Tanaidaurum kachinensis, Pazinato & Müller & Haug, 2023

Pazinato, Paula G., Mueller, Patrick & Haug, Joachim T., 2023, New species of Tanaidacea from Cretaceous Kachin amber, with a brief review of the fossil record of tanaidacean crustaceans, Fossil Record 26 (1), pp. 39-50 : 39

Tanaidaurum kachinensis sp. nov.

Type material.

Holotype SNHMB.G 8008.

Etymology.

“Tanaid” in reference to Tanaidacea + " aurum " ( “gold” in Latin) in reference to the golden appearance of the preservation. The specific epithet “kachinensis” refers to the Kachin State in Myanmar, where the specimens come from.

Diagnosis.

Body cylindrical, surface smooth. Shield with set off triangular sclerite postero-laterally on each side. Antennula with four elements. Antenna uniramous, four elements. Appendages of post-ocular segment 7 ( ‘cheliped’) with ischium not separated from basipod. Appendages of post-ocular segments 8-10 (trunk appendages 1-3; ‘pereopods’ 1-3) slender, ischium separated from basipod; distal part of dactylus ( ‘unguis’) distinctly sclerotized, elongated, separated from proximal part of dactylus. Appendages of post-ocular segments 11-13 (trunk appendages 4-6; ‘pereopods’ 4-6) stouter, ischium separated from basipod; one pair of distal spines on merus, carpus and propodus; dactylus curved, specialized as a claw, no distinct separate part ( ‘unguis’’) apparent. Group of short setae in the lateral margin of pleon segments. Uropods with endopod and exopod differing slightly in length, two elements in both rami with terminal long setae.

Description.

Overall morphology. Body longer than wide, differentiated into functional head and trunk, the latter further subdivided into an anterior part and posterior part. Functional head ( ‘cephalothorax’) composed of eight segments: six segments of the ancestral eucrustacean head (ocular segment + post-ocular segments 1-5: segments of antennula and antenna accessible, mandible, maxillula and maxilla not accessible) + two anterior trunk segments (post-ocular segments 6-7: segments of maxilliped and ‘cheliped’ accessible). Segments of functional head dorsally conjoined forming a distinct shield ( ‘carapace’). Anterior part of the trunk ( ‘pereon’) composed of six segments (post-ocular segments 8-13, ‘pereon’ segments 1-6). Posterior part of the trunk (pleon) composed of five free segments (post-ocular segments 14-19, pleon segments 1-5) and a pleotelson (post-ocular segment 14, pleon segment 6 conjoined to the telson) (Figs 1 - 3 ).

Functional head (cephalothorax). Dorsally accessible, one-fourth of the length of the entire body (body length measured from anterior edge of shield to tip of telson (Table 1 ); surface of shield smooth, postero-lateral edges with set off triangular sclerites. Compound eyes, protruding antero-laterally (Figs 2B , 4 ). Appendages of functional head. Antennula and antenna (appendages of post-ocular segments 1, 2) arising anterior-medially. Most mouthparts not accessible (appendages of post-ocular segments 3-5). Two distal elements of maxilliped (appendage of post-ocular segment 6) and cheliped (appendage of post-ocular segment 7).

Antennula longer, broader and dorsally displaced in relation to antenna; four elements, proximal-most element longest and broadest, subsequent elements progressively shorter than proximal one, at least one long seta arising in the connective joint between elements 1 and 2, as well as 3 and 4, at least five shorter setae in the anterior part of distal-most element.

Antenna shorter and slenderer than antennula, four elements, proximal-most elements 1 (coxa?) and 2 (basipod?) shorter and broader than subsequent elements (endopod?), element 3 longest, at least one long seta arising in the connective joint between elements 3 and 4, at least two shorter setae in the anterior part of distal-most element.

Maxilliped with two distal elements accessible, anterior margin of distal-most element convex with at least five setae. ‘Cheliped’ with five elements accessible most likely representing basipod, merus, carpus, propodus and dactylus; basipod wider than long (2 ×), merus sub-triangular in ventral view, carpus longer than wide (2 ×), propodus with cone-like projection (fixed-finger) longer than wide (2 ×), fixed finger with five setae on inner surface, dactylus as long and wide as fixed finger, tapering distally (Figs 1 , 4 ).

