All specimens were inspected dry with a Nikon SMZ 1500 stereomicroscope under cross-polarized light. Specimens were photographed dry under cross-polarized light. Only in very rare cases did wetting specimens and photographing them under reflected light (high angle) without polarizers provide better contrast. Images were taken with a Canon Rebel T3i camera, equipped with either an EF-S 18-55 mm lens or a MP-E 65 mm macro lens. Lighting was provided by a MeiKe FC 100 LED macro ring flash and additional fiber light sources. Images were often recorded as a stack at different focal planes, especially for close-ups. Stacks were fused with CombineZM or CombineZP. Overview images, in particular, were taken as several photographs in different x- and y-positions and then stitched together with Adobe Photoshop CS3 or Microsoft Image Composite Editor (for details on composite imaging see 15). Images were color- and contrast-optimized in Adobe Photoshop CS3 or GIMP 3.0. Unsharp mask filters were applied in Adobe Photoshop CS3. The 3D computer model was reconstructed using Blender.

The great appendage has been interpreted as being homologous to the chelicera of Chelicerata sensu stricto (4514 but see, for example, 16). The distal two elements of a chelicera form a chela with a proximal fixed finger (digitus fixus) and a distal movable finger (digitus mobilis) (Figure 1A). The megacheiran great appendage comprises several functional chelae, in which an element forms a movable finger against the one proximal to it, while forming a fixed finger in relation to the one distal to it (Figure 1B). Thus the normal terminology for chelicerae is inadequate. We refer to this type of claw with more than two fingers as a 'multi-chela'. This term is applicable not only to the great appendage of megacheirans but also, for example, to the pedipalps of thelyphonid uropygids.

The movable finger in the great appendage is usually a single element, i.e., a spine arising from the next distal element, but it may include more than one. Such an arrangement occurs not only in Leanchoilia, but also, e.g., in different (sub-)chelate appendages in various crustaceans and insects (Figure 1C) and in the first appendage in Amplectobelua17. The different functional chelae of a multi-chela can be described independently: that in Leanchoilia superlata, for example, is composed of a proximal chela with a movable finger including a single element (the movable finger is the spine arising from the next distal element) and a distal chela where the movable finger includes two elements (Figure 1D).

In order to allow direct comparison of structures in different megacheirans, we propose a standardized description. Clearly if an interesting structure is discovered in one species, it is necessary to check for its occurrence in others (Hennig’s principal of reciprocal illumination). Even if a structure were present in closely related species, it may not have been considered sufficiently important to warrant a mention. A morphological re-interpretation is usually triggered in this way - with the discovery of a new detail in a related species.

New discoveries and new interpretations often make it necessary to re-work species descriptions 2. However, this process should not be restricted to supplementing or modifying data matrices, new interpretations of morphological characters should be properly explained and justified. This could be achieved by handling descriptions in a similar way to matrices, with a systematic treatment of character states and presence/absence information. Comparable approaches have been used in database projects like DELTA (DEscription Language for TAxonomy; 18) although this program is neither readily accessible nor appropriate in its recent form.

Here we follow a simple 'descriptive matrix' approach, compiling the description in an xls.-file in OpenOffice with appropriate rows and columns (see Additional file 1). Following this approach will ensure that subsequently described species will be checked for all characters. When new characters come to light they can be added, but need to be checked on all species. The application of such an approach will result in a comparable set of descriptions that can be converted easily into a cladistic matrix (some characters may be dependent or reciprocal, and therefore omitted). Yet, the whole process must remain an iterative one: with the addition of each new species, new characters will be introduced, which then will have to be described in already named species. The descriptive matrix approach is established here for the short great-appendage arthropods, a small group of morphologically similar species. Expanding this approach to a larger group such as Megacheira will require a clear discussion of assumed character homologies. Even though such a process will be labor-intensive, the descriptive matrix provides a tool to make the enterprise transparent and comprehensible.

The descriptive matrix approach generates a data set that can be readily converted to a “plain language” description of the species (term from DELTA) which is easily compared to other descriptions using the same matrix. We will use the re-description of Leanchoilia superlata generated in this way here as a basis for preparing descriptions of other leanchoiliid species. Although it is straightforward to generate text directly from the matrix, we provide such a description here, but shortened to avoid repetitions.

