Thalassocnus

Amson, Eli, de Muizon, Christian & Gaudin, Timothy J., 2017, A reappraisal of the phylogeny of the Megatheria (Mammalia: Tardigrada), with an emphasis on the relationships of the Thalassocninae, the marine sloths, Zoological Journal of the Linnean Society 179, pp. 217-236 : 232

THALASSOCNUS WITHIN THE TARDIGRADA

The present analysis is the first to place Thalassocnus (considered as such) amongst megatheriids. A Megatheriidae that includes Thalassocnus , is supported by nine unambiguous synapomorphies (and up to 36 depending on the optimization), amongst which four are postcranial: rounded and distally positioned medial epicondyle of the humerus [ch. 3 (0)1); CI = 1/2, RI � 0.9; Fig. 1C, D ], laterodistal process of radius extending far distally [ch. 8 (0)1); nonhomoplastic; Fig. 2C, D ], a strongly distinct odontoid process of the astragalus [ch. 35 (1)2); CI = 2/5, RI = 2/3], and sustentacular and cuboid facets of the calcaneum widely confluent [ch. 41 (0)1); CI = 1/2, RI = 4/5]. The craniodental synapomorphies of the Megatheriidae are: an elongate condyloid process [ch. 112 (2)0); CI = 2/5, RI = 2/3], a plane of the condylar articular surface that changes mediolaterally [ch. 121 (0)1); CI = 1/3, RI = 2/3], an elongate symphysis [ch. 123 (2)3); CI = 2/3, RI � 0.9], moderately developed symphyseal spout [ch. 129 (1)2); CI = 2/5, RI = 2/3], and the absence of clear demarcation between symphysis and horizontal ramus [ch. 130 (0)1); nonhomoplastic]. Furthermore, the Megatheriinae and Thalassocnus are united by 15 unambiguous synapomorphies (and up to 57 depending on the optimization), the postcranial ones are: short humerus [ch. 1 (0)1); CI = 1/2, RI � 0.7], fovea capitis only partially included in the femoral head articular surface [ch. 27 (0)1); CI = 1/2, RI � 0.8; Fig. 5C, D ], anterior border of medial and lateral facets of the proximal tibia at same level [ch. 34 (0)1); CI = 1/2, RI � 0.9], right angle between the odontoid and discoid facets of the astragalus in distal view [ch. 36 (0)1); CI = 1/3, RI = 1/2], and strong development of the proximal processes of the tuber calcis [ch. 40(0)1); CI = 1/2, RI = 0.9; Fig. 7C, D ]. For this last character, Planops , positioned in our results as the sister taxon to all other included megatheriids, features an interesting condition ( Fig. 7B ). Because its lateroproximal process extends more distally than that of nonmegatheriid megatherioids ( Fig. 7A ), it can be viewed as having an intermediate condition when compared to those of other megatheriids ( Thalassocnus included), in which this process and the medioproximal process are more developed distally ( Fig. 7C, D ). The megatheriines and Thalassocnus also share ten unambiguous craniodental apomorphies, amongst them: toothrow horizontal in lateral view [ch. 64 (2)0); CI = 1/2, RI = 3/ 5], tympanic fused dorsally [ch. 265 (0)1); CI = 1/3, RI = 3/4], and hemispherical glenoid [ch. 338 (0)1); CI = 1/2, RI = 4/5].

OTHER RELATIONSHIPS AMONGST MEGATHERIOIDEA

Although not the focus of the present study, some comments can be made regarding the other nodes of the tree produced by our analysis. As in previous phylogenetic analyses ( Gaudin, 2004; Pujos et al., 2007; and references therein), the Megatheria, a clade comprising the megatheriids and the nothrotheriids, is recovered. Whereas this clade was supported by only four unambiguous synapomorphies in Gaudin (2004), seven unambiguous synapomorphies are obtained here (and up to 31 depending on the optimization). Only two of these are postcranial synapomorphies. This could suggest that the inclusion of Thalassocnus itself in an analysis that comprises both families of Megatheria further substantiates the recognition of this clade, although the modification of the taxonomic sample when compared to the analysis of Gaudin (2004) cannot be ruled out as an alternative cause of the increase of unambiguous synapomorphies for the Megatheria. Concerning their postcranium, the Megatheria are defined by the medially projecting bicipital tuberosity of the radius [ch. 6 (0)1)] and the prominent anterior extension of the medial trochlear ridge of the femur [ch. 31 (1)0)]. Furthermore, they are unambiguously defined by parallel lateral edges of the mandibular spout [ch. 133 (1)0); CI = 1/2, RI = 3/4], a posterior external opening of mandibular canal that opens laterally on the horizontal ramus [ch. 136 (0)1); nonhomoplastic], fused vomerine wings, leaving the overlying ethmoid unexposed [ch. 260 (0)1); CI = 1/ 3, RI = 3/5], medial expansion of entotympanic dorsal to floor of basicranium [ch. 292 (1)0); CI = 1/4, RI � 0.6], stylomastoid foramen connected to nearby ventral opening of canal for occipital artery by a strong groove [ch. 321 (1)3); CI � 0.4, RI � 0.6], and occipital artery completely enclosed within a canal [ch. 331 (1)3); CI = 0.3; RI � 0.7].

