Research Article |
Corresponding author: Vicente D. Crespo ( vidacres@gmail.com ) Academic editor: Torsten Scheyer
© 2022 Vicente D. Crespo, Francisco J. Goin, Martin Pickford.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Crespo VD, Goin FJ, Pickford M (2022) The last African metatherian. Fossil Record 25(1): 173-186. https://doi.org/10.3897/fr.25.80706
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Morotodon aenigmaticus gen. et sp. nov. (Mammalia, Metatheria, ?Herpetotheriidae) from the early or early-middle Miocene of equatorial Africa (Moroto II locality, Moroto District, northeastern Uganda) is characterized by a short anterior cingulum, a buccal shelf, a well-developed hypoconulid in a central position, and a trigonid and talonid with similar mesio-distal lengths. Its small size and morphology suggest mostly insectivorous-faunivorous feeding habits. The faunal association of Moroto II, as well as previous palaeoenvironmental analyses, suggest that Morotodon lived in open woodland and bushland areas surrounded by grasses. Morotodon aenigmaticus shows several features reminiscent of early herpetotheriids, such as Golerdelphys stocki (late Paleocene of North America), and Amphiperatherium ambiguum (Eocene of Europe); this suggests an origin for its lineage previous to the Oligocene. In summary, its affinities lie with Northern Hemisphere herpetotheriids, and, most probably, with European ones.
Africa, Herpetotheriidae, Metatheria, Miocene, Moroto II, Uganda
The record of extinct African metatherians is scarce and, up to now, restricted to the Paleogene of its northernmost portion (Fig.
Cenozoic African metatherians (or putative metatherians) described up to now. The type localities of all these taxa are shown in the map of Fig.
Species | Family | Age and locality | Author | Observations | |
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1 | ?Garatherium todrae | Adapisoriculidae (Eutheria) | Late Paleocene, Morocco | Gheerbrant et al. (1998) | See Seifert (2010), |
2 | Garatherium mahboubii | Adapisoriculidae (Eutheria) | Early Eocene, Algeria |
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See |
3 | Kasserinotherium tunisiense | ?Peradectidae (Metatheria) | Early Eocene, Tunisia |
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See |
4 | Ghamidtherium dimaiensis | ?Chiroptera, fam. indet. (Eutheria) | Late Eocene, Egypt |
|
See |
5 | Peratherium africanum | Herpetotheriidae (Metatheria) | Early Oligocene, Egypt and Oman | Simons & Bown (1984) | See |
6 | Morotodon aenigmaticus gen. et sp. nov. | Herpetotheriidae (Metatheria) | Early-middle Miocene, Uganda | This work | See |
Map of Africa indicating the occurrences of extinct metatherians, or taxa previously regarded as Metatheria. Bottom left, map of Uganda indicating the Moroto II locality. Right, geologic time column indicating the age of each taxon. References: E, Equator; 1,?Garatherium todrae; 2, Garatherium mahboubii; 3, Kasserinotherium tunisiense; 4, Ghamidtherium dimaiensis; 5, Peratherium africanum; 6, Morotodon aenigmaticus gen. et sp. nov. See the text and Table
The other undisputed African metatherian, K. tunisiense, is solely known from two upper molars, and was included by
Other extinct African taxa referred to the Metatheria are more probably eutherians (see Table
Ghamidtherium dimaiensis Sánchez-Villagra, Seiffert, Martin, Simons, Gunnell, & Attia, 2007 was recovered from the Late Eocene of the Fayum Depression (Quarry L-41; Egypt).
Here we describe a third taxon unambiguously referrable to the Metatheria, on the basis of a single, isolated lower molar. It has two unusual features: it comes from equatorial Africa (Uganda), and represents the youngest African metatherian so far known (early Neogene). The specimen was previously studied by
The type specimen was obtained while screen-washing sediments at the Moroto II locality. Comparisons were made with original specimens, casts, photographic stereopairs, drawings, photographs, and SEM micrographs provided in the literature. Measurements were done with a microscope with measuring table to 0.01 mm resolution. Dental terminology is provided in Fig.
The series of fossil localities of Moroto II are located near Kogole Hill, north of Nakiloro Village, in Moroto District, northeastern Uganda (Fig.
