Stevens et al., 2006a). Some authors rely on putative differences in the morphology of the dens, with those of Cryptopygus long and slender, rather than the shorter and stouter form found in Proisotoma ( Stach, 1947; Gisin, 1960; Fjellberg, 1980).

Proisotoma communis is the only known species of Cretaceous Spanish Collembola ascribable to an extant genus. Interestingly, the genus is also known in Burmese and Canadian ambers ( Christiansen and Pike, 2002a; Christiansen and Nascimbene, 2006), and as noted by those authors, it is possible that if finer details of the sensory structures were discernible, then the Cretaceous representatives might better be classified in a separate genus. However, in the absence of such data we have adopted the conservative position of considering them congeneric. Despite this, extant genera are not unheard of from Cretaceous ambers. Mesozoic representatives of still-surviving hexapod genera include examples from among the rove and bark beetles ( Cognato and Grimaldi, 2008; Chatzimanolis et al., 2013), zorapterans ( Engel and Grimaldi, 2002), biting midges ( Borkent, 2001; Szadziewski and Arillo, 2003; Pérez-de la Fuente et al., 2011), among others. Perhaps the most remarkable example is the genus Alavesia Waters and Arillo , originally described from fossils in Spanish amber ( Waters and Arillo, 1999; Peñalver and Arillo, 2007) and then recorded in Burmese amber ( Grimaldi et al., 2002), which was recently discovered alive and well in Namibia, southern Africa ( Sinclair and Kirk-Spriggs, 2010). Such bradytely is likely attributable to the conservatism and long-term consistency of their microhabitat preferences.

Genus and Species Indeterminate

Figure 7

Entomobryomorpha form 1 (fig. 7): Specimen MCNA 9560 is preserved in a piece of clear yellow amber trimmed to 0.90 × 0.50 × 0.05 cm (in an epoxy trapezoid of dimensions 2.15 × 1.30 × 0.10 cm), and without syninclusions. The specimen is observable laterally as well as dorsally and ventrally, but not much more than an external profile is visible, preventing suitable comparison. Nonetheless, the morphology of the antennae and legs correspond to a morphotype distinct from those described above. The body length as preserved is 407 µm. The head shape is cylindrical, distinctly narrower than the body, and 79 µm in length as preserved. The well-preserved antennae are slightly longer than the head, with four antennomeres; the first and fourth antennomeres are subequal in length; the second and third antennomeres are subequal in length, together not reaching the length of the first or fourth antennal segments; the fourth antennomere is slightly swollen, with a few slender, short, mostly weakly curved and acuminate setae; the antennomere lengths in µm are approximately: IV, 32; III, 11; II, 10; I, 30. The tibiotarsus and femur are subequal in length, and the trochanter is distinctly longer and inflated; some slender setae, apparently not clavate or truncate, are visible distally on the tibiotarsus, and although the unguis and unguiculus are not clear, they are apparently simple, without teeth, very short, and acuminate. Other features cannot be seen because of the poor state of preservation of this specimen. Hopefully further, more finely preserved, material will be discovered at a later date and permit a full characterization and identification of this form.

Figures

FIGURE 5. Microphotographs of the holotype and two paratypes of Proisotoma communis, new species, in Late Albian amber from northern Spain (images combining consecutive photographs taken at successive focal planes). A. Holotype (MCNA 9273.1) in lateral habitus, and paratype (MCNA 9273.2) in ventral habitus. B. Paratype (MCNA 9273.3) in dorsal habitus.

FIGURE 6. Camera lucida drawings of the holotype and two paratypes of Proisotoma communis, new species. A. Paratype (MCNA 9273.2) in ventral habitus. B. Holotype (MCNA 9273.1) in lateral habitus. C. Detail of right antenna of holotype (MCNA 9273.1). D. Paratype (MCNA 9273.3) in dorsal habitus. E. Ventral detail of furca of paratype (MCNA 9273.3).

FIGURE 7. Microphotograph in dorsal habitus of specimen MCNA 9560 (Entomobryomorpha form 1) (image combining consecutive photographs taken at successive focal planes).