Anterior trunk (free thorax, ‘pereon’). Six free segments, dorsally each with a tergite; tergites sub-equal in shape, anterior and posterior margins straight, lateral margins semi-circular; tergite of trunk segments 3-5 (post-ocular segments 8-13) gradually increasing in length, tergites of trunk segments 6 and 7 (post-ocular segments 11 and 12) longest, tergite of trunk segment 8 (post-ocular segment 13) slightly shorter than previous segment; width not accessible. Surface of tergites smooth, except for transversal rim near the posterior margin; sclerites of ventral surface (sclerites) smooth.

Anterior trunk appendages ( ‘pereopods’). Appendages of trunk segments 3-5 sub-equal in shape, each with seven elements: coxa, basipod, ischium, merus, carpus, propodus and dactylus; coxa wider than long (1.8 ×), basipod longest element, longer than wide (3.5 ×), ischium wider than long (1.5 ×), carpus, merus and propodus sub-equal, longer than wide (1.5 ×), dactylus almost as long as basipod, proximal part of dactylus short, as long as wide, distal part of dactylus ( ‘unguis’) slender and elongated, tapering distally, at least one seta in carpus/propodus and one in propodus/dactylus connective joints; trunk appendages 6-8 sub-equal in shape, stouter than trunk appendages 3-5, six elements, from proximal to distal: possibly basipod, ischium, merus, carpus, propodus and dactylus; basipod longest and widest element, longer than wide (2 ×), ischium ring-like, merus, carpus and propodus sub-equal in length and shape, longer than wide (2.5 ×) with two spines latero-distally positioned, dactylus without distinct differentiation (no ‘unguis’ apparent), dactylus sickled-shaped (specialized as claw), at least four setae in propodus/dactylus connective joint (Figs 2C, D , 4 ).

Posterior trunk (pleon). Five free segments, dorsally with tergites; tergites sub-equal in shape, each half of the length of the tergites of the first free thorax segment and same width of these anterior and posterior margins straight, lateral margins semi-circular with group of short setae (Fig. 3 ), surface smooth; sclerites in ventral surface (sternites) with mid furrow in antero-posterior direction; pleotelson about twice the length of last free pleon segment, semi-circular posterior margin, dorsal and ventral surfaces smooth.

Pleon appendages (pleopods). Each free pleon segment bears one pair of appendages (pleopods), only distal portion of pleopods accessible, distal margin semi-circular bearing many setae. Unclear if uniramous or biramous; pleotelson bearing one pair of uropods (appendage of pleon segment 6) latero-posteriorly arising from pleotelson; uropods with basipod bearing two rami - endopod and exopod; basipod shorter and wider than rami; endopod longer than exopod (1.18 ×), two elements: proximal element longer than wide (5 ×), distal element longer than wide (2.5 ×), at least four long setae distally; exopod with two elements: proximal element longer than wide (3 ×), distal element longer than wide (5 ×), at least two long setae distally (Fig. 3B ).

Figures

Figure 1. Digital microscopy images of Tanaidaurum kachinensis gen. et sp. nov .. Specimen SNHMB. G 8008 in dorsal (A) and ventral (B) views. Specimen BUB 4227 - 1 in lateral view, left (C) and right (D) sides. Co-axial polarized light with black (A, D) and white (B, C) backgrounds.

Figure 2. Digital microscopy images of Tanaidaurum kachinensis gen. et sp. nov. Specimen BUB 4227 - 2 in ventral view (A), ring light with white background. Detail of anterior part of functional head, showing compound eye, antennula and antenna (B). Detail of trunk appendage 1 (C), detail of trunk appendage 4 (D) of the holotype.

Figure 3. Digital microscopy images of Tanaidaurum kachinensis gen. et sp. nov. Holotype, ventral view of the pleon (A). Detail of uropod (B). Detail of pleon segments (C, D), arrow heads point to bundles of setae in the lateral margin of the pleon segments.

Figure 4. Digital drawing of Tanaidaurum kachinensis gen. et sp. nov. Holotype in dorsal (A) and ventral (B) views. Abbreviations: at: antenna, an: antennula, ce: compound eyes, cp: ' cheliped', mp: maxilliped, pl 1 - 5: pleon segment 1 - 5, plp 1 - 5: pleopod 1 - 5, plt: pleotelson, s: shield (' carapace'), sc: sclerite, ta 3 - 8: trunk appendage 3 - 8, tam: trunk appendage muscle tissue, ts 3 - 8: trunk segment 3 - 8, ur: uropod.