The area of the geological succession is in the vicinity of Kogole; it is underlain by basement complex gneisses (Mozambique Belt;
The age of Moroto II is controversial, with two different opinions in the literature: one, based on the geology, dates the locality as 21–20 Ma (early Miocene), around the transition between the Aquitanian and the Burdigalian; it is regarded as older than Napak (Uganda, Faunal Set I, ca 20.5–19.5 Ma;
Mammalia Linnaeus, 1758
Metatheria Huxley, 1880
?Herpetotheriidae Trouessart, 1879
Morotodon aenigmaticus gen. et sp. nov.
“The mysterious tooth from Moroto”. Moroto II is the fossil locality where this taxon was found; “-odon”, from odontos, genitive of odous, ancient Greek for tooth; gender is masculine; aenigmaticus, from the Latin aenigma (mystery), refers to the unexpected finding of a metatherian near the Equator in the Neogene of Africa.
Lower molar series of various marsupial taxa belonging to Peradectidae (A) and Herpetotheriidae (B–F). A. Peradectes russelli, CB 1027 (CL), right m4 in lingual, occlusal, and labial views; B. Amphiperatherium lamandini, ECA 3104 (UM), fragment of right mandible with m3-4 in occlusal view; C. Peratherium elegans (type of Peratherium agmardi Filhol), QU 8216 (MNHN), right mandible with c and p1-m4 in occlusal view; D. Peratherium perrierense (holotype), PRR 2524, right mandible with i1-m4 in occlusal view; E. Peratherium cayluxi, QU 8217 (MNHN) (holotype), left mandible with p2-m1 and m3-4 in occlusal view; F. Amphiperatherium ambiguum, PLA 1042 (UM), left m4 in occlusal and lingual views. Figures after
Total length, 1.63 mm (1 mm trigonid length, 0.63 talonid length); trigonid width, 0.93 mm; talonid width, 0.94 mm (from
Moroto II, north of Nakiloro Village, Moroto District, northeastern Uganda (Fig.
?Herpetotherid metatherian with lower molars having a short anterior cingulid, a buccal shelf, and a trigonid and talonid with subequal length and width; the m4 has a vertical, well-developed hypoconulid in a central position. The specific diagnosis extends to the genus by monotypy.
Specimen
The specimen was originally described as a m1 or m2 (
Specimen
Afrosoricids such as tenrecs and golden moles (of which at least members of the former were contemporaneous with Morotodon), were already discarded on the basis of the number of talonid cusps, three in metatherians and a single, elongated one in tenrecs (
Chiropterans can also be ruled out because of the morphology and position of the hypoconulid, which in Morotodon is more developed and more centrally placed at the distal edge of the tooth; additionally, chiropterans have a buccal shelf or cingulid which is mesiodistally complete, linking the anterior and posterior cingulids. Morotodon differs from the probable chiropteran Ghamidtherium dimaiensis Sánchez-Villagra, Seiffert, Martin, Simons, Gunnell, & Attia, 2007 in that the anterior cingulid is shorter and does not extend distally at the crown base; the metaconid is anteriorly positioned with respect of the protoconid; the entoconid is less developed; it lacks a posterior cingulid; the hypoconulid is larger, higher and less dorso-ventrally compressed, and it is not placed immediately distal to the entoconid but instead buccal to it; finally, the oblique cristid in Morotodon is less parallel to the dental axis.
Morotodon differs from the Eulipotyphla in the presence and morphology of the hypoconulid. In their lower molar morphology representatives of the Soricidae and Talpidae have some similarities with Morotodon. However, the anterior cingulid in soricids is better developed and may continue posteriorly towards the buccal surface of the crown, and in the talonids the hypoconulid is reduced (or, if not reduced, is placed very low regarding the entoconid) and located immediately posterior to the entoconid. Generalized erinaceids such as Galerix lack a hypoconulid, and, in the last molar, the paraconid is crest-like, and the oblique cristid is parallel to the dental axis. Among other more derived soricomorphs, the living Solenodon, for instance, has extremely reduced talonids and mesio-distally compressed trigonids.
