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Stylolophus minor
#1
Stylolophus minor

Temporal range: Eocene (Ypresian)

Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Clade: Paenungulata
Order: †Embrithopoda
Genus: †Stylolophus
Species:Stylolophus minor

Embrithopoda ("heavy-footed") is an order of extinct mammals known from Asia, Africa and eastern Europe. Most of the embrithopod genera are known exclusively from jaws and teeth dated from the late Paleocene to the late Eocene, but the order is best known from its terminal member, the elephantine Arsinoitherium.




Ancient Moroccan dental remains elucidate history of long-lost African fauna

Date: June 28, 2018
Source: Cell Press

[Image: 180628151910_1_900x600.jpg]
This figure show the lower jaw of Stylolophus minor, holotype of the new species. C is 3-D model reconstructed from CT scans. It shows by transparency the teeth roots, and especially those of the anterior incisors that are enlarged and oriented (tilted) horizontally as in the early proboscidean Phosphatherium. Length of M1-3 series: 38.5 mm. Scale bar, 10 mm.
Credit: Photographs by Philippe Loubry (MNHN). Drawing by Charlène Letenneur (MNHN)

Long before rhinoceros, giraffes, hippos, and antelopes roamed the African savannah, a group of large and highly specialized mammals known as embrithopods inhabited the continent. The most well known is Arsinoitherium, an animal that looked much like a rhinoceros but was in fact more closely related to elephants, sea cows, and hyraxes. Now, researchers reporting in Current Biology on June 28 offer a glimpse into this ancient past with the discovery of the earliest and most ancient embrithopod yet described.

The approximately 55-million-year-old fossilized dental remains found in the first lower Eocene levels of the Ouled Abdoun phosphate basin in Morocco represent two new species in the genus Stylolophus, the researchers report. The earliest embrithopods were previously known from 48-million-year-old fossils collected in Africa and Turkey.

"The embrithopods were large and strange extinct mammals that belonged, together with hyraxes and elephants, to the early megaherbivorous mammalian fauna that inhabited the island Africa, well before the arrival about 23 million years ago of the Eurasian ungulate lineages such as the artiodactyls, including giraffes, buffalos, hippopotamus, and antelopes, and the perissodactyls, including zebras and rhinoceros," says Emmanuel Gheerbrant of CNRS-MNHN in Paris, France. "They belong to the old endemic African fauna."

Gheerbrant said that the origins of embrithopods had been uncertain, with two known co-existing families: one in Africa and the other in Turkey and Romania. It's been unclear what the exact relationships of the embrithopods were with respect to sea cows and elephants.

The new phylogenetic study of the two species of Stylolophus found in Morocco confirms that they are basal embrithopods. It also shows that the extinct Embrithopod order is ancient, predating the divergence of the sea cows and elephants.

"Comparative anatomy of the new Moroccan species shows that the highly specialized embrithopod teeth derived from the ancestral dental morphology of all paenungulates, a clade including elephants, sea cows, and hyraxes, with the W-crested molars seen in some of the oldest hyracoids," the group including hyraxes, Gheerbrant says. "The specialized design of the teeth with two transverse ridges, known in the most advanced forms such as Arsinoitherium, is a convergence of the embrithopods and the extant group of tethytheres, including manatees and elephants, towards leaf eating, which was favored by the ancient herbivorous niches available on the African island."

The new species S. minor -- which was unusually small at about the size of a sheep -- is also the first to show the presence in embrithopods of enlarged and anteriorly inclined incisors, in the form of incipient tusks, as seen in the early ancestors of the group including elephants.

The early age and primitive state of Stylolophus, together with the high-level relationships (paenungulate and afrotherian), all support an African origin of the order Embrithopoda, the researchers say. The findings suggest that the Paleoamasiidae family found in Turkey arrived on the Eurasian shores of the Tethys Ocean (an ocean during much of the Mesozoic Era and the Paleogene period located between the ancient continents of Gondwana and Laurasia), after an early dispersal of an African ancestor resembling Stylolophus across the sea.