Small arthropod with an elongate body differentiated into head, segmented trunk and non-somitic telson (Figure 2). Body with 16 segments comprising an ocular and 15 post-ocular appendage-bearing segments (Figure 2A, B). Ocular segment and post-ocular segments 1–4 incorporated into the head, their dorsal area contributing to the head shield. Post-ocular segments 5–15 (corresponding to trunk segments 1–11) forming tergites dorsally.

Head shield with pronounced axial region and lateral ‘wings’ (Figure 3A), each wing region about one quarter of head shield width. Head shield shape almost triangular in dorsal view, about as long as wide at the posterior margin. Anterior rim of head shield drawn out into dorsally curved, hook-like rostrum (Figure 3E). Rim of head shield with up to 13 serrations on each side (Figures 2A, 3A). Posterior region of head shield (probably corresponding to post-ocular segment 4) resembles trunk tergites [cf. (Figure 4)] in the presence of a pair of carinae on the latero-dorsal area of the axis (Figure 3B, E). Carinae spine-like, pointing posteriorly.

Length about one fifth that of the head shield. Total width slightly greater than that of the head shield. Width of axial region similar to that of the head shield. Tergo-pleural region on each side about one quarter entire width, almost straight, curving slightly posteriorly. Lateral rim with 7 to 8 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width and width of axial region slightly greater than those of preceding segment. Tergo-pleural region as in previous segment. Lateral rim with about 7 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width and width of axial region slightly greater than those of preceding segment. Tergo-pleural region on each side slightly more than one quarter entire width, curving slightly posteriorly at roughly 20°. Lateral rim with about 7 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width and width of axial region slightly greater than those of preceding segment. Tergo-pleural region on each side slightly more than one quarter of entire width, curving posteriorly at roughly 30°. Lateral rim with 7 to 8 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width slightly greater than that of preceding segment. Axial region slightly narrower than that of preceding segment. Tergo-pleural region on each side more than one quarter of entire width, curving posteriorly at roughly 40°. Lateral rim with 7 to 8 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width slightly greater than that of preceding segment, largest of series. Axial region slightly narrower than that of preceding segment. Tergo-pleural region on each side slightly less than one third of entire width, curving posteriorly at roughly 40°. Lateral rim with 7 to 8 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width slightly less than that of the preceding segment. Axial region slightly narrower than that of preceding segment. Tergo-pleural region on each side about one third of entire width, curving posteriorly at roughly 45°. Lateral rim with 7 to 8 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width less than that of the preceding segment. Axial region slightly narrower than that of preceding segment. Tergo-pleural region on each side about one third of entire width, curving posteriorly at roughly 50°. Lateral rim with about 7 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width less than that of the preceding segment. Axial region narrower than that of preceding segment. Tergo-pleural region on each side slightly less than one third of entire width, curving posteriorly at roughly 60°. Lateral rim with probably 7 serrations (Figures 2A-C, 4).

Length about one sixth of the head shield. Total width about three-quarters of preceding segment. Axial region narrower than that of preceding segment. Tergo-pleural region on each side slightly less than one third of entire width, curving posteriorly at roughly 70°. Lateral rim with probably 7 serrations (Figures 2A-C, 4).

Length as in preceding segment. Total width less than two thirds of the preceding segment. Axial region narrower than that of preceding segment. Tergo-pleural region on each side slightly less than one third of entire width, curving posteriorly at roughly 85°. Lateral rim with 6 to 7 serrations (Figures 2A-C, 4).

Telson elongate triangular in dorsal view (Figure 5A-E), scimitar-shaped in lateral view (Figure 5F-G), tip pointed. Setae around the entire distal and lateral margin, 11 on each side, uniform in size.

Lateral eyes with short stalks arising from the antero-ventral region of the head (Figure 3D, F-H). Each eye consisting of two lobes.

Hypostome elongate drop-shaped, situated ventrally between the insertions of the eyes (anteriorly), appendage 1 (antero-laterally) and appendage 2 (postero-laterally) (Figure 3C). Presumably carrying the mouth opening posteriorly.