Being more similar in overall morphology (but see below), a more detailed comparison of Morotodon with individual adapisoriculid taxa is worthwhile. Morotodon differs from Afrodon gheerbranti De Bast & Smith, 2017 in having a more lingual paraconid, more developed protoconid, presence of a buccal shelf (or cingulid), better developed hypoconid and entoconid, and a more centrally placed hypoconulid. It differs from Afrodon chleuhi Gheerbrant, 1988 in having a longer trigonid, a better developed protoconid, trigonid and talonid of similar width, and a less developed, more anteriorly placed hypoconulid. It differs from Bustylus marandati (Crochet and Sigé 1983) in having a narrower anterior cingulum, longer trigonid, more centrally placed metaconid, a developed buccal shelf, a larger entoconid, and an independent hypoconulid. It differs from the todralestid Todralestes variabilis Gheerbrant, 1991 in having a less reduced talonid, the presence of a buccal shelf, and a larger hypoconulid.
Morotodon aenigmaticus compares best with metatherian mammals, especially with Marsupialiformes (most metatherians except the early clade Deltatheridia). The best known early marsupialiform (i.e., non deltatheroid) metatherian is Kokopellia juddi Cifelli, 1993, from the medial Cretaceous of Utah in North America (see Cifelli and Muizon 1997 for a detailed description of the dentition of Kokopellia). Molars of Kokopellia represent the generalized condition for almost all Cenozoic metatherians. Morotodon aenigmaticus differs from Kokopellia juddi in that its m4 lacks a posterior cingulid, and a shorter talonid (clearly longer in Kokopellia in all lower molars), a less posteriorly placed metaconid, a smaller hypoconulid (in Kokopellia it is larger and closer to the entoconid), and a smaller and less anteriorly placed hypoconid. Both Morotodon and Kokopellia share a well-developed buccal shelf (or cingulid), and a similarly oriented oblique cristid, which ends anteriorly at a point below the metacristid notch.
Most Cenozoic Holarctic metatherians belong either to the Peradectidae or to the Herpetotheriidae, so a detailed comparison with species of these two groups is needed in order to clarify the affinities of Morotodon aenigmaticus. Most representatives of both families are known from the Northern Hemisphere. In North America, peradectids and herpetotheriids are known from the Late Cretaceous to the Miocene, while in Eurasia they span the early Eocene to the Miocene (in Europe, peradectids are restricted to the Eocene). In Africa, peradectids had been known only for the early Eocene (Kasserinotherium), .while herpetotheriids are known for the early Oligocene (Peratherium). Our allocation of Morotodon to herpetotheriids expands the group to the Miocene.
Peradectes. Morotodon aenigmaticus differs from Peradectes louisi Crochet, 1979 in having a proportionally longer talonid, less difference in height between the trigonid and the talonid, and in that the hypoconid is more salient. Differs from Peradectes californicus (Stock, 1936) (m4 of this species is unknown) in that the metaconid is more anteriorly placed than the protoconid, and the talonid is proportionally narrower. Differs from Peradectes chesteri Gazin, 1952 (although the m4 of this species is unknown) in having a larger paraconid which is not as close to the metaconid, and a proportionally larger hypoconulid. Differs from Peradectes coprexeches Williamson & Taylor, 2011 in having a narrower trigonid, the oblique cristid is not subparallel to the preentocristid (in such a way that the talonid is anteriorly narrower), the hypoconulid is more developed and less paired to the entoconid; finally, a buccal cingulid is present. Differs from Peradectes minor Clemens, 2006 and Peradectes mutigniensis Crochet, 1979 in having a paracristid which is less transverse to the dental axis, the hypoconulid is farther from the entoconid, it lacks a posterior cingulid and has a buccal shelf or cingulid. Differs from Peradectes pauli (Gazin, 1956) in having a more salient hypoconid, while the oblique cristid is less parallel to the dental axis. Differs from Peradectes protinnominatus McKenna, 1960 in having a longer talonid, a larger hypoconulid which is farther from the entoconid, and in that the oblique cristid is less parallel to the dental axis. Differs from Peradectes russelli Crochet, 1979 in that the paraconid and metaconid are less close to each other, the entoconid is smaller, the hypoconulid is farther from the entoconid, and the oblique cristid is less parallel to the dental axis.