The researchers say that they'll continue to search for paleontological evidence elucidating the evolutionary history and relationships of African ungulate-like mammals and insectivore-like afrotherian mammals, including golden moles, elephant shrews, tenrecs, aardvarks, and hyraxes. They'll also continue the search for the enigmatic early roots of all placental mammals in Africa, going back even further in time to the Cretaceous Period.

Story Source: Cell Press. "Ancient Moroccan dental remains elucidate history of long-lost African fauna." ScienceDaily. www.sciencedaily.com/releases/2018/06/180628151910.htm (accessed July 2, 2018).




Journal Reference:
Emmanuel Gheerbrant, Arnaud Schmitt, László Kocsis. Early African Fossils Elucidate the Origin of Embrithopod Mammals. Current Biology, 2018; DOI: 10.1016/j.cub.2018.05.032

Highlights
•Earliest embrithopods (new genus)
•African origin of embrithopods
•Old African Tethythere offshoot predating the sea-cows and elephants divergence
•Repeated lophodont specializations in the Tethytheria

Summary
Modern mammals rapidly evolved in the early Cenozoic in all continental provinces, including in Africa, with one of the first placental branches, the Afrotheria [1, 2]. Afrotherian evolution is at the origin of the major radiation of African ungulate-like mammals, including extant hyrax, elephant, and sea cow orders, which all belong to the Paenungulata. The paenungulate radiation also includes the extinct order Embrithopoda of uncertain interordinal relationships, which is best known for the giant and strangely specialized Oligocene genus Arsinoitherium. The Ouled Abdoun basin, Morocco, yielded exceptional Paleocene-Eocene fossils documenting the early paenungulate evolution [3, 4, 5, 6, 7, 8]. Here we report two new small Ypresian species, Stylolophus minor n.g., n.sp. and cf. Stylolophus sp., which are the earliest and most primitive embrithopods. The cladistic analysis relates the Embrithopoda to crown paenungulates as the stem-group of the Tethytheria, which makes crown tethytherians restricted to extant elephant and sea cow orders. The Embrithopoda is therefore an early tethytherian offshoot predating the elephant and sea cow divergence. The resulting phylogeny supports a strictly African early radiation of the paenungulates excluding the Phenacolophidae and Anthracobunia. It sustains an at least early Paleocene African origin of the Embrithopoda. The unique tooth pattern of the embrithopods (hyperdilambdodont and pseudolophodont molars) is resolved as evolving early and directly from the dilambdodont (W-shaped labial molar crests) ancestral paenungulate morphotype. The specialized upper molar morphology with two transverse crests is convergent and non-homologous in embrithopods and crown Tethytheria. These convergences for specialized folivorous diet were driven by free herbivorous African niches in the early Paleogene.

Results and Discussion
Placentals comprise the largest radiation of living mammals, as the result of an evolutionary history dating back to the Cretaceous-Paleogene (K-Pg) transition. Molecular phylogeny identified the first major splitting clades at the base of placentals [1, 2]. Unfortunately, the fossil record of the initial placental radiation remains poor, especially for the key K-Pg period of evolution and for some major continental centers of evolution. This is especially true for the Arabo-African Island, which was the center of radiation of one of the main placental clades, the Afrotheria. The Ouled Abdoun phosphate series, in Morocco, yielded the only known Paleocene and early Eocene fossil remains of stem and early crown paenungulate afrotherians [3, 4, 5, 6, 7, 8]. The paenungulates, today represented by orders of hyraxes (Hyracoidea), elephants (Proboscidea), and sea cows (Sirenia), were major components of the African endemic herbivorous megafauna during the Paleogene. The paenungulates also include a remarkable extinct order, the Embrithopoda. Embrithopods are strangely specialized and large, ungulate-like mammals known in the Paleogene of Arabo-Africa and Eurasia. The best known is the rhino-sized Arsinoitherium zitelli, discovered in the early Oligocene of the Fayum [9, 10]. It is characterized by a highly specialized dentition with hyperdilambdodont molars [11] and by a pair of huge hollow nasal horns [9].

