Neither leg nor jaw-nor always the same: a critical revision of the eumalacostracan maxilliped Author Grams, Markus Author Klinger, Michael Author Richter, Stefan text Zoological Journal of the Linnean Society 2023 2023-04-01 197 4 965 1004 https://academic.oup.com/zoolinnean/article/197/4/965/6795015 journal article 10.1093/zoolinnean/zlac083 f2620f0f-c702-454b-ae46-83f90899b86b 0024-4082 7797605 MEGANYCTIPHANES NORVEGICA The first pair of thoracopods ( Fig. 7A ) to Thp7 are all similar, whereas Thp8 is vestigial. The following descriptions are based on a detailed examination of the right Thp1 of three adult females of M. norƲegica . Cuticle and skeletal structures of thoracopods in M. norvegica The following description refers collectively to Thp1–7, if not specified otherwise. The ventral cuticle of the first thoracomere is coalesced with that of the cephalon. The density of the cuticle of this cephalothoracic sternum appears higher than in the posterior thoracomeres. Anteriorly, the thoracic cuticle articulates with the maxilla, which is tightly fitted against the anterior wall of the coxa and basis of Thp1. In all thoracomeres, the posterior thoracic cuticle is folded in an anterior direction, forming a broad roof-like apodeme (partly dashed curved arrow in Fig. 7B, C ) of moderate density that hangs over the posterior half of the proximal foramen of the leg (i.e. of the coxa). Medially, the cuticle of the thorax forms a posterolaterally and dorsally directed finger-like extension (dashed black outline on Th in Fig. 7B ). Figure 5. Muscles in the protopod of thoracopod 1 (Thp1; right) of Anaspides richardsoni (female); micro-computed tomography volume rendering. A, B, similar orientation with different muscles blanked out; dotted white lines emphasize the outline of the basis, the ventral margin of the proximal epipod, the thorax, the proximal endite and the endoskeletal ligament in the coxa; posterior coxa is coloured light blue; intrabasis is coloured light grey. A, posterior view; partly dashed curved arrow emphasizes the posteromedial apodeme of the thorax. B, posterior view with proximal endoskeleton. Abbreviations: Ba, basis; dEp, distal epipod; dis, distal; Ex, exopod; InB, intrabasis; lat, lateral; Lg, ligament in coxa; med, medial; pCo, posterior coxa; pEn, proximal endite; pEp, proximal epipod; pEs, proximal endoskeleton between thorax and coxa; prox, proximal; Th, thorax. Numbered muscle labels are as in Table 2. Figure 6. Muscles in the endopod of thoracopod 1 (Thp1; right) in Anaspides richardsoni (female); micro-computed tomography volume rendering. A, posterior view; dashed line emphasizes the proximal margin of the basis. B, anterior view; dashed line outlines concealed muscle CD1; dactylus indicated only by white outline; muscle PD2 blanked out. C, posterolateral view. Abbreviations: ant, anterior; Ba, basis; Ca, carpus; Da, dactylus; dis, distal; Is, ischium; lat, lateral; Me, merus; Pr, propodus; prox, proximal. Numbered muscle labels are as in Table 2. The protopod of M. norƲegica comprises a coxa (onethird) and a subdivided basis (two-thirds of the total length of the protopod). The coxa is predominantly made up of its more solid distal margin, which forms a half-ring reaching from posterior over medial to anterolateral (Co in Fig. 7D ), where it connects to a plate of less dense cuticle. From the anterolateral corner of this plate, a solid cuticular beam reaches dorsad (Cb in Fig. 7C, D ) and articulates with the thorax (*l in Fig. 7B–D ). Posterolateral on the coxa, an epipod arises, which on Thp1 is plate-like (Ep in Fig. 7A, B ), extending somewhat dorsad, but mostly ventrad (almost reaching the exopod). On the posterior thoracopods, the epipod splits into two coiled branches (one dorsal and one ventral) that ramify into several smaller filaments; on Thp6–8, the epipods show the highest complexity, with an additional ventral off-branch that ramifies in a similar way. Medially attached to the coxa of Thp1, a setose endite is present (En, Fig. 7A, C ). The endite forms the medial articulation point with the thorax, immediately beneath the finger-like extension (*m in Fig. 7B ). On the following thoracopods, the endite is much smaller, if present at all ( Fig. 7E ). Apart