The Phylogenetic Affinities of the Enigmatic Mammalian Clade Gondwanatheria

Abstract

Gondwanatheria is a group of extinct mammals known from the Cretaceous and Paleogene of Gondwana. Resolution of the phylogenetic affinities of gondwanatherians has proven problematical, with the group currently considered Mammalia incertae sedis. We briefly review the morphology of known gondwanatherians, and argue that isolated upper premolars and a partial dentary preserving a blade-like p4 originally referred to the ferugliotheriid gondwanatherian Ferugliotherium windhauseni but subsequently identified as Multituberculata incertae sedis do indeed belong to F. windhauseni. We also suggest that the recently described ?cimolodontan multituberculate Argentodites coloniensis, based on an isolated lower premolar, may in fact be an unworn p4 of Ferugliotherium or a closely related taxon. We present the first phylogenetic analyses to include gondwanatherians, using maximum parsimony and Bayesian methods. Both methods place Ferugliotherium and sudamericid gondwanatherians in a clade with cimolodontan and “plagiaulacidan” multituberculates, although relationships within this clade are largely unresolved. The Gondwanatheria + Multituberculata clade supported here may reflect the convergent evolution of similar dental features, but it is the best supported hypothesis based on currently available data. However, denser sampling of multituberculate taxa and the discovery of more complete gondwanatherian fossils will be required to clarify the precise relationship between gondwanatherians and multituberculates, specifically to determine whether or not gondwanatherians are members of Multituberculata. We hypothesize that the anterior molariforms of sudamericid gondwanatherians evolved from blade-like precursors similar to the p4 of Ferugliotherium, possibly in response to the appearance of grasses in Gondwana during the Cretaceous.

Appendices

Appendix 1

List and description of characters from the matrix of Rougier et al. (2007) that can be scored for Ferugliotherium and the sudamericid genera Sudamerica and Gondwanatherium.

6. Mandibular symphysis:

Feruglotherium: ?

Sudamerica: 1 – Unfused

Gondwanatherium: ?

Notes: the Ferugliotherium dentary does not preserve the symphyseal region. The symphysis of Sudamerica is unfused (Pascual et al. 1999). Bonaparte (1990a) tentatively assigned an extremely fragmentary, edentulous partial dentary (MACN Pv-RN 228) to Gondwanatherium. Pascual et al. (1999) questioned the referral of this specimen to Gondwanatherium, but Gurovich (2001; 2006) presented new evidence that the referral is correct and we have followed her conclusions here. However, the symphysis cannot be identified with certainty in this specimen (Gurovich 2006), and hence we score Gondwanatherium as unknown (‘?’) for this character.

16. The pterygoid fossa on the medial side of the mandible (Gambaryan and Kielan−Jaworowska 1995):

Ferugliotherium: ?

Sudamerica: 1 – Present.

Gondwanatherium: ?

Notes: the Ferugliotherium dentary does not preserve the pterygoid region. Pascual et al. (1999) argued that the pterygoid fossa was extensive in Sudamerica. The Gondwanatherium dentary does not preserve the pterygoid fossa (Gurovich 2006).

26. Alignment of ultimate molar to the anterior margin of the dentary coronoid process:

Ferugliotherium: ?

Sudamerica: 0 - Ultimate functional molar is medial to the coronoid process.

Gondwanatherium: ?

Notes: the dentary of Ferugliotherium does not preserve the coronoid process. The anterior margin of the coronoid process is lateral to the molar row of Sudamerica (Pascual et al. 1999). This character follows the revised definition of Rougier et al. (2007). The Gondwanatherium dentary does not preserve the coronoid process (Gurovich 2006).

29. Ultimate lower premolar—arrangement of principal cusp a, cusp b (if present) and cusp c (we assume the cusp to be c if there is only one cusp behind the main cusp a):

Ferugliotherium: 3 - Premolar multicuspate and blade−like

Sudamerica: ?

Gondwanatherium: ?

Notes: Following Kielan-Jaworowska and Bonaparte (1996), we identify the bladelike premolar preserved in the dentary of Ferugliotherium as p4, i.e., the ultimate lower premolar. The homologies of the molariforms of Sudamerica and Gondwanatherium with the dental loci of other taxa are controversial (see text), and so neither of these taxa has been scored for this character.

31. Ultimate lower premolar—outline:

Ferugliotherium: 1 - Bladelike, close to twice as long as wide

Sudamerica: ?

Gondwanatherium: ?

Notes: see character 29. This character follows the revised definition of Rougier et al. (2007).