Embrithopods are enigmatic placentals with uncertain phylogenetic relationships because of their unusual autapomorphic morphology. They are included in the Paenungulata [12], but with debated interordinal relationships. The order initially related to the Hyracoidea [9] is most consensually placed as the sister group to the Proboscidea [13, 14, 15], although it is sister group to the Sirenia in some recent works [16]. Here, we report the discovery of the oldest and most primitive known embrithopods from Ypresian Ouled Abdoun phosphate levels. They shed new light on the origin and early history of the Embrithopoda. Their phylogenetic analysis helps to clarify the basal radiation and interordinal phylogeny of the Paenungulata and Tethytheria, as well as the evolution of key embrithopod synapomorphies.

Systematic Paleontology
Placentalia Owen, 1837.
Afrotheria Stanhope et al., 1998.
Paenungulata Simpson, 1945.
Tethytheria McKenna, 1975.
Embrithopoda Andrews, 1906.
Family indet.

Stylolophus minor n. g., n. sp. Gheerbrant
LSID urn:lsid:zoobank.org:act:9B31342F-7FBA-44E7-8B46-4C5D5341FBA5.

LSID urn:lsid:zoobank.org:act:B4367769-7BE7-4EBF-A9EA-2E18E409D80E.

Holotype
OCP DEK/GE 668, right dentary bearing roots of I1–2, alveolae of I3, C1, and P2, and crown of P1, P3–4, and M1–3.

[Image: gr1.jpg]

Three specimens documenting upper and lower dentition: holotype; OCP DEK/GE 667, a damaged skull rostrum (maxillary and premaxillary fragments, with cheek teeth); and MNHN.F PM30, fragment of left dentary with broken dP4 and M1 and tooth germs of M2 and P4.

Etymology
Generic name from stylos (Greek “pillar”), styles, and lophos (Greek “crest”), lophs, in reference to the upper molar lophs corresponding to the transverse development of the labial crests linking the enlarged parastyle and mesostyle; species name refers to the small size.

Localities and Horizon
Morocco, northeast Ouled Abdoun basin (Grand Daoui and Sidi Chennane quarries), intercalary phosphate beds II/I and possibly phosphate bed I, early Ypresian.

Diagnosis
S. minor shares with the Embrithopoda the hyperdilambdodont morphotypic molar pattern. It differs from all known embrithopods, including Palaeoamasia, by its very small size and primitive state of hyperdilambdodonty, with less lingual paracone and metacone and correlatively shorter pseudolophs (preparacrista, premetacrista) and with retention of more developed postparacrista, postmetacrista, and pseudohypocone; cheek teeth with brachyodont low crown; M1–2 with one lingual root and with separated paracone and protocone roots; P2 single-rooted and simple; and P2 double-rooted and simple. Anterior incisors are enlarged (especially their root), and I1 is partially hypsodont.

Description
See characters 0–208 in Data S1 (detailed description in preparation; E.G., A.S., and L.K., unpublished data). The molar series of S. minor (Figures 1 and 2) is about 20% longer than that of Phosphatherium escuilliei [17]. We inferred from tooth size [18] a 20–31 kg body mass for S. minor.

[Image: gr3.jpg]
Referred Material (Figures 3 and S1)
MNHN.F PM53, several pieces of a broken skull of the same individual, including a fragment of the snout with anterior part of parietals, right frontal and nasal, left maxillary with cheek teeth, and broken petrosals.

Locality and Horizon
Morocco, northeast Ouled Abdoun Basin (unknown quarry), upper phosphate horizons, probably sillons A–B, middle Ypresian (Data S1B).