from the cuticular half-ring constituting the distal margin of the coxa, its anterior and posterior parts are mostly membranous, with two (posterior and posterolateral) invaginations. The posterolateral invagination forms a tight proximodistal fold, which expands into the central space of the coxa (curved arrows in Fig. 7D ), where it forms a solid and highly complex endoskeletal structure (dark blue part of Co in Fig. 7B–D ; see also the Supporting Information, Video S1). Starting from a posterolateral arm, the structure reaches into the centre of the coxa, where it expands into the dorsal and ventral directions. Dorsally, it reaches onto the roof-like apodeme of the thorax. Ventrally, it reaches onto the proximal margin of the basis by extending another two straight arms in anterior and posterior directions. Table 2. Extrinsic and intrinsic muscles in thoracopod 1 (Thp1; right) of Anaspides richardsoni (female)

Muscle	Size	Location of proximal attachment site (origin)	Location of distal attachment site (insertion)
TC1 (Fig. 5A)	//	Posteromedial on roof apodeme of thorax	Medial on posterior invagination of coxa
TC2 (Fig. 5A)	//	On posterolateral body wall of thoracomere	Posteromedial on proximal endite of coxa
TC3 (Fig. 5B)	//	Anterior on body wall of thoracomere	Anteromedial on proximal margin of coxa
TC4 (Fig. 5B)	//	Anterior on body wall of thoracomere	Anterior on proximal margin of coxa
TC5 (Fig. 5A)	///	Anterolateral on body wall of thoracomere (pleuron)	Posterolateral on posterior invagination of coxa
TC6 (Fig. 5B)	///	Anterolateral, on infolding of lateral wall of thoracomere; close to proximal margin of coxa	Anterior on endoskeletal ligament plane be- tween anterior coxa and posterior invagination
TEp1 (Fig. 5A)	/	On posteromedial body wall of thoracomere; close to margin of coxa	Posterolateral on posterior coxa plate; close to the origin of distal epipod
TIn1 (Fig. 5B)	///	Lateral on body wall of thoracomere (pleuron)	Anterolateral on proximal margin of intrabasis
TB1 (Fig. 5A, B)	///	On anteromedial extension of proximal endoskel- eton; close to edge of thorax	Anteromedial on proximal margin of basis
TB2 (Fig. 5A)	///	Posteromedial on roof apodeme of thorax	Posterior on proximal margin of basis
TB3 (Fig. 5A)	///	Posteromedial on roof apodeme of thorax; close to M4	Posteromedial on proximal margin of basis
CEp1 (Fig. 5A, B)	/	On posteromedial extension of proximal endo- skeleton; close to proximal margin of coxa	Lateral on anterior coxa, close to the origin of proximal epipod
CIn1 (Fig. 5A)	//	Posteromedial on margin of proximal endite	Distal on anterolateral wall of intrabasis
CIn2 (Fig. 5B)	//	Posteromedial, on posterior coxa plate and anteromedial extension of proximal endoskel- eton	Anterolateral on proximal margin of intrabasis; close to M8
CB1 (Fig. 5B)	///	On anteromedial extension of proximal endoskel- eton of coxa	Posteromedial on proximal margin of basis; close to M21
CB2 (Fig. 5A)	//	Posteromedial on posterior coxa plate	On posterior margin of basis
CB3 (Fig. 5A)	///	Posteromedial on posterior invagination of coxa	Posterolateral on proximal margin of basis
CB4 (Fig. 5A)	///	Posterolateral on posterior invagination of coxa	Posterolateral on proximal margin of basis
CB5 (Fig. 5B)	/	On anterior extension of proximal endoskeleton; close to proximal margin of coxa	Posteromedial on proximal margin of basis
InB1 (Fig. 5A)	//	Anterolateral on distal margin of intrabasis	Posterolateral on proximal margin of basis
BIs1 (Fig. 6A)	//	Posterolateral on wall of basis	Posterolateral on proximal margin of endopod (ischium)
BIs2 (Fig. 6A)	///	On medial half of posterior wall of basis	Posteromedial on proximal margin of endopod (ischium)
IsM1 (Fig. 6A, B)	///	Posteromedial to posterolateral at proximal margin of ischium	Posterior on proximal margin of merus
IsM2 (Fig. 6B)	//	Anterior at proximal margin of ischium	Anterior on proximal margin of merus
MC1 (Fig. 6B)	//	Anterior to anteromedial at proximal half of merus	Anterior on proximal margin of carpus
MC2 (Fig. 6A–C)	///	As two strands medial and lateral at proximal margin of merus	Posterior on proximal margin of carpus

Table 2. Continued

Muscle	Size	Location of proximal attachment site (origin)	Location of distal attachment site (insertion)