32. Labial cingulid of the ultimate lower premolar:

Ferugliotherium: 0 - Absent or vestigial

Sudamerica: ?

Gondwanatherium: ?

Notes: see character 29.

33. Lower premolars lingual cingulid:

Ferugliotherium: 0 - Absent or vestigial

Sudamerica: ?

Gondwanatherium: ?

Notes: see character 29.

38. Alignment of main cusps of the posterior lower molar(s) (m3 or more posterior if present):

Ferugliotherium: 3 - Multiple longitudinal multicuspate rows

Sudamerica: 3 - Multiple longitudinal multicuspate rows

Gondwanatherium: 3 - Multiple longitudinal multicuspate rows

Notes: all molars/molariforms referable to Ferugliotherium and sudamericids exhibit multiple longitudinal multicuspate rows.

42. Precise opposition of the upper and lower molars (either one−to−one or occluding at the opposing embrasure or talonid):

Ferugliotherium: 2 - Present: one lower molar sequentially contacts more than one upper molar

Sudamerica: 2 - Present: one lower molar sequentially contacts more than one upper molar

Gondwanatherium: 2 - Present: one lower molar sequentially contacts more than one upper molar

Notes: wear patterns indicate that both Ferugliotherium (Krause et al. 1992) and sudamericids (Koenigswald et al. 1999; Pascual et al. 1999) had a palinal power stroke, as in undoubted multituberculates. Following Rougier et al. (2007), this character was ordered.

43. Relationships between the cusps of the opposing upper and lower molars:

Ferugliotherium: 4 - Lower multicuspate rows alternately occlude between the upper multicuspate rows

Sudamerica: 4 - Lower multicuspate rows alternately occlude between the upper multicuspate rows

Gondwanatherium: 4 - Lower multicuspate rows alternately occlude between the upper multicuspate rows

Notes: Gurovich (2006) reconstructed the occlusion of Ferugliotherium and sudamericids.

51. Labiolingual compression of the primary functional cusps of the lower molars (at the level of the cusp base but above the cingulid):

Ferugliotherium: 0 – Absent

Sudamerica: 0 – Absent

Gondwanatherium: 0 – Absent

52. Posterior lingual cingulid of the lower molars:

Ferugliotherium: 0 – Absent or weak

Sudamerica: 0 – Absent or weak

Gondwanatherium: 0 – Absent or weak

Notes: Following Rougier et al. (2007), this character was ordered.

55. Mesial transverse cingulid above the gum:

Ferugliotherium: 0 – Absent

Sudamerica: 0 – Absent

Gondwanatherium: 0 – Absent

Notes: This character follows the revised definition of Rougier et al. (2007).

57. Postcingulid (distal transverse cingulid) on the lower molars:

Ferugliotherium: 0 – Absent

Sudamerica: 0 – Absent

Gondwanatherium: 0 – Absent

58. Interlocking mechanism between two adjacent lower molars

Ferugliotherium: 0 – Absent

Sudamerica: 0 – Absent

Gondwanatherium: 0 – Absent

Notes: This character follows the revised definition of Rougier et al. (2007).

84. Upper molars interlock:

Ferugliotherium: 0 – Absent

Sudamerica: 0 – Absent

Gondwanatherium: 0 – Absent

85. M1—number of cusps within the main functional straight cusp row (if there are multiple rows, the labial row is designated):

Ferugliotherium: ?

Sudamerica: 1 - Four main functioning cusps or more

Gondwanatherium: ?

Notes: Gurovich (2006) identified only one isolated tooth (MACN-Pv RN 248) from Los Alamitos as possibly representing an MF1 of Ferugliotherium (Krause et al. 1992 had earlier suggested it was a right MF1, but Gurovich 2006, argued that it is from the left side); however, the number of cusps in the labial row is uncertain, and Gurovich (2006) could not rule out the possibility that the tooth was in fact a right mf1. All isolated Sudamerica teeth identified by Gurovich (2006) as representing MF1 exhibit four labial lobes, corresponding to four cusps. Gurovich (2006) did not consider any of the isolated Gondwanatherium teeth in the MACN collection to represent MF1, and so Gondwanatherium has not been scored for this character.

87. Outline of m1:

Ferugliotherium: 5 - Rectangular (or slightly rhombdoidal)

Sudamerica: 5 - Rectangular (or slightly rhombdoidal)

Gondwanatherium: 5 - Rectangular (or slightly rhombdoidal)

88. Aspect ratio of M1:

Ferugliotherium: 4 - Rectangular or nearly so

Sudamerica: 4 - Rectangular or nearly so

Gondwanatherium: ?

