Platicrista, Pilato, 1987

Gąsiorek, Piotr, Blagden, Brian, Morek, Witold & Michalczyk, Łukasz, 2024, What is a ‘ strong’ synapomorphy? Redescriptions of Murray’s type species and descriptions of new taxa challenge the systematics of Hypsibiidae (Eutardigrada: Parachela), Zoological Journal of the Linnean Society 202 (1), pp. 1-63 : 34-36

Composition of Platicrista

With body lengths exceeding 800–900 μm, several species of Platicrista are among the largest eutardigrades and are the largest hypsibiids. In light of this, the only species described from the tropics, P. borneensis , is evidently the most divergent in terms of adult body size (<< 500 μm) and buccopharyngeal tube, which is in agreement with its sister position with respect to all other Platicrista spp. ( Fig. 2 ). The two Nearctic species, P. brunsoni and P. cheleusis , evolved a surprisingly similar type of dorsal sculpturing and thus might be closely related. We sequenced an additional, unnamed species from the Peruvian Andes that is a sister taxon of P. angustata ( Fig. 2 ). It exhibits a weakly developed reticulum on the caudal body portion, not the irregular wrinkling present in P. angustata or P. carpathica .

Taxonomic key to the genus Platicrista

Remarks: Platicrista affine ( Mihelčič, 1951) has an erroneous original description, because it is shown with macroplacoids forming a rhomb-like structure in the pharynx; such an arrangement remains unseen in the Itaquasconinae , which are characterized by pharyngeal structures arranged in parallel. We designate the species as objectively invalid, because it describes and depicts a mounting artefact or a morphological aberration and, consequently, we have not included it in the key. Platicrista itaquasconoide is assigned a nomen inquirendum status given that the description was shown to contain an error (the presence of a microplacoid was not confirmed; Marley 2006) and, with an example of I. serratulum , we show here that the presence of ‘spurs’ on posterior claw bases is a morphological aberration, not a reliable taxonomic character. Finally, the claw IV bases of both P. itaquasconoide and P. angustata are indented. Consequently, given that the species lacks any morphological characteristics separating it from P. angustata , it is excluded from the key.

1. Cuticular bars on legs present .................................................................................................................................................................... 2

– Cuticular bars on legs absent ...................................................................................................................................................................... 5

2. Cuticle sculptured with polygonal tubercles ........................................................................................................................................... 3

– Cuticle smooth .............................................................................................................................................................................................. 4

3. Internal bars I absent, claw IV bases indented ........................................................................................................ Platicrista brunsoni

– Internal bars I present, claw IV bases smooth .......................................................................................................... Platicrista cheleusis

4. Only median and posterior bars present, pt of the stylet insertion point <90% .......................... Platicrista borneensis sp. nov.

– Only internal bars I–III present, pt of the stylet insertion point> 95% .............................................................. Platicrista ramsayi

5. Buccal tube striation present ............................................................................................................................................ Platicrista aluna

– Buccal tube striation absent ........................................................................................................................................................................ 6

6. Caudal granulation present only on the lateralmost portions of the caudal trunk ........................ Platicrista carpathica sp. nov.

– Caudal granulation absent or present on the entire caudal trunk ....................................................................................................... 7

7. Claws IV with smooth bases .............................................................................................................................. Platicrista nivea sp. nov.

– Claws IV with indented bases .................................................................................................................................................................... 8

8. Pseudolunulae IV smooth ......................................................................................................................................... Platicrista angustata

– Pseudolunulae IV indented ........................................................................................................................................ Platicrista horribilis

Figures

Figure 2. Phylogenetic relationships within Hypsibiidae in the Bayesian and maximum likelihood analyses (identical topologies; the scale refers to the Bayesian tree and represents substitutions per position; posterior probability values are provided above the nodes, whereas bootstrap values can be found below the nodes; *, maximal support; #, no support).Calohypsibius ornatus constitutes an outgroup. Newly added species and populations are in bold.