Mimoperadectes. Differs from Mimoperadectes labrus Bown & Rose, 1979 in having a less developed anterior cingulid, a shorter trigonid, a paraconid that is farther from the metaconid and the hypoconulid and the entoconid are more detached (for this reason, the talonid is wider posteriorly).
Nanodelphys. Differs from Nanodelphys hunti (Cope, 1873) in having a narrower trigonid and shorter talonid, a hypoconulid that is set farther from the entoconid, and the oblique cristid not being subparallel to the preentocristid.
Armintodelphys. Differs from Armintodelphys dufraigni Smith & Smith, 2013 in having a wider anterior cingulid, a less reduced paraconid, a slightly posteriorly placed paraconid (relative to the protoconid), a less straight oblique cristid, a hypoconulid that is farther from the entoconid, and in the presence of a buccal shelf. Differs from Armintodelphys dawsoni Krishtalka & Stucky, 1983 in having a more developed paraconid which is less mesio-distally compressed, and a narrower talonid. Differs from Armintodelphys blacki Krishtalka & Stucky, 1983 in having an anteriorly placed metaconid with respect to the protoconid, the anterior half of the oblique cristid not being parallel to the dental axis, a smaller hypoconulid that is farther from the entoconid, and a narrower talonid basin.
Asiadidelphis. Morotodon aenigmaticus differs from Asiadidelphis zaissanense Gabunia, Shevyreva, & Gabunia, 1990 (described in Ziegler et al. 2007; fig. 3.3) in having a wider anterior cingulid, oblique cristid less parallel to the dental axis, and a more buccally placed hypoconulid. Differs from Asiadidelphis tjutkovae Emry, Lucas, Szalay, & Tleuberdina, 1995 in its smaller size, a more centrally positioned hypoconulid on the posterior edge of the talonid, and a larger entoconid. Differs from Asiadidelphis (= Indodelphis) luoi (Bajpai, Kapur, Thewissen, Tiwari, & Das, 2005) in having a more developed anterior cingulid, a proportionally higher protoconid relative to the metaconid, a mesio-distally less compressed paraconid, a narrower talonid an oblique cristid that is less parallel to the dental axis.
Swaindelphys. Differs from Swaindelphys encinensis Williamson & Taylor, 2011 in having a shorter anterior cingulid, narrower trigonid and talonid, a less developed, lower hypoconulid, and, in occlusal view, a straight but not curved oblique cristid. Differs from Swaindelphys cifelli Johanson, 1996 in having a more developed anterior cingulid, in its hypoconulid which is farther from the entoconid, a smaller hypoconulid, and a talonid that is longer relative to the trigonid.
Thylacodon. Differs from Thylacodon montanensis Williamson, Brusatte, Carr, Weil, & Standhardt, 2012 in having (although the m4 was not preserved in the latter) a narrower entoconid, a hypoconulid that is farther from the entoconid, and in the absence of a posterior cingulid. Differs from Thylacodon pusillus (Archibald, 1982) in having a better developed anterior cingulid, smaller entoconid, and a hypoconulid that is not twinned to the entoconid.
Golerdelphys. Differs from Golerdelphys stocki Williamson & Lofgren, 2014 in lacking a posterior cingulid and in that the entoconid is proportionally smaller.
Copedelphys. Differs from Copedelphys titanelix (Matthew, 1903) in having a shorter and wider anterior cingulid, a proportionally smaller trigonid, a paraconid that is not mesio-distally compressed, an oblique cristid obliqua is not straight, a buccally more salient hypoconid, and in that the hypoconulid is less posteriorly projected. Differs from Copedelphys stevensoni (Cope, 1873) in having a shorter and wider anterior cingulid, a less antero-posteriorly compressed paraconid, an oblique cristid obliqua that is less parallel to the preentocristid, and in the presence of a buccal shelf (or cingulid).