Description
See characters 0–208 in Data S1 (detailed description in preparation; E.G., A.S., and L.K., unpublished data). This species is most closely related to the younger species S. minor in dental morphology. It differs most remarkably in its larger size (30%–40% larger in tooth size). It also differs by derived traits of the molars such as longer pseudolophs and smaller postparacrista, postmetacrista, and pseudohypocone. The body mass of cf. Stylolophus sp. indet. is estimated from tooth size as 60–88 kg, i.e., about 3.6 times heavier than S. minor.

Comparisons
The Moroccan Ypresian species S. minor and cf. Stylolophus sp. share several key morphotypic dental features of the order Embrithopoda, such as the hyperdilambdodont pattern (enlarged W-shaped ectoloph linked to enlarged styles); presence of two sharp pseudolophs (elongated preparacrista and premetacrista); hypoconulid cingulid-like and lingually located; M3 hypoconulid lobe small, compressed laterally and lingually set; cristid obliqua lingual on the trigonid; and reduced entocristid (postfossid opened lingually). Comparisons emphasize the primitive morphology of the two Moroccan species within the Embrithopoda. They differ from all other embrithopods, including Palaeoamasiidae, by their small size and plesiomorphic traits, such as a primitive hyperdilambdodont state with less lingual paracone and metacone and correlatively shorter pseudolophs, and by retention of prominent postparacrista, postmetacrista, and pseudohypocone (metaconule in Paenungulata [7]). Other primitive features of Stylolophus with respect to palaeoamasiids are the lower crown of cheek teeth, M1–2 with only one lingual root and with separated paracone and protocone roots, P2 single rooted and simple, and P2 double rooted and simple. In addition, the petrosal of cf. Stylolophus sp., although damaged, shows a likely separated foramen for the aquaeductus cochleae, in contrast to Arsinoitherium, in which this foramen and the fenestra cochleae remain undivided as a single perilymphatic foramen [19]. S. minor reveals a remarkable morphotypic condition of the Embrithopoda in the presence of enlarged anterior incisors with hypertrophied root (Figures 1A and 2C). I1–2 are large and procumbent (Figure 2C), and I1 is partly hypsodont, as shown by its open root in the holotype (adult individual) (Figure 2D).

Enlarged and procumbent anterior lower incisors are most likely representative of the ancestral embrithopod morphotype. However, the anterior dentition remains poorly known in the order, even in Arsinoitherium, in which the first incisor is unknown. Andrews [9] reconstructed in A. zitelli a quite distinctive homodont anterior tooth battery with all teeth, from P1/1 to I1/1, of similar size, orientation, and morphology. It displays especially subvertical I1–2, in contrast to Stylolophus. However, some specimens of A. zitelli, such as MNHN.F LBE 579, display large and procumbent alveoli of I1–2, in a quite similar disposition to the holotype of S. minor. If Andrews’s reconstruction [9] is correct, it suggests that the small and vertical I1–2 is secondary in the order, as are some other remarkable characters of Arsinoitherium, such as the orbit posterior and the narrow zygomatic arches.

The dental morphology of the new Moroccan species is clearly closer to that of the Palaeoamasiidae than to the more derived Arsinoitheriidae Arsinoitherium and Namatherium, which have more specialized hyperdilambdodonty. Arsinoitheriids differ indeed in the absence of a pseudohypocone (fully regressed), P3–4 with postcingulum extended posteriorly, and M3 more offset lingually with respect to M1–2. In addition, Namatherium has a lingual cingulum on the upper premolars and a more anterior zygomatic process, and Arsinoitherium has molarized premolars with a hypocone.

Our comparative anatomical study unambiguously confirms unique specialized embrithopod features in Stylolophus, but also its plesiomorphic morphology within the order.

