Following the concept of Isotomidae as outlined by Folsom (1937a) and Potapov (2001), as well as the last suprageneric classification of Entomobryomorpha by Soto-Adames et al. (2008; whose treatment of Isotomidae followed Potapov, 2001), all the entomobryomorphan springtails from the Spanish Cretaceous amber are assignable to Isotomidae. Collectively, this is based on: 1, the absence of scales, trochanteral organs, and postocular trichobothria; 2, the relatively short antennae, with four antennomeres; 3, the fourth abdominal segment usually subequal to or longer than the third (erroneously cited as “shorter than the third” in Soto-Adames et al., 2008), with one never more than one and one-half times as long as the other; 4, the last two or three abdominal segments sometimes fused (e.g., in Folsomia Willem, Folsomina Denis, and Pectenisotoma Gruia); 5, trichobothria usually absent; and, 6, a general covering of simple setae, the largest of which are often serrate or fringed, but never clavate or fringed on all margins. In addition, Folsom (1937a) also considered the following characters as diagnostic for Isotomidae: 1, prothorax membranous and without setae (except in Guthriella Borner); 2, mesothorax not projecting over the head; 3, postantennal organs almost invariably present, each consisting externally of a simple tubercle; 4, pretarsi with an unguis and sometimes an unguiculus; 5, inner edge of unguis always simple, never basally split or doubled; 6, furcula present (except in Anurophorus Nicolet and its allies); 7, manubrium with setae, seldom bare ventrally; and, 8, integument generally smooth and with distinct sclerites, except in some taxa in which it can be tuberculate and with less distinctive sclerites. Most of the characters utilized by Potapov (2001) in his system are related to chaetotaxy. The present samples share with his characterization the absence of dorsal prothoracic setae, the body covered with abundant setae (i.e., polychaetotic), and a weak morphological chaetotaxic differentiation expressed by the plesiomorphic condition of two features: a weak differentiation of chaetalike components, and uniformity of the chaetotaxy of body segments.

Isotomidae are organized into three subfamilies: Isotominae Schärfer, 1896, Anurophorinae Börner, 1901, and Pachyotominae Potapov, 2001 (Potapov, 2001). The three species erected below can be placed among the Anurophorinae owing to the combination of: 1, absence of secondary granulation over the body; 2, chaetotaxy oligo- to polychaetotic; 3, empodium sometimes reduced or outright lacking; 4, tibiotarsi with seven setae in the distal ring; 5, manubrium with or without a few anterior setae; 6, well-developed dens that is continuously narrowed apically, with its posterior side crenulate or tuberculate; and 7, mucro, if present, usually bidentate. Anal spines may or may not be present among anurophorine taxa. As will be observed from the descriptions below, all the addressed genera have a combination of traits that best suits the Anurophorinae and are, in fact, close to the extant genus Proisotoma Borner (with one new species placed within that particular genus).

Burmisotoma Christiansen and Nascimbene, 2006: 340. Type species: Burmisotoma lamellifera Christiansen and Nascimbene, 2006, by original designation.

Diagnosis: Aside from the general traits of the genus (outlined by Christiansen and Nascimbene, 2006), the new species differs from the type species (Burmisotoma lamellifera Christiansen and Nascimbene) by the presence of a long, thick, and straight femoral spine, as well as a ventrally crenulate manubrium.

Description: Total body length as preserved 473 µm. Body slender, thickened in posterior half, of typical isotomid shape. Dorsal integument with distinct, fine reticulation; all body setae smooth.

Head length as preserved 122 µm, about 0.26× length of body; covered largely with sparse, somewhat curved, slender setae; antennae with four antennomeres; fourth antennomere not swollen, densely setaceous, with slender, mostly weakly curved, acuminate, long setae, accompanied by some thick, truncate, cylindrical, long setae on apical half of antennomere; first to third antennomeres mostly obscured preventing measurements. Ommatidia not visible. Postantennal organ not visible (likely not present).

Thorax highly distorted, maximum width 87 µm, without visible setae; prothoracic segment membranous; meso- and metathoracic segments subequal in length.