Herpetotherium. Differs from Herpetotherium youngi (McGrew, 1937) in having a shorter anterior cingulid, an oblique cristid that meets the trigonid more lingually, a more developed hypoconulid that is not twinned to the entoconid, and a more salient hypocone. Differs from Herpetotherium fugax Cope, 1873 in having a shorter and wider anterior cingulid, a proportionally narrower trigonid, a paracristid that is less transversal to the dental axis, an oblique cristid that is not subparallel to the preentocristid, and a hypoconulid that is farther from the entoconid. Differs from Herpetotherium comstocki (Cope, 1884) in having a shorter anterior cingulid, an oblique cristid that is subparallel to the dental axis, a proportionally larger hypoconulid that is farther from the entoconid; in turn, this last cusp is proportionally smaller. Differs from Herpetotherium edwardi (Gazin, 1952) in having larger paraconid and hypoconulid, the latter smaller and farther from the entoconid, and an oblique cristid that is less parallel to the dental axis. Differs from Herpetotherium marsupium Troxell, 1923 in having a smaller entoconid, more buccal hypoconulid, and a straighter oblique cristid that is less parallel to the dental axis. Differs from Herpetotherium merriami (Stock & Furlong, 1922) in having (although the m4 was not preserved) a larger hypoconulid which is placed farther from the entoconid. Differs from Herpetotherium tabrumi Korth, 2018 in having (although the m4 is not present) a larger hypoconulid which is farther from the entoconid, a shorter preentocristid, and an oblique cristid that is less parallel to the dental axis. Differs from Herpetotherium valens (Lambe, 1908) in having (although the m4 was not preserved) a shorter and wider anterior cingulid, relatively narrower talonid, a paracristid that is less transversal to the dental axis, paraconid and metaconid clearly set apart from each other, a smaller entoconid and a proportionally larger hypoconulid that is farther from the entoconid.
Peratherium (Fig.
Amphiperatherium (Fig.
Rumiodon. Differs from Rumiodon inti Goin & Candela, 2004 in having a wider anterior cingulid, distinct hypoconulid that is larger and not twinned with the entoconid, and an oblique cristid that is less parallel to the dental axis.
Estelestes. Differs from Estelestes ensis Novacek, Ferrusquía-Villafranca, Flynn, Wyss, & Norell, 1991 (early Eocene; referred by the authors to the “Didelphidae”), in that it lacks a postcingulid (in Estelestes the postcingulid extends anteriorly forming a buccal cingulid basal to the hypocone), the hypocone is more buccally salient, the hypoconulid is farther from the entoconid and less posteriorly tilted; finally, an oblique cristid is less parallel to the dental axis.
Orhaniyeia. Differs from Orhaniyeia nauta Métais, Coster, Kappelman, Licht, Ocakoğlu, Taylor, & Beard, 2018 (middle Eocene of Turkey) in being much smaller, has less bunoid molars, the anterior cingulid is better developed, the paraconid is placed farther from the metaconid, the paracristid is less transverse to the dental axis, the talonid is shorter, the hypoconid is much more distally placed, an oblique cristid that is less parallel to the dental axis; finally, it lacks multiple cuspids on the pre-entocristid.
Morotodon aenigmaticus differs from the Protodidelphidae (early to middle Eocene) in being much smaller, has less bunoid molars, and smaller and narrower talonids. Differs from the Derorhynchidae (Paleogene of South America and Antarctica) in having a longer talonid, no posterior cingulid, smaller entoconid and larger hypoconulid. Differs from species of Gaylordia (early Eocene) in having a less developed anterior cingulid, longer talonid, less lingually placed paraconid; finally, an oblique cristid that is less parallel to the dental axis. Differs from species of Marmosopsis (early Eocene) in lacking a posterior cingulid and an oblique cristid that is less subparallel to the dental axis. Differs from species of Minusculodelphis (Eocene) in its larger size, better developed talonids and hypoconid, and in the persistence of the hypoconulid. Differs from species of Monodelphopsis (early Eocene) in having a narrower talonid, an oblique cristid that is less subparallel to the dental axis, and better developed entoconid and hypoconulid which are less closely set to each other. Differs from species of Carolopaulacoutoia (early Eocene) in its shorter talonid, less parallel oblique cristid to the dental axis, more salient hypoconid, and smaller and more centrally placed hypoconulid. Differs from species of Itaiboraidelphys (early Eocene) in having a poorly developed anterior cingulid, the paraconid is more distant from the metaconid, an oblique cristid that is less parallel to the dental axis, and the hypoconulid is more centrally placed. Differs from species of Didelphopsis (Paleocene-early Eocene) in having a shorter anterior cingulid, proportionally longer trigonid, the paraconid is placed farther from the metaconid, an oblique cristid that is less parallel to the dental axis, and the hypoconulid is larger and farther from the encotonid. Differs from Pucadelphys andinus Marshall & Muizon, 1988 (early Paleocene) in having a larger hypoconulid and an oblique cristid that is less parallel to the dental axis.
