Maldybulakia malcolmi, Edgecombe, 1998

Edgecombe, Gregory D., 1998, Early myriapodous arthropods from Australia: Maldybulakia from the Devonian of New South Wales, Records of the Australian Museum 50 (3), pp. 293-313 : 299-305

Maldybulakia malcolmi n.sp.

Figs. 3-8

Maldybulakia new species 1 Edgecombe, 1998: fig. 1.

Etymology. For MaIcolm Young.

Diagnosis. Maldybulakia with relatively minor serial variation in B-pleurotergites; lacking long paratergal spines, posteromedian spines, and tuberculation; caudal pleurotergite with lateral lobes set offby shallow furrows; median projection on caudal pleurotergite blunt, conical, rather than spinose.

Types. HOLOTYPE articulated trunkAM F.102533 (Fig. 3a­ d,f,g), composed of one T-tergite, four B-pleurotergites, a caudal tergite, disarticulated telson sclerites; and possibly associated T-tergite; length from anterior end of T-tergite to posterior end of caudal pleurotergite 31.8 mm. Boyd Volcanic Complex (late Givetian or early Frasnian), Saltwater Creek Road , East Boyd State Forest, NSW (37°09'S 149°57'E). Collected by Z. Iohanson. PARATYPES AM F.102534-102556, 102581, from type locality .

Description. Trunk up to 115 mm long, as inferred from size of largest known B-pleurotergite scaled to articulated specimen.

T-tergite(s) of length up to 16.4 mm in known specimens, but inferred to be larger based on relative size of B-pleurotergites; smallest disarticulated specimen 5.1 mm long medially. Length 38-44 percent of width; of even, moderate convexity (tr.). Anterior part of prozonite a forward-sloping scarp that evenly lengthens medially to about 45 percent the length of the prozonite. Stricture moderately deep and approximately transverse medially, then weakly curving backwards laterally, abruptly shallowing and recurving forward, then inward, such that lateral part of stricture is C-shaped. Metazonite 64-75 percent length (sag.) of prozonite; gently convex (sag.). Lobate posterior projections lie against anterodistal socketlike processes on underlying ring tergite, serving as point of articulation. Posterior margin gently convex forwards between posterior projections. Doublure wide beneath lateral and posterolateral edges ofT-tergite, shortening and scalloped on each side of a flange-like median section that has a transverse course; doublure abruptly truncated anteriorly, with straight anteromedial margin.

Ring tergite (Fig. 5b,d-f) divided by sharp transverse furrow into short, crest-like anterior band of even length and long, elevated posterior band; furrow strongly shallow or effaced near distal margin. Anterior band extended laterally well outside posterior band as slender, pointed process; anterodistal edge folded as a small socket-like process. Posterior band lenticular in outline, convex (sag.), moderately arched transversely. Ring tergite overlapping prozonite of first B-pleurotergite, with distal edge of posterior band extending to inner edge of anterior process on prozonite of B-pleurotergite.

Posterior tagma composed of four B-pleurotergites and caudal pleurotergite, widest across first pleurotergite then gently, evening narrowing posteriorly; largest known disarticulated specimen (second B-pleurotergite) of length 25.8 mm, width 45 mm; smallest disarticulated specimen (fourth B-pleurotergite) 7.5 mm long, but as short as 6 mm in articulated specimens. Relative length and width of pleurotergites variable (compare Figs. 3a and 5i for relatively narrow and wide examples, respectively); each pleurotergite longer relative to its width than the preceding one. Furrows separating prozonite and metazonite on all four pleurotergites of similar, moderate depth, long medialIy, narrower but with comparable depth against lateral lobes. In sagittal profile, prozonites gently convex, metazonites flat or weakly convex along most of length; median lobe raised well above lateral lobes. Lateral lobes of all four pleurolergites triangular, with short, blunt posterolateral projections, in some specimens developed as weak spines. Metazonites progressively more strongly arched (tr.) posteriorly in trunk. Diagonal furrows separating lateral and median lobes of equal impression on each B-pleurotergite, gently convex backward.

