Protoisotoma autrigoniensis, 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 : 8-10

Protoisotoma autrigoniensis , new species

Figures 3–4

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 subequal 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).

Genus Proisotoma Börner, 1901

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

Figures

FIGURE 3. Microphotograph in dorsal habitus of the holotype (MCNA 12788.2) of Protoisotoma autrigoniensis, new species, in Late Albian amber from northern Spain (images combining consecutive photographs taken at successive focal planes).

FIGURE 4. Camera lucida drawing of the holotype (MCNA 12788.2) of Protoisotoma autrigoniensis, new species, in dorsal habitus.