With only two exceptions (the deltatheroidan Gurbanodelta kara, from the Paleocene of China, and the anatoliadelphids, from the middle Eocene of Turkey), all other Cenozoic Afro-Eurasian metatherians (36 species) have been referred either to the Peradectidae (six species) or to the Herpetotheriidae (the remaining ones). It is obvious to us that Morotodon aenigmaticus is neither a deltatheroidan nor an anatoliadelphid. On the contrary, its molar pattern is much more similar to that of “opossum-like marsupials” such as herpetotheriid and peradectid marsupialiforms. Herpetotheriids differ from peradectids in several aspects of their respective molar structure. Regarding the lower ones,
A comparison of dental features of the last lower molar in Morotodon aenigmaticus and in species of Peradectes, Peratherium and Amphiperatherium.
Features in m4 | Morotodon | Peradectes | Peratherium | Amphiperatherium |
---|---|---|---|---|
1. Paracristid inclination | 45° | 45°, or less | 45° or less | 45°, less than 45°, more than 45° |
2. Metaconid position relative to the protoconid | Slightly posterior | Posterior, or at the same level | Same level, or metaconid posterior | Same level, or metaconid posterior, or protoconid posterior |
3. Hypoconulid position on the rear of the entoconid / proximity with the entoconid | Central | Almost central, or far lingual, or almost twinned to entoconid | Almost central, or far lingual, or almost twinned to entoconid | Almost central, or far lingual, or almost twinned to entoconid |
4. Postcingulid (present /absent /variable) | Absent | Variable | Variable | Variable |
5. Buccal cingulid (pres/abs/var) | Present | Variable | Variable | Variable |
6. Relative heights, entoconid /hypoconulid | Hypoconulid higher | Hypoconulid higher, or equal | Hypoconulid higher, or equal, or entoconid higher | Hypoconulid higher, or equal, or entoconid higher |
7. Width of the talonid at its anterior edge | Narrow | Less narrow, or wide | Narrow | Narrow, or less narrow |
8. Mesiodistal compression of paraconid | Not compressed | Compressed, or not compressed | Compressed, or not compressed | Compressed, or not compressed |
9. Entoconid position relative to hypoconid | Entoconid slightly anterior | Same level, or ent. Slightly anterior, or ent. Slightly posterior | Same level, or ent. slightly anterior | Same level, or ent. slightly anterior |
10. Width of distal edge of the talonid | Wide | Wide, or reduced but wider than the anterior edge | Narrow, or almost as wide as the trigonid | Almost as wide as the trigonid, or narrow |
11. Postentocristid linking entoconid with hypoconulid | Absent | Present | Absent | Absent |
12. Metacristid slope (almost vertical, gently sloping) | Almost vertical | Almost vertical | Almost vertical | Almost vertical, one species gently sloping |
Ladevéze et al. (2012) studied a large sample of dental materials (around 400 specimens) belonging to herpetotheriid metatherians from the earliest Eocene of Dormaal (Belgium). Previous reviews of the herpetotheriids from Dormaal led to the recognition of only two species: Peratherium constans Teilhard de Chardin, 1927 and Amphiperatherium brabantense Crochet, 1979. Ladevéze et al. (2012) concluded that, due to the dental variability shown by the dental materials, the two species are indistinguishable, therefore they referred both to Peratherium constans. Actually, they not only questioned the validity of the Amphiperatherium species but also the status of the genus Amphiperatherium itself. While we agree that several features used to distinguish the two genera are subtle, and subject to variability (see above), we prefer to maintain the validity of Amphiperatherium until a thorough review of all Northern Hemisphere (North American and Eurasian) herpetotheriids is made.