First B-pleurotergite (Figs. 3e, 6c-e) with outer margin of lateral lobe oriented posterolaterally in dorsal view, gently convex outward, this margin posteriorly directed on the succeeding three pleurotergites with a corresponding posterior redirection of the posterolateral projections. Prozonite lenticular, with a strong process at anterodistal edge that is lacking on succeeding pleurotergites. Lateral edge of tergum sharply folded down as vertical scarp; lenticular pleurite separated by sharp, curved pleural furrow; pleural furrow effaced posteriorly; separation of pleuron and tergum not marked by suture on posterior part of pleurotergite; posteroventral edge of pleurotergite turned out as a rounded shelf.

Second B-pleurotergite ( Fig. 6 g-j) having anteroventral edge of prozonite forming a narrow, steep rim; anterolateral corner of tergite with ledge-like protrusion markedly distal to pleuron; small ventrally directed pleural process at anterior corner of metazonite, ventral margin of pleuron abruptly flexed ventrally immediately behind this process; pleuron bisected by anterodorsally oriented groove; posterior part of pleuron lensoid in outline, gently sloping outward; pleural furrow indistinct on posterior part of pleurotergite.

Third B-pleurotergite (Fig. 3b,c,f) with pleural furrow expanded anteriorly to form a shallow, elongate basin within which a deep, elliptical canal (spiracle) projects inwards; pleurite narrow beneath spiracular region, then abruptly widening; rounded ventral projection on anterior part of pleurite as on preceding diplopleurotergite; sharp, shallow suture between pleurite and pleural furrow anteriorly.

Fourth B-pleurotergite with lateral lobes of metazonite terminating as posteriorly-directed angulations. Weak nodose swelling usually distinguishable on posteromedian part of median lobe of metazonite (Figs. 3a, 5k), occasionally discernible on other B-pleurotergites. Pleural furrow deep, incised as a V-shaped groove along most of juncture between tergum and pleuron, abruptly shallowing near posterior margin. Pleurite clavate, narrowest anteriorly, projecting only slightly outside lateral edge of tergum.

Caudal tergite (Figs. 3c,g, 5i,l) with prozonite longer than on preceding B-pleurotergites, weakly convex (sag.); furrow between prozonite and metazonite shallow medially, distinctly deeper against lateral lobes; lateral lobes flat, set off by weak furrow; lateral edge of tergum forming a slenderridge that gently bulges beyond otherwise triangular outline of metazonite in dorsal view; pleural furrow not impressed; posteromedian process large, conical, triangular or elliptical in section posteriorly, gently turned up; posterolateral margin vertically inclined, gently convex.

Two small telson sclerites (Figs. 3g, 4g). Anterior telson sc1erite divided into a depressed anterior band that resembles an articulating edge and a longer, convex, strongly arched (tr.), posterior band; about 40 percent width of caudal pleurotergite. Posterior telson sc1erite divided by a sharp transverse furrow into short anterior band and flattened, broadly quadrate posterior plate; anterior band overlapped by anterior telson sclerite.

Dorsal surface of cuticle densely covered with honeycomb-like, shallow, polygonal pits; density of polygons the same on prozonite, stricture, and metazonite. Cuticle replaced with chalcedony, divided into a thin outer layer (0.04 mm) and thicker main layer (0.45 mm), measured in thin section; internal fabric of main layer obliterated by chalcedony aggregates. Large, densely arranged canals run through cuticle perpendicular to its surface (Fig. Sa), opening to variable-sized, rounded pits on ventral surface (Fig. Sb).