CP1	//	Posterior at proximal margin of carpus	Posterior on proximal half-ring at margin of
(Fig. 6B, C)	propodus
CP2	//	Lateral at proximal half of carpus	Anterolateral on proximal half-ring at margin of
(Fig. 6C)	propodus
CP3	/	Anterolateral at about half of carpus	Anterolateral on proximal margin of propodus
(Fig. 6C)
CP4	//	Anterior at proximal margin of carpus	Anteromedial on proximal half-ring at margin of
(Fig. 6B)	propodus
CD1	/	Posterior on proximal margin of carpus	Posterior on proximal margin of dactylus; to-
(Fig. 6B, C)	gether with M31
PD1	//	Posterior at proximal margin of propodus; closely	Posterior on proximal margin of dactylus; around
(Fig. 6B, C)	proximal to M30	M30
PD2	/	Anterior at proximal margin of propodus	Anterior on proximal margin of dactylus
(Fig. 6C)

Muscles are characterized by their proximal and distal attachment sites. Relative size (compared with one another, based on circumference) is specified by one (/ = ‘small’) to three (/// = ‘large’) slashes. The basis comprises a solid distal part (Ba in Fig. 7A–E ), whereas its proximal part (InB in Fig. 7C–E ) consists of three posterior and lateral plates and a broad anterior part. Owing to the resemblance to the anaspidan intrabasis, the proximal part(s) of the basis are referred to as the intrabasis in the following description. The anterior part of the intrabasis is separated from the larger distal basis by an inconspicuous furrow (dashed white line in Fig. 7C, E ). Anteriorly, it forms a small bulb at its proximal margin ( Fig. 7E ), into which the anterior arm of the coxal endoskeleton reaches. Proximal to the anterior bulb, at the proximal margin of the intrabasis, sits a somewhat triangular plate (white outline on InB and partly dashed curved arrow in Fig. 7C ), which articulates with the coxa (a + in Fig. 7C ). Another articulation point is realized between the coxa and the posteromedial plate of the intrabasis (p + in Fig. 7D ), resulting overall in a bicondylar coxa–intrabasis articulation (ax Co-InB in Fig. 7B ). The medial cuticle of the distal basis appears robust, whereas laterally it is mostly membranous. In this area, two rigid, finger-like structures emerge (one posterolateral and another distoanterolateral) and articulate with two hook-like structures of the exopod ( Fig. 7B, C ; see also Supporting Information, Video S1). The posterolateral articulation is supported by a longish sclerite positioned immediately distal to the posterolateral finger. The medial cuticle of the distal basis appears robust and carries numerous setae. Distally, the basis is coalesced with the endopod (i.e. with the ischium), with a clear suture but no interpodomere membrane or articulation points ( Fig. 7C, D ). Within the lateral membrane of the distal basis, the exopod attaches (Ex in Fig. 7A–D ). Its peduncle comprises a short posteroventral stem and a much longer distal portion, followed by the flagellum. The peduncle is anteroposteriorly flattened and has a prominent distodorsal pointed protrusion. Proximally, the peduncle articulates with the basis both anterolaterally and posterolaterally via hook-like structures (one from the peduncle stem and one from the distal portion) that are interlocked with the finger-like structures of the distal basis (short arrow in Fig. 7B ; see also Supporting Information, Video S1). The endopod consists of the ischium, merus, carpus, propodus and dactylus. The ischium (Is in Figs 7A , 8A ) is about twice as long as the protopod. Anteriorly, the ischium forms an articulation with the merus (arrow in Fig. 8D ). However, posteriorly an articulation point is not clearly identifiable. Nevertheless, the proximal margin of the merus forms a prominent posterolateral protrusion, which overlaps slightly with the distal margin of the ischium (dashed line in Fig. 8D ). Figure 7. Cuticle of thoracopod (Thp) 1 and Thp3 (right) of Meganyctiphanes norƲegica (female); micro-computed tomography volume rendering. A–D, Thp1; colour code of podomeres as in Figure 1, but with coxal endoskeleton highlighted in darker blue. A, B, epipod emphasized by white outline. C, D, epipod blanked out. A, anterior view. B, dorsal view of protopod; endopod blanked out; partly dashed curved arrow emphasizes the posterior apodeme of