If molars in general are subject to some degree of variability, the case of m4 is more evident. Unlike several eutherians, metatherian molars invariably erupt successively, from front to back; the hypoconulid of the preceding molar serves as a guide for the alignment of the succeeding one. The hypoconulid of the former fits in the hypoconulid notch of the latter – a groove delimited lingually by the mesial end of the anterior cingulid, and buccally by the paraconid. In such a way, the succeeding molar erupts in precise alignment in the molar row. In mammals with mostly orthal jaw movements, such as those with insectivorous, faunivorous or carnivorous feeding habits, it is understandable that selective pressures strongly favour stable morphologies among these structures – any malocclusion could lead to the breaking of teeth. Not having a succeeding molar behind, the m4 in metatherians, and especially the talonid, is subject to a larger degree of variability (see, e.g.,
Table
Following Ladéveze et al. (2012),
Among herpetotheriids, Morotodon is more similar to some species of Peratherium, such as P. sudrei, P. perrierense, P. constans and P. cayluxi, but differs in the position of the hypoconulid, the size of the hypoconid and the length of the anterior cingulid. More remarkable is the similarity with Amphiperatherium goethei; among other features, the two taxa share a proportionally similar size and position of the hypoconulid. Among North American metatherians, we note a general similarity with Golerdelphys stocki, even though the materials referred to this species have not preserved the m4. However, judging from its preserved molars, similar morphologies can be observed: short and wide anterior cingulid, not mesiodistally compressed paraconid, proportionally large and vertical hypoconulid, laterally compressed entoconid, and, probably, presence of a buccal shelf (or cingulid; see fig. 4E in
Most of the similarities between the m4 of Morotodon and that of several herpetotheriids are based on features which are generalized for the family: relatively large hypoconulid, non-reduced talonid, paraconid in a mesio-lingual location, not appressed to the metaconid. The already mentioned species of European genera that show these similarities are all from the Eocene except P. cayluxi, from the basal Oligocene (
The hypothesis of an Eurasian origin of Morotodon is reinforced by the tectonic, climatic and eustatic events that happened by the latest Paleocene and earliest Eocene, which may have facilitated these faunal exchanges. Evidence of a land bridge at northwestern Iran and southeastern Turkey is yielded by the Gercüs Formation; these floodplain/lacustrine/lagoonal/coastal plain sediments were deposited during the first phases of the collision between Africa and Eurasia (
The sediments of the southern layers of Moroto II were deposited in a shallow valley, similar to the present day Karamoja Plain (
The plentiful presence of the terrestrial snail Nothapalinus, suggests the presence of grassy areas (
Following the palaeoecological analysis of some herpetotheriid species made by
No potential conflict of interest was reported by the authors.
F.J.G. and V.D.C. thank Agencia (Foncyt, Mincyt; PICT 2019-03283) and CONICET (PIP KB2 - 11220200100150CO).
We thank the Uganda Museum, Kampala (Rose Mwanja, Sarah Musalizi, Christopher Sebuyungo) for access to fossil material in its care, and the Uganda National Council for Science and Technology for authorization to carry out research in the country. Pierre Mein measured the specimen from Moroto and made preliminary identifications. Hugo Salais (Metazoa Studio) for the 3D reconstruction, Agustín Ruella for a previous version of the 3D reconstruction, Marcela Tomeo for the realisation of Figs
3-D video of a digital reconstruction of Morotodon aenigmaticus gen. et sp. nov.
Data type: 3-D video (avi file)
Explanation note: 3-D video of a digital reconstruction of Morotodon aenigmaticus gen. et sp. nov.
Morotodon aenigmaticus gen. et sp. nov. Stereoscopic pairs and schematic drawing of specimen
Data type: Stereoscopic pairs and schematic drawing
Explanation note: A–E, Morotodon aenigmaticus gen. et sp. nov. Stereoscopic pairs and schematic drawing of specimen
Reconstruction of the palaeoenvironment of Moroto II
Data type: Images
Explanation note: Reconstruction of the palaeoenvironment of Moroto II. Left to right: the anthracothere Brachyodus sp.; the primate Afropithecus turkanensis Leakey & Leakey, 1986 ; the deinothere Deinotherium hobleyi Andrews, 1911; the creodont Hyainailouros sulzeri Biedermann, 1863 and the metatherian Morotodon aenigmaticus gen. et sp. nov.