Discussion. The description of this species regards the trunk as being composed of pleurotergites, the pleurites being fused to the tergal metazonites ( Fig. 7 ). The two components are set off by a deep groove, described as a pleural furrow. That the pleuron is actually fused to the tergum is most evident in the posterior part of the first (Fig. 3b,e) and second ( Fig. 6i ) pleurotergites, where they are in direct continuity, the first pleurotergite without even a trace of a suture behind the abruptly-effaced pleural furrow. The morphology of the pleuron is frequently complicated by crushing, on occasion giving the appearance of several pleurites. In the holotype in particular, this appearance is compounded by fracture along the lateral edge of the tergum on the first three pleurotergites (Fig. 3b­ d). Comparison with other specimens (Figs. 3e, 6h,i) which have an intact down-folded edge of the tergum clarifies the structures ( Fig. 7 ). Every B-pleurotergite (even disarticulated ones) preserves the pleuron in association with the tergites, and they have characteristic morphologies on each of the four B-pleurotergites. Were the pleurites separate, unfused elements (e.g., as in centipedes) this invariant association with the tergum would not be maintained. Whether the pleural component of the pleurotergum may consist of multiple, fused pleurites is not certain. In at least some pleurotergites (Fig. 3c, particularly the fourth Bpleurotergite) the pleuron consists of a simple, elongate band without sutures, and seems likely to be composed of a single pleurite.

Serial variation in the B-pleurotergite series includes a progressive decrease in the height of the lateral edge of the tergum, and an increase in the length of the pleural furrow (short on the first B-pleurotergite versus the entire length of the fourth B-pleurotergite). The pit interpreted as a spiracle is present on a single pleurotergite, and is only distinguishable on the holotype (Fig. 3b,f). This opening is not a preservational artifact because it is entirely lined with cuticle, forming a distinct, ovate canal. The position of such a canal at the juncture between the tergum and pleuron resembles that of some tracheates (e.g., pleurostigmophoran chilopods, hexapods). The existence of only one such opening, rather than a series along the trunk, does not negate identity as a spiracle, although it is peculiar given the substantial size of Maldybulakia ; presence of only one or two spiracles in extant myriapods (e.g., Symphyla and some lithobiomorphs) is reasonably regarded as linked to minute body size. To consider alternative interpretations, the tube in Maldybulakia is situated in the appropriate position for an ozopore, the opening for the repugnatorial glands in helminthomorph diplopods (see Shear, 1993: fig. 2 for a fossil example). Such an identity is considered less likely than a spiracle because an ozopore would be expected to be smaller, whereas myriapod spiracles can be very large (e.g., in scolopendrid chilopods; see lG.E. Lewis et al., 1996).

A lesser degree of serial variation within the two trunk tagmata is a character uniting Maldybulakia malcolmi and M. mirabilis to the exclusion of M. angusi . Suppressed variation is, however, presumed to be a primitive condition (with outgroup comparison to other myriapods), and not indicative of a sister species relationship between M. malcolmi and M. mirabilis . Given that M. malcolmi has only four B-pleurotergites (not counting the caudal pleurotergite), it is likely that the four articulated Bpleurotergites in the holotype of the very similar M. mirabilis ( Tesakov & Alekseev, 1992: fig. la,b) represent the entire B-pleurotergite series. Maldybulakia malcolmi is distinguished from M. mirabilis by its pronounced transverse furrow on the T-tergite (as opposed to a pair of chevron-shaped, longitudinally aligned furrows in M. mirabilis ), much finer granulation on the tergites, shallow (versus deep) furrows setting off the lateral lobes on the caudal pleurotergite, and a less spinose median projection on that sclerite.

Figures

Figure 6. Maldybulakia malcolmi n.sp. Scale bars 10 mm. Ca) dorsal view of T-tergite AM F.102549. Cb) ventral view of T-tergite AM F.102550, showing doublure. (c) dorsal view of first B-pleurotergite AM F.102551. (d) dorsal view of first B-pleurotergite AM F.102552. (e) dorsal view of first B-pleurotergite AM F.102553. (j) dorsal view of B-pleurotergite AM F.102554. (g,h) dorsal and left lateral views of second B-pleurotergite AM F.102555. (i,j) right lateral and dorsal views of second B-pleurotergite AM F.102556.

Figure 7. Maldybulakia malcolmi n.sp. Reconstruction of trunk in lateral view, showing serial variation in structures of the pleurotergum. Based mostly on AM F.102533 (Fig. 3b-d).