Prehensile great appendage, differentiated into peduncle and claw (Figure 6A-D), separated by a hinge joint. Peduncle of two elements (Figure 6A) which are slightly shorter than wide (diameter). Claw a multi-chela with four elements (Figure 6A), the two distalmost forming a divided movable finger [cf. (Figure 1D)]. Claw element 1 short, as wide (diameter) as long, curved inward, with an elongate, medio-distal spine, directed distally, 3x as long as the basal part of the element (almost as long as the head shield). The diameter of the spine at its base is about 1/3 that of the basal part of the element. The spine curves slightly away from the other fingers, more strongly towards the tip, and extends into a feeler-like multi-annulated structure (flagellum) (Figure 7F, G). The basal part of the element contains a possible excretory gland (Figure 6D). Claw element 2 short, as wide (diameter) as long, straight, with an elongate, medio-distal spine, directed distally, 3x as long as the basal part of the element. The diameter of the spine at its base is about 1/3 that of the basal part of the element. The spine is curved slightly outward, more strongly towards the tip, and extends into a feeler-like multi-annulated structure (flagellum). Claw element 3 is almost half the diameter of the preceding element and only slightly shorter than the medio-distal spines of the preceding claw elements. It extends latero-distally into a feeler-like multi-annulated structure (flagellum; Figure 7D-F). Claw element 4 is a small hemispherical structure with four inwardly-curving (hook-like) spines differentiated in size (Figure 7A-C). The most distal spine (s1) is the largest (length about twice the width of element 4 at the base), the most proximal (s2) is the second largest (length similar to the width of element 4), the sub-terminal (s3) is the third largest (length about half the width of element 4), the second proximal spine (s4) is the smallest (length about one third the width of element 4).

Significantly smaller than the more posterior appendages: only one third the length of appendage 3 (Figure 8A, C). Consists of basipod (?), endopod and exopod. The endopod consists of 6 elements (?) (Figure 8B). Elements 1 and 2 are longer than wide (diameter), unclear whether they are setose. Element 3 slightly longer than wide (diameter), with a long medio-distal seta. Element 4 as long as wide (diameter), with a long medio-distal seta. Element 5 very small, forming the base for element 6, with two long setae, one medio-distal, one latero-distal. Element 6 small, extending into an elongate distally pointing seta. Exopod paddle-shaped, about twice as long as wide, unclear whether bipartite. Exopod about 30% longer than endopod. Extrapolation based on the spacing of the approximately 5 preserved setae, indicates that there were orginally about 16.

Significantly larger than appendage 2, length about two thirds that of the head shield. Basipod with 3 (?) groups of robust spines arranged from proximal to distal along the median edge (Figure 8D). Spine groups apparently triplets, probably with one central and two adjacent spines arranged in a line. Arthrodial membrane with three large folds occupies a medial notch in the basipod where it articulates with the body (Figure 9A). Endopod of 6 or 7 elements, their length similar to or slightly longer than width (Figure 8E), tapering progressively distally. One seta arises medio-distally from each element and an additional latero-distal seta is present on element 6. Element 7 about twice as long as element 6, length 5x width (diameter), curving slightly inwards. Exopod slightly longer than endopod (?), length about 2.5x width, bipartite, with a triangular proximal part and a paddle-shaped distal part (Figure 8D). Proximal part articulates with basipod and bears 3 spine-like setae laterally. Distal part articulates with endopod element 1 (?), and bears 16–18 spine-like setae around the margin (about 4 median, 1 distal, 11–13 lateral).

Similar in morphology (Figure 9A) but larger than appendage 3, length similar to that of head shield. Basipod with 4 groups of robust spines arranged from proximal to distal along the median edge (Figure 9B). Spine groups apparently triplets, probably with one central and two adjacent spines. Basipod-body joint the same as that in appendage 3. Endopod consists of 7 elements, their length similar to or slightly longer than width (Figure 9A), tapering progressively distally. One seta arises medio-distally from each element (Figure 9C) and an additional latero-distal seta is present on element 6. Element 7 about twice as long as element 6, length 5x width (diameter), curving slightly inwards. Exopod slightly shorter than endopod, length about 2x width, bipartite, with a triangular proximal part and a paddle-shaped distal part (Figure 9D). Proximal part articulates with basipod and bears 4 spine-like setae laterally. Distal part articulates with endopod element 1 and preserves 16 spine-like setae of an estimated 19 (3 median, 1 distal, 12 (estimated 15) laterally).