Legs with trochanter poorly visible, with a few short, weakly curved, acuminate setae; femur length 33 µm, bearing a thick and straight spine, about as long as femoral diameter; tibiotarsus length 43 µm, with several slender setae, setae apparently not clavate nor truncate; unguis and unguiculus not clearly visible, apparently simple, without serration or denticles; unguis length 23 µm, acuminate.

Abdomen swollen and somewhat ovoid, maximum width 153 µm, distinctly wider than thorax, with sparse, straight to somewhat curved, slender setae, such setae becoming longer and more numerous posteriorly; abdominal segments not fused; third and fourth abdominal segments subequal in length, fourth segment only slightly longer than third segment; lengths of abdominal segments III–VI in µm, approximately: III, 43; IV, 49; V, 27; VI, 20. Length, exclusive of appendages 107 µm.

Furcula long and slender (visible in lateral view); manubrium length 36 µm, ventrally crenulate, without visible setae; dens length 84 µm, tapering distally, without visible setae; mucro poorly visible, apparently bidentate.

Holotype: MCNA 12583 (fig. 1), virtually complete, visible dorsally, ventrally, and laterally. Preserved in a clear-yellow, turbid piece of amber trimmed to 1.05 × 0.80 × 0.05 cm (set into an epoxy trapezoid of dimensions 2.10 × 1.20 × 0.10 cm), and included with fungal hyphae, and many arthropod (e.g., scales) and plant remains (e.g., stellate hairs).

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 from the Latin spinula (meaning, “small spine”) and fero (meaning, “bear” or “carry”), and refers to the femoral spine distinctive of the species.

Remarks: The genus Burmisotoma was described originally from a single specimen in mid-Cretaceous amber from Myanmar (Burma). Burmisotoma spinulifera, n. sp., shares with the type species, B. lamellifera, a well-developed furcula lacking ventral manubrial setae, strongly tapered dentes, and the antennae bearing stout, cylindrical, truncate setae. Unfortunately, the ommatidia, details of the setae and tubercles of the dens, and the morphology of the mucrones were not clearly visible in B. lamellifera, preventing comparison of these structures with the present specimen. In the new species, the body shape (slender and thickened in the posterior half), general chaetotaxy of the body, antennal structure, and the tarsal complex are all very similar to B. lamellifera as illustrated by Christiansen and Nascimbene (2006); there- fore, the most reasonable is to consider them congeneric. However, B. spinulifera is remarkable in having a thick and straight femoral spine (fig. 2), and a ventrally crenulate manubrium. The dorsal reticulation of the integument is also distinctive of the new species. These features are, however, merely autapomorphic and the establishment of a separate genus based on these comparatively minor differences would serve only to proliferate the number of monospecific genera for otherwise quite similar Cretaceous springtails.

Protoisotoma Christiansen and Pike, 2002a: 171. Type species: Protoisotoma micromucra Christiansen and Pike, 2002a, by original designation.

Diagnosis: The new species agrees with the characters of the genus (as outlined by Christiansen and Pike, 2002a), but differs from its congeners by the longer third antennomere that is subequal to the fourth antennomere, and the fourth abdominal segment being subequal to or slightly longer than the third segment.

Description (based on holotype): Total body length as preserved 1273 µm. Body slender, not thickened in posterior half, about 5.12× as long as wide, of typical isotomid shape. Dorsal integument smooth; all body setae smooth.

Head length as preserved 192 µm, about 0.15× length of body, largely with long, somewhat curved, slender setae; antennae with four antennomeres; fourth antennomere swollen, length 107 µm, with a few slender, mostly weakly curved and acuminate, short setae; third antennomere about as long as fourth; first and second antennomeres mostly obscured by head, preventing measurements. Ommatidia not visible. Postantennal organ not visible (perhaps not present).

Thorax length 335 µm, about 0.26× length of body, maximum width 235 µm; prothoracic segment indistinct; meso- and metathoracic segments subequal in length; segments densely covered with short, straight to somewhat curved, slender setae.

Legs poorly visible except for protibiotarsus: tibiotarsus length 109 µm, with a few short, weakly curved, acuminate setae, apparently not clavate nor truncate; unguis simple, without serration or denticles, relatively long, length 58 µm, nearly straight and acuminate; unguiculus simple, half length of unguis (26 µm), nearly straight and acuminate.