Notes: as noted for character 85, MF1 is currently unknown for Gondwanatherium.

90. Lower molars with multicuspate rows—connection between cusps (character modified from Luo et al. 2002 and Rougier et al. 2007):

Ferugliotherium: 2 - prominent transverse ridges and furrows connecting labial and lingual cusp rows

Sudamerica: 2 - prominent transverse ridges and furrows connecting labial and lingual cusp rows

Gondwanatherium: 2 - prominent transverse ridges and furrows connecting labial and lingual cusp rows

Notes: This character is modified from Luo et al. (2002) and Rougier et al. (2007). We have included an additional state for this character to account for the most distinctive feature of the molars of gondwanatherians, namely the presence of prominent transverse ridges and furrows connecting the labial and lingual cusp rows.

93. Functional development of occlusal facets on individual molar cusps:

Ferugliotherium: 2 - Wear facets matches upon the eruption of teeth (inferred from the flat contact surface upon eruption)

Sudamerica: ?

Gondwanatherium: ?

Notes: known molariforms of Gondwanatherium and Sudamerica are too worn to score this character. Following Rougier et al. (2007), this character was ordered.

94. Topographic relationship of wear facets to the main cusps:

Ferugliotherium: 2 - Multicuspate series, each cusp may support 2 wear facets

Sudamerica: 2 - Multicuspate series, each cusp may support 2 wear facets

Gondwanatherium: 2 - Multicuspate series, each cusp may support 2 wear facets

Notes: This character follows the revised definition of Rougier et al. (2007).

105. Number of lower incisors:

Ferugliotherium: 1 - Two or fewer

Sudamerica: 1 - Two or fewer

Gondwanatherium: 1 - Two or fewer

Notes: this character is scored based on the presence of only a single incisor alveolus in the dentaries of both Ferugliotherium and Sudamerica. Gurovich (2006) also tentatively identified a single incisor alveolus in the Gondwanatherium dentary.

108. Total number of the lower premolars

Ferugliotherium: 3 - Two or fewer

Sudamerica: 3 - Two or fewer

Gondwanatherium: ?

Notes: Ferugliotherium is scored on the basis that the bladelike premolar in the dentary is the ultimate (and therefore only) premolar. Following Rougier et al. (2007), this character was ordered. The Gondwanatherium dentary is edentulous (Bonaparte 1990a; Gurovich 2006), and so Gondwanatherium has been scored as unknown (‘?’) for this character.

110. Number of lower molars or molariform postcanines:

Ferugliotherium: ?

Sudamerica: 1 - 4 to 5 molars (or molariforms)

Gondwanatherium: ?

Notes: Gurovich (2006) hypothesised that Ferugliotherium possessed only two lower molars, as known lower molars of Ferugliotherium fell into two classes; however, very few teeth of Ferugliotherium are currently known, and it is possible that this taxon may have had more than two molars but that the posterior molars have yet to be found. Pascual and Ortiz-Jaureguizar (2007, figure 11) suggested that Ferugliotherium has four lower molars, but this was based on the a priori assumption that it has the same number as Sudamerica. In the absence of unequivocal evidence for the postincisive dental formula of Ferugliotherium, we have not scored it for this character. There are four alveoli (two still containing molariforms) in the dentary of Sudamerica, and Koenigwald et al. (1999) and Gurovich (2006) both identified four classes of isolated lower molariforms of Sudamerica. Gurovich (2006) also identified four classes of isolated lower molariforms of Gondwanatherium, but this was based on the a priori assumption that Gondwanatherium has the same number of lower molariforms as Sudamerica; we have instead chosen to score Gondwanatherium as unknown (‘?’) for this character. This character follows the revised definition of Rougier et al. (2007) and was ordered, again following Rougier et al. (2007).

111. Lower postcanine roots:

Ferugliotherium: 2 - Multiple roots (more than three)

Sudamerica: 0 - Root division is incipient or incomplete

Gondwanatherium: 0 - Root division is incipient or incomplete

Notes: Krause et al. (1992) described two pairs of roots in MACN-Pv RN 174, which Gurovich (2006) identified as a left m1 of Ferugliotherium. The roots of Sudamerica and Gondwanatherium molariforms are single, and the root tips are open except in older individuals. Following Rougier et al. (2007), this character was ordered.