Similar in morphology (Figure 9D) but slightly larger than appendage 4, length similar to that of head shield. Neither basipod nor its body joint preserved but probably similar to those of appendage 4. Endopod consists of 7 elements, their length similar to or slightly longer than width (Figure 10A). Elements 1–6 tapering progressively distally. One seta arises medio-distally from each element and an additional latero-distal seta may be present on element 6. Element 7 about twice as long as element 6, length 5x width, curving slightly inwards. Exopod slightly shorter than endopod, length about 2x width, bipartite, with a triangular proximal part and a paddle-shaped distal part. Proximal part articulates with basipod and bears 4 spine-like setae laterally (Figure 9D). Distal part articulates with endopod element 1 (?) and bears 19 spine-like setae (estimated).

Similar in morphology (Figure 9D) and size to appendage 5. Neither basipod armature nor body joint preserved, but presumably as in appendage 4. Endopod consists of 7 elements, their length similar to or slightly longer than width (Figure 10B). Elements 1–6 tapering progressively distally. One seta arises medio-distally from each element and an additional latero-distal seta may be present on element 6. Element 7 about twice as long as element 6, length 5x width, curving slightly inwards. Exopod slightly shorter than endopod, length about 2x width, bipartite, with a triangular proximal part and a paddle-shaped distal part. Proximal part articulates with basipod and bears 4 (?) spine-like setae laterally. Distal part articulates with endopod element 1 (?) and bears 19 spine-like setae (estimated).

Similar in general aspect and size to appendage 6 (Figure 9D). Basipod with 3 or 4 groups of robust spines arranged from proximal to distal along the median edge. Spines grouped apparently as triplets, with one central and two adjacent spines (Figure 10C). Arthrodial membrane, possibly with three large folds, occupies a medial notch in the basipod where it articulates with the body (Figure 10C). Endopod consists of 7 elements (Figure 10C, D), 1–6 tapering progressively distally. All elements as long or slightly longer than wide. One seta arises medio-distally from each element and an additional latero-distal seta may be present on element 6. Element 7 about twice as long as endopod element 6, length 5x width, curving slightly inwards. Exopod slightly shorter than endopod, length about 2x width, bipartite with a triangular proximal part and a paddle-shaped distal part. Proximal part articulates with basipod and bears 4 (?) spine-like setae laterally. Distal part articulates with endopod element 1 and bears 16 spine-like setae (estimated 19).

Similar in general aspect and size to appendage 7 (Figure 9D). Neither basipod armature nor body joint preserved, but presumably as in appendage 7. Endopod and exopod as in appendage 7 (Figure 10F). Distal part bears 19 spine-like setae.

Similar in general aspect but slightly smaller than appendage 8; slightly shorter than the head shield. Basipod armature not preserved, presumably as in appendage 7. Basipod-body joint as in appendage 7 (Figure 10E). Endopod as in appendage 8. Exopod as in appendage 8 (Figure 9D), but proximal part bears only 3 (estimated) spine-like setae laterally. Distal part bears 17 spine-like setae (estimated 18).

Similar in general aspect but slightly smaller than appendage 9. Neither basipod armature nor body joint preserved, but presumably as in appendages 7 and 9, respectively. Endopod details not preserved, presumably as in appendage 9. Exopod presumably as in appendage 9, but with 16 (estimated) spine-like setae (Figure 9D).

Similar in general aspect but slightly smaller than appendage 10; length about four fifths of the head shield (Figures 9D, 10E). Neither basipod armature nor body joint preserved, but presumably as in appendages 7 and 9, respectively. Endopod as in appendage 8. Exopod presumably as in appendage 10.

Similar in general aspect but slightly smaller than appendage 11; length about three quarters of the head shield. Neither basipod armature nor body joint preserved, but presumably as in appendages 7 and 9, respectively. Endopod as in appendage 8, but elements 1 and 3 have one adjacent seta each (Figure 10G). Exopod presumably as in appendage 10.

Appendage 13–15 not preserved with details. Presumably similar in general aspect but slightly smaller than preceding appendages (Figure 2D).