We provide additional documentation and a formal description of Srokalarva berthei morphology in the Additional file 1, where important structural details on a tagma-by-tagma and segment-by-segment basis
are listed and described. Relevant details regarding the taxonomic affiliation of
this specimen with particular early-appearing lineages of Holometabola also are provided.
The head and mouthparts
The head is less well-preserved than previously thought. In the original reconstruction,
head structures 6], 14] were divided into substructures such as cephalic capsule sclerites, antennal articles
and palpal segments. Under certain angles of illumination weak subdivisions of these
structures are apparent, but other angles of illumination reveal alternative demarcations.
Neither surface texture or ornamentation, nor color provides a clear guide for the
presence of cephalic sutures (Additional file 1). It is likely that the cephalic capsule indeed was subdivided, yet their exact delimitation
remains indeterminate. Fundamental head structures are evident, such as the antennae,
clypeus and its surrounding sutures, some primary segments, and the mouthparts and
their subelements.
Two darker spots positioned anterodorsally most likely represent antennal insertions.
Impressions of the more distal parts of the elongate, filiform antennae appear to
be directed toward these dark spots, one antenna of which overlies the proximal clypeal
region. An additional indication that these spots are antennal insertions is presence
of an encircling ridge that originally was interpreted as a molting suture. Anatomically
below the antennal insertions and anterior to the mandible is the clypeo-labral complex
which bears well-resolved surface texture and a suture that likely was articulatory
(Fig. 2g, h). Under cross-polarized light the clypeus is shorter and is inserted further dorsally
whereas the labrum appears significantly longer than originally reconstructed (Fig. 2a–f).
Fig. 2. Head structures of Srokalarva berthei and some derived artifacts. a, Unpolarized, reflected light. b, An interpretative version of (a). c and e, Low angle unpolarized light, from multiple source directions. d and f, Interpretative versions of C and E. g, Red–cyan stereo anaglyph under cross-polarized light. (For best visual results,
use red–cyan glasses for viewing.) h, Interpretative version of (g). Note how the mandible shape depends on variation in lighting. Only the stereo anaglyph
provides a neutral evaluation of head structure. Abbreviations: an, antenna; “ceâ€, presumptive compound eye; cl, clypeus; hp?, possible hypopharynx of the intercalary segment; lb, labium; “lrâ€, labrum interpreted by Kukalová-Peck 5]; lr, labrum interpreted by us; “mdâ€, mandible interpreted by Kukalová-Peck (1997); md, mandible interpreted by us; mx, maxilla; su, suture, possibly the epicranial suture
The most prominent mouthpart element is a large, triangular mandible, identifiable
by shape and position. The mandible is ill defined and the inner surface apparently
has partly collapsed while the outer border has remained intact. Our interpretation
of the mandible is a more massive structure than provided in the initial account 6]. The mandible under reflected light from above (Fig. 2a, b) or at other angles of incidence (Fig. 2c, d) would suggest a surface bounded by a proximal border, indicating a less massive
structure. However, at different light angles (Fig. 2e, f), the proximal border appears positioned further dorsal and the general shape is
more slender. The expanded size of the mandible is corroborated by stereomicroscopic
imaging (Fig. 2g, h). Above the dorsal mandible margin is a structure more challenging to interpret.
This structure is broad, lobate, well-sclerotized and originally was interpreted as
an eye, possibly compound. This structure is prominently upraised (Fig. 2), but with no indication it contains ocular features, and appears to have been partly
compressed under the mandible. These features suggest that it is related to the mouthparts;
the most plausible interpretation is a hypopharynx. A less likely possibility is that
it represents a large, projecting condyle of the opposite mandible. (Although it appears
the specimen lacks compound eyes, a cluster of miniscule, circular structures may
be stemmata (Fig. 1a–f), but their identity is ambiguous.) The two serial structures that are posterior
to the mandible (Fig. 2g, h) likely represent head segmental regions with ventral appendages. Based on structure
and position, they are interpreted as the maxilla and labium, as originally described.