[Image: gr4.jpg]

Phylogenetic Relationships and Character Evolution in Embrithopoda, Tethytheria, and Paenungulata
We investigated the relationships of S. minor and cf. Stylolophus sp. among various lophodont and stem ungulate-like placentals using parsimony analysis with TNT [20] of 209 dental and cranial characters, 68% of which are known in S. minor (Data S1). It unambiguously places S. minor and cf. Stylolophus sp. as the basal-most embrithopods, arranged in sequence with S. minor being the first branch (Figure 4A). S. minor and cf. Stylolophus sp. are therefore the most primitive known embrithopods. Although being very plesiomorphic within the order, their embrithopod relationship is indicated by noticeable specialized traits. The most remarkable is the hyperdilambdodont pattern (Figures 1 and 3). Other embrithopod molar synapomorphies of Stylolophus are homoplastic, but they form altogether a suite of characters exclusive to the Embrithopoda: hypoconulid lingual, cristid obliqua very lingual on the trigonid, small and lingually located hypoconulid lobe in M3, and talonid widely open lingually. Some embrithopod skull synapomorphies are also identified in S. minor, such as the palate concave between the premolars. In addition, cf. Stylolophus sp. shows a remarkable intranasal rostrum pattern through the development of large paranasal sinuses (Figure S1), which is reminiscent of the very large hollow air space developed in the nasal horn of Arsinoitherium. As a result, S. minor shows that the ancestral morphotype of the order Embrithopoda was well established by the earliest Eocene, even in early small plesiomorphic species.

Within the Embrithopoda, the cladistic analysis does not include the new Moroccan species S. minor and cf. Stylolophus sp. in a single generic clade. cf. Stylolophus sp. most remarkably differs by its larger size, consistently with its slightly higher stratigraphic provenance (Data S1B). Otherwise, their close morphological affinity argues against a distinct genus for the larger Ouled Abdoun species that is provisionally referred to an unnamed new species of Stylophus. S. minor and cf. Stylolophus sp. are regarded as successive chronospecies of a new Ypresian mammal lineage found in Ouled Abdoun series. The intraordinal relationships of other embrithopods remain poorly solved in our analysis, and especially for the poorly known palaeoamasiids Hypsamasia and Crivadiatherium, as previously found by Erdal et al. [21]. It should be particularly noted that the node grouping the palaeoamasiids Hypsamasia and Crivadiatherium with arsinoitheriids, to the exclusion of Palaeoamasia (Figures 4A and 4B), relies mostly on character optimizations, and it is poorly supported with low Bremer support.

At higher, interordinal systematic rank, our cladistic analyses (analyses 1–10; Table S1 and Data S1) recover two topologies within Tethytheria: (1) a clade (Embrithopoda (Proboscidea, Sirenia)) and (2) a clade (Embrithopoda, Proboscidea). The sister-group relationship of the Embrithopoda to both the Sirenia and Proboscidea, i.e., its stem tethytherian position, is found in nearly all our analyses (Table S1 and Data S1). It is supported by analyses constraining the Afrotheria and Afroinsectiphilia (analyses 7–10), and it is also found in unconstrained analysis (analysis 1) and in analysis with TNT’s “implied weighting” option (analysis 2; Figure 4A). Of note, it is most usually associated with the clade Paenungulatomorpha [7]. The alternative, previously consensual sister-group relationship of the Embrithopoda and the Proboscidea is found marginally in three analyses (analyses 1, 3, and 7), especially with unordered features (analysis 3).

The relationship of Eritherium within Tethytheria is noticeably unstable in our analyses, depending on relationships of the Proboscidea with respect to the Sirenia and the Embrithopoda. Eritherium is found as a stem proboscidean, as initially established [6], only in cladograms recovering the clade (Proboscidea, Embrithopoda). In other MPTs, it is sister group either to both the Proboscidea and Sirenia (most frequent topology) or to all tethytherians. Consequently, in this study, the Eritherium position is unexpectedly unresolved within Tethytheria (polytomy in consensus trees or low Bremer support).