115. Procumbent and enlargment of the anteriormost lower incisors:

Ferugliotherium: 1 - Both procumbent and enlarged (at least 50% longer than second functional incisors)

Sudamerica: 1 - Both procumbent and enlarged (at least 50% longer than second functional incisors)

Gondwanatherium: 1 - Both procumbent and enlarged (at least 50% longer than second functional incisors)

Notes: although both Ferugliotherium and sudamericids possess only a single lower incisor, this tooth is clearly procumbent and large; hence, both taxa have been scored as 1 for this character.

117. Enlarged diastema in the lower incisor premolar region (rodentiform):

Ferugliotherium: 1 - Present

Sudamerica: 1 - Present

Gondwanatherium: 1 - Present

Notes: scored based on the presence of diastemata in the dentaries of Ferugliotherium, Sudamerica and Gondwanatherium. This character follows the revised definition of Rougier et al. (2007).

274. Direction of jaw movement during occlusion

Ferugliotherium: 2 - Dorsoposterior movement

Sudamerica: 2 - Dorsoposterior movement

Gondwanatherium: 2 - Dorsoposterior movement

Notes: wear patterns indicate that both Ferugliotherium (Krause et al. 1992) and sudamericids (Koenigswald et al. 1999; Pascual et al. 1999) had a palinal power stroke.

278. Position of the posteriormost mental foramen:

Ferugliotherium: 0/1 (polymorphic) - below the canine and anterior premolariform region/ below the penultimate premolar

Sudamerica: 0/1 (polymorphic) - below the canine and anterior premolariform region/ below the penultimate premolar

Gondwanatherium: ?

Notes: the mental foramen is far forward in the dentaries of both Ferugliotherium and Sudamerica, below the postincisive diastema. As neither canines nor penultimate premolars are identifiable in either taxon, we have scored both as polymorphic (either state 0 or 1) for this character, on the assumption that the canine and penultimate premolar (if they were present) would have occupied the diastemal region.

279. Enamel prism shape:

Ferugliotherium: ?

Sudamerica: 1 – arc

Gondwanatherium: 1/2 (polymorphic) – arc/enclosed

Notes: Krause et al. (1992) noted that the ‘shape of the prism boundaries (i.e. whether circular or arc-shaped’ could not be discerned in the Ferugliotherium specimens they examined. Koenigswald et al. (1999) reported incomplete prism sheaths in Sudamerica but stated that the prism sheath in Gondwanatherium is ‘normally incomplete’, hence Gondwanatherium has been scored as polymorphic for this character. Following Rougier et al. (2007), this character was ordered.

280. Enamel prism seams:

Ferugliotherium: ?

Sudamerica: 0 – present

Gondwanatherium: ?

Notes: Koenigswald et al. (1999) reported prism seams in Sudamerica

282. Interprismatic matrix:

Ferugliotherium: 2 - prisms ‘‘shoulder to shoulder’’, little IPM

Sudamerica: 0 - on all sides, widely separated prisms

Gondwanatherium: 0 - on all sides, widely separated prisms

Notes: Krause et al. (1992) stated that there is ‘comparatively little interprismatic material’ in the enamel of Ferugliotherium, whilst Koenigswald et al. (1999) identified the interprismatic matrix as ‘thick’ in Gondwanatherium and ‘unusually thick’ in Sudamerica. Following Rougier et al. (2007), this character was ordered.

283. Outer aprismatic zone:

Ferugliotherium: 1 - absent

Sudamerica: 0/1 (polymorphic) – present/absent

Gondwanatherium: ?

Notes: enamel prisms appear to extend to the outer enamel surface in Ferugliotherium (Krause et al. 1992). Koenigswald et al. (1999) stated that in Sudamerica ‘most of the outer enamel lacks prisms’, and so we have coded Sudamerica as polymorphic for this character.

285. Staggered incisor:

Ferugliotherium: 0 – absent

Sudamerica: 0 – absent

Gondwanatherium: 0 - absent

Notes: we have scored Ferugliotherium and sudamericids as ‘0’ for this character because Rougier et al. (2007) scored plagiaulacidan and cimolodontan multituberculates (which, like the three gondwanatherian taxa, have a single lower incisor) as ‘0’. However, it could be argued that this character is inapplicable for taxa that have a single incisor. Regardless, a staggered incisor is an apomorphy of Metatheria (marsupials and their stem-relatives) and should have no bearing on the relationships of gondwanatherians.

Appendix 2

All character scores for the taxa in the matrix of Rougier et al. (2007) plus Ferugliotherium, Sudamerica, and Gondwanatherium. 0 to 5 = character states; ? = missing or inapplicable information. Polymorphic scorings are: a = 0, 1; b = 0, 2; c = 0, 3; d = 1, 2; e = 0, 1, 2; f = 1, 2, 3.