A suture separating these two, posterior, segmental regions from the rest of the head
capsule was not observed.
The thorax and legs
The thoracic segments do not differ markedly from the abdominal segments in the original
interpretation. Our observations contradict this view. Three, well-delineated, nonoverlapping
and noninterlinking regions of thoracic sclerites are apparent from an assessment
of surface relief and color (Fig. 1a, d; Additional file 1: Figure S1A,D). The three thoracic legs are significantly more robust, longer and
possess a greater diameter than the abdominal leglets. Originally thoracic legs were
reconstructed with seven elements, whereas we found five major elements with a possible
sixth element bearing terminal paired claws that are variably preserved (Fig. 3a–d). Although dark lines occur on sclerite surfaces and were interpreted originally
as setae 6], and Mazon Creek fossils occasionally preserve fine hairs 15], we found no evidence for hirsute integument. Areas between the sclerites appear
to preserve softer cuticle. These observations indicate that the thorax extends further
rearward than the original reconstruction, corresponding to the anterior five postcephalic
segments of Kukalová-Peck 6], and is more differentiated from the abdomen than originally reconstructed. The abdominal
segments are in register (body segments matching respective leglets), but with overlapping
tergites, the exact intersegmental boundaries are difficult to discern. This is borne
out by a lack of an exact match of tergites between part and counterpart.
Fig. 3. Thoracic and abdominal appendages of Srokalarva berthei. a to d, Second (mesothoracic) and third (metathoracic) appendages; (e) to (g), True first abdominal appendage. a, e, Directed low angle light. b, Undirected reflected light. Arrows mark patterns resembling supposed claw. c, Red-blue anaglyph of a virtual surface reconstruction. (Use red-cyan glasses to
view.) d, Interpretative version of (c). f, Red-cyan stereo-anaglyph under cross-polarized light. (Use red-cyan glasses to view.)
g, Interpretative version of (f). Abbreviations: “clâ€, originally interpreted claw; tr, tergite rim
The abdomen and leglets
The original interpretation listed eleven abdominal segments (Fig. 4a) 6]. This number was arrived at by a miscount, with the two anteriormost “abdominalâ€
segments 6] actually combined into our posteriormost metathoracic segment. Also, the initial
count failed to recognize an inconspicuous but preserved segment behind the presumptive
‘cerci’. Consequently, ten abdominal segments are recognized in the current restoration
(Fig. 4b). The first eight of these have ventral appendicular leglets (Figs. 1, 3e–g, Additional file 1: Figure S2). Originally, the last abdominal segment bore a pair of segmented ‘cerci’.
This structure is apparent in relief and color, but its presumptive segmental subdivision
likely is an artifact caused by irregularities of nonbiological surfaces. Due to the
segmental mismatch in the original reconstruction, the supposed cerci do not arise
from the eleventh, but from our ninth abdominal segment and are interpreted as ventrally
positioned, paired, precursor genitalia ontogenetically comparable to urogomphi.
Fig. 4. Reconstructions of Srokalarva berthei. a, The original interpretation, simplified from Kukalová-Peck (1991). b, A new reconstruction based on the present study. The grey hue indicates softer,
more weakly sclerotized regions between sclerites. Dotted lines indicate possible
intersegmental junctures which have been preserved
The abdominal appendages differ in certain aspects from the original interpretation.
The abdominal appendages are inserted more dorsad along the abdominal sidewall, or
pleurite, than formerly recognized, and are partially covered by the body. This indicates
longer appendages than originally reconstructed, but they are somewhat shorter than
the thoracic ones. None of these appendages preserve unequivocally a distal claw.
As the exact subdivision of these appendages is difficult to discern, there appears
to be two broader proximal elements and up to five smaller, distally tapering ones.
The abdominal appendages therefore are composed of more elements and are more markedly
gracile than the thoracic ones.