The stem tethytherian position of the Embrithopoda is the best supported phylogenetic hypothesis found here, both in our parsimony analyses and in character transformation within paenungulates. It implies that the crown-group Tethytheria only includes extant orders. It is most consistent with an ancestral dilambdodont tooth morphotype of the paenungulates [7, 8] from which evolved the specialized hyperdilambdodont and pseudolophodont molar pattern of the embrithopods (Figure 4C). Such a paenungulate ancestral dilambdodont morphotype is supported by our MPTs (Figure 4A) recovering the clade Paenungulatomorpha, with a stem paenungulate relationship of early dilambdodont genera Ocepeia and Abdounodus [7], and also by the dilambdodont morphology of early crown paenungulates such as Eritherium and Seggeurius. Embrithopods’ hyperdilambdodont and pseudolophodont molars evolved by enlargement of labial molar structures of the “primary trigon” such as the styles, stylar shelf, and ectoloph, at the expense of the lingual structures [11]. The stem tethytherian position of the Embrithopoda found here allows to identify a remarkable opposite early trend of the crown Tethytheria (Sirenia, Proboscidea) to reduce labial molar structural elements (stylar shelf and cusps, ectoloph), combined with development of the lingual lophs and true bilophodonty (Figure 4C). As a result, S. minor helps to elucidate two divergent and non-homologous lophodont specializations at the tethytherian root (Figure 4C), with morphological and functional emphasis of either (1) labial molar crests and cusps in the hyperdilambdodont and pseudolophodont pattern of embrithopods or (2) lingual molar crests and cusps in the true lophodont pattern of crown tethytherians (extant elephant and sea cow orders). These two structural trends are mutually exclusive in the Tethytheria; by contrast, the hyraxes order retained both structural trends with the evolution of more or less specialized and combined dilambdodonty (selenodonty) and true bilophodonty in several lineages [22].

Significance of Stylolophus
The discovery of the earliest and basal-most embrithopods in the early Eocene of Morocco sheds new light on the basal diversification of the Tethytheria and the early evolution of African ungulate-like mammals. The phylogenetic trees including Stylolophus exclude any relationships of the Asian phenacolophids to the Embrithopoda, in agreement with enamel microstructure [23]. More generally, they support a strictly African endemic early radiation of the paenungulates and tethytherians, independently from early non-African bilophodont ungulates such as phenacolophids, anthracobunids, and desmostylians that are instead related to Laurasian Euungulata and Perissodactyla (Figure 4A), as found in recent studies [24]. They also show that the order Embrithopoda is an extinct stem tethytherian branch that evolved before the divergence of the extant orders Proboscidea and Sirenia (Figure 4A). It supports at least an early Paleocene origin and early evolution of the Embrithopoda, taking into account the age of earliest crown tethytherians [25, 26]. Phylogenetic analysis of Stylolophus also elucidates the evolution of major dental specializations in Paenungulata and Tethytheria, including the most specialized one of the Embrithopoda (hyperdilambdodonty) that is homologized here with the dilambdodont ancestral morphotype of the paenungulates (Figure 4C) and is shown to correspond to an early lophodont functional convergence with the crown Tethytheria. The repeated lophodont specializations for the folivorous diet within tethytherians and paenungulates was driven by remarkable favorable herbivorous niches of the Island Africa during the Paleogene.

By the onset of the Eocene, embrithopods already had the morphotypic hyperdilambdodont dental pattern and specialized skull characters such as a developed nasal sinus system. They display distinctive specializations and are larger with respect to coeval (and sympatric) crown tethytherians from the early Ypresian of Ouled Abdoun basin such as Phosphatherium.

Our cladistic analysis supports the Embrithopoda monophyly [21]. The embrithopods distribution in both Arabo-Africa (Stylolophus, Arsinoitheriidae) and Eurasia (Palaeoamasiidae) [21, 27] is the most demonstrative mammalian evidence of trans-Tethyan dispersals involving Africa (Figure S2). The basal embrithopod position and early age of Stylolophus, as well as the supraordinal relationships to the Afrotheria/Paenungulata, all indicate at least an earliest Paleogene African center of origin for the Embrithopoda. This supports the embrithopod colonization of