According to Gaudin (2004), the clade Megatherioidea includes the Megatheriidae , Nothrotheriidae , and a third family, the Megalonychidae (which comprises the extant two-toed sloth Choloepus ), along with several Santacrucian taxa whose relationships are not entirely resolved, namely Schismotherium , Pelecyodon , Hapalops , and Analcimorphus . Our results yield an unambiguous resolution of the relationships amongst these early megatherioids and the three megatherioid families. Schismotherium and Pelecyodon form a clade that represents the sister group of all other Megatherioidea, herein called ‘clade A’. This clade is not well supported (branch support value of 2), but it is noteworthy that it was also found in one of the MPTs of Gaudin (2004). It is defined by six unambiguous synapomorphies: upper and lower caniniforms (C1 and c1) slightly depressed ventrally relative to the remaining molariforms [ch. 64 (0)2); CI = 1/2, RI = 3/5], elongate diastema [ch. 67 (0)1); CI = 1/2, RI � 0.8], sphenopalatine foramen situated well anterior and ventral to sphenorbital fissure/optic foramen [ch. 222 (1)0); CI = 1/3, RI � 0.7], squamosal with lateral bulge at root of zygoma [ch. 228 (0)1); CI = 1/3, RI � 0.7], nuchal crest overhangs occiput posteriorly [ch. 245 (0)1); nonhomoplastic], and rugose tympanic external surface [ch. 263 (0)1); CI = 1/2, RI = 4/5]. Hapalops is positioned here as sister taxon of a clade consisting of Analcimorphus and megalonychids, all forming the ‘clade B’ ( Fig. 10 ). This clade is not well supported either (branch support value of 1), but was also recovered in some of the analyses of Gaudin (2004), depending on the character weighting scheme. The ‘clade B’ is defined by seven unambiguous synapomorphies: no contact between lunar and unciform [ch. 12 (1)0); CI = 1/2, RI = 1/2), median position of astragalar process for navicular in distal view [ch. 38 (1)0); nonhomoplastic], 18 or more thoracic vertebrae [ch. 51 (1)0); nonhomoplastic], elongate and narrow coronoid process of dentary [ch. 108 (2)0); CI = 1/4, RI � 0.6], one posteriorly projecting point on distal portion of descending process of jugal [ch. 215 (1)0); CI = 1/4, RI = 1/2], median ridge of occiput extends dorsally onto the roof of the skull [ch. 246 (0)1); CI = 1/2, RI = 1/2], and occipital condyles with distinct neck [ch. 254 (0)1); CI = 1/3, RI � 0.8]. The Nothrotheriidae are recovered as monophyletic and well supported (branch support value of 9), with 11 unambiguous synapomorphies, amongst them the presence of a contact between the pterygoid and the vomer [ch. 193 (0)1); CI = 1/2, RI = 3/4], the vomer bearing an elongate asymmetrical ventral keel and extending posteriorly into nasopharynx [ch. 261 (0)1), nonhomoplastic], and a very large exposure of the vomer, which covers the presphenoid and much of the basisphenoid [ch. 262 (0)1), nonhomoplastic].

Figures

Figure 1. Anterior view of the distal epiphysis of the right humerus amongst megatherioid sloths. A, Hapalops longiceps (from Scott, 1903–1904); B, Nothrotheriops shastensis; C, Thalassocnus littoralis; D, Megatherium americanum (from Owen, 1858). Not to scale.

Figure 2. Anterior view of the distal epiphysis of the right radius amongst megatherioid sloths. A, Hapalops longiceps (from Scott, 1903–1904); B, Nothrotheriops shastensis; C, Thalassocnus littoralis; D, Megatherium americanum. Not to scale.

Figure 5. Posterior view of the right femur amongst megatherioid sloths. A, Hapalops sp.; B, Planops martini; C, Thalassocnus littoralis; D, Megatherium americanum. Not to scale. Abbreviations: 3rd troch., third trochanter; fov. cap., fovea capitis; great. troch., greater trochanter; lat. cond., lateral condyle; lat. epic., lateral epicondyle; less. troch., lesser trochanter; med. cond., medial condyle; med. epic., medial epicondyle.

Figure 7. Plantar view of the left calcaneum amongst Megatheria. A, Nothrotheriops shastensis; B, Planops martini; C, Thalassocnus natans; D, Megatherium americanum. Not to scale.

Figure 10. Phylogeny of megatherioid sloths. The PAUP heuristic search (Swofford, 2002) resulted in a single most parsimonious tree (consistency index = 0.47, retention index = 0.62). The numbers at the nodes are the branch support values. Abbreviations: Moi., Megatherioidea; Myc., Megalonychidae.