Abdomen length 749 µm, about 0.59× length of body, not swollen, maximum width 255 µm, only slightly wider than thorax; abdominal segments not fused; third and fourth abdominal segments sub equal in length, fourth segment only slightly longer than third segment; lengths of abdominal segments III–VI in µm, approximately: III, 187; IV, 200; V, 76; VI, 43. Abdominal segments densely covered with short, straight to somewhat curved, slender setae, as well as some long, erect pseudotrichobothria.

Furcula appressed against body (and therefore poorly visible); manubrium without visible setae; dens long and slender, without visible setae; mucro minute, poorly visible.

Holotype: MCNA 12788.2 (fig. 3), virtually complete, visible dorsally and ventrally. Preserved in a clear-yellow, turbid piece of amber trimmed to 1.00 × 0.90 × 0.10 cm (set into an epoxy trapezoid of dimensions 2.20 × 1.50 × 0.20 cm), and accompanied by much debris and arthropod remains (e.g., scales). Syninclusions include three acari and the holotype of the symphypleonan Pseudosminthurides stoechus Sánchez-García and Engel (2016). Inclusions in piece MCNA 12788 and a further springtail plus three acari in piece MCNA 12787 were originally part of a single piece of amber that was divided into two fragments for optimal study.

Additional material: MCNA 12787.1, virtually complete, visible in profile, with blackened cuticle somewhat altered due to fossilization; preserved in a clear-yellow, turbid piece of amber trimmed to 1.30 × 0.70 × 0.10 cm (set into an epoxy trapezoid of dimensions 2.80 × 1.80 × 0.10 cm), with syninclusions as detailed for MCNA 12788.2. This specimen agrees in most respects with the holotype but is noticeably smaller (total length 809 µm) and exhibits at least two pairs of anal spines. The shared shape of the furcula, antennae, and chaetotaxy of the body, as well as its preservation as syninclusion with the holotype, all make it seem likely that they represent the same taxon, but this remains unresolved, particularly as the anal spines cannot be discerned in the holotype (owing to its state of preservation).

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 based on the region of Autrigonia, home of the pre-Roman Autrigones who lived in the area of the type locality before their eventual integration into the Empire.

Remarks: Overall, this species exhibits all those traits characteristic of the genus Protoisotoma, previously described from species preserved in Cretaceous Canadian and Burmese ambers (Christiansen and Pike, 2002a, 2002b; Christiansen and Nascimbene, 2006). Distinguishing features within the subfamily include the swollen fourth antennomere, simple ungues and unguiculi, elongate dentes, minute mucrones, and a body densely covered with curved, acuminate, smooth setae as well as erect pseudotrichobothria on the abdomen. Unfortunately, the postantennal organ, ommatidia, and details of the furcula cannot be seen, preventing more extensive comparison. Although the dens is elongate in P. autrigoniensis, n. sp., no crenulation or setae are visible owing to the nature of preservation of the type. The new species can be distinguished from its congeners (P. micromucra Christiansen and Pike from Canadian amber, and P. burma Christiansen and Nascimbene from Burmese amber) by its longer third antennomere, which is subequal to the fourth, and the fourth abdominal segment subequal to or slightly longer than the third abdominal segment (rather than the subequal or shorter fourth abdominal segment relative to the third abdominal segment in P. micromucra and P. burma) (fig. 4). It is remarkable that the other putative specimen of this species, MCNA 12787.1, exhibits two pairs of anal spines, a feature not observed in all other specimens of Protoisotoma from Burmese and Canada ambers (this is not a statement of the character's absence from such specimens, merely a condition of it not being observable in such specimens owing to the nature of preservation).

Proisotoma Börner, 1901: 133. Type species: Isotoma minuta Tullberg, 1871, subsequent designation by Borner (1903). Refer to Potapov (2001) and Potapov et al. (2006) for a complete summary of extant synonyms.

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; mesoand 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 x 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 clear-yellow, 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 clear-yellow, 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, dark-orange, 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.

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.