the thorax; black dashed line emphasizes the finger-like posterodorsal extension of the thorax; white dashed line emphasizes anterior invagination of the intrabasis, proximal to the anterior bulb; black lines emphasize the outlines of the posterior thorax apodeme, individual parts of the coxa and the proximodorsal hook of the exopod (small arrow). C, anterior view; dashed white line emphasizes Figure 8. Cuticle of endopod of thoracopod 1 (Thp1; right) of Meganyctiphanes norƲegica (female); micro-computed tomography volume rendering. A, anterior view; colour code of podomeres as in Figure 1. B, C, details of carpus–propodus joint; white lines emphasize the distal margin of the carpus; orange lines emphasize the proximal margin of the propodus. B–D, macerated samples; volume rendering using PHYSICS.ICOL COLORMAP (see colour scale at bottom of D). B, distolateral view. C, proximomedial view; medial wall of carpus blanked out. D, dorsal view of ischium–merus joint, from inside the ischium; white line emphasizes the distal margin of the ischium; dashed line emphasizes the prominent protrusion of the proximal merus margin. Abbreviations: ant, anterior; ax, articulation axis between […]; Ca, carpus; Da, dactylus; dis, distal; Is, ischium; lat, lateral; Me, merus; Pr, propodus. The merus (Me in Fig. 8A ) appears slightly longer than the ischium. Two long sclerites/tendons emerge laterally and medially from the proximal margin of the merus and reach far into the ischium. A bicondylar articulation with the carpus is formed by a medial and a lateral articulation point, respectively (ax Me-Ca in Fig. 8A ). The carpus (Ca in Fig. 8A ) measures approximately one-third of the total length of the merus. One short and two long sclerites/tendons emerge anteriorly (long) and posteriorly (long and short) from the proximal margin of the carpus and reach far into the merus. The distal margin forms anteriorly a noselike condyle that articulates with a broad anterior depression at the proximal margin of the propodus ( Fig. 8B, C ). A thin, longish sclerite articulates laterally with the distal margin of the carpus and posteriorly with the proximal margin of the propodus ( Fig. 8B, C ). Combined, these structures probably realize a somewhat less restricted bicondylar articulation. the furrow between the intrabasis and basis; black lines emphasize individual plates of the coxa; white lines and partly dashed curved arrows emphasize the posterior thorax apodeme and the proximal anterior triangular plate of the intrabasis. D, posterior view; dashed white line emphasizes the posterior plates of the intrabasis; dashed white circle indicates the anterior bulb of the intrabasis; black lines emphasize individual parts of the coxa and the margins of the intrabasis; curved arrows indicate posterolateral coxa invagination. E, Thp3, anterior view; dashed line emphasizes the furrow between intrabasis and basis. Abbreviations: a*, anterior articulation point; ant, anterior; ax, articulation axis between […]; Ba, basis; Ca, carpus; Cb, proximolateral cuticular beam of coxa; Co, coxa; Da, dactylus; dis, distal; En, endite; Ep, epipod; Ex, exopod; InB, intrabasis; Is, ischium; l* and *l, lateral articulation point; lat, lateral; *m, medial articulation point; Me, merus; med, medial; p*, posterior articulation point; Pr, propodus; prox, proximal; Th, thorax. The propodus (Pr in Fig. 8A ) is about twice as long as the carpus. One short and three long sclerites/ tendons emerge from the anteromedial, posteromedial, anterolateral (all three long) and posterolateral (short) corners of the proximal margin of the propodus and reach into the carpus. The propodus articulates with the dactylus by a medial and a lateral articulation point, respectively (ax Pr-Da in Fig. 8A ). The dactylus is about half the length of the propodus in Thp1 (Da in Figs 8A , 9E ) and somewhat longer than half the propodus in the posterior thoracopods. Two long sclerites/tendons emerge anteriorly and posteriorly from the proximal margin of the dactylus and reach into the propodus. The podomere tapers from its proximal to its distal margin, except for Thp2, where it is more lateromedially flattened, forming a slight distoposterior bulge before tapering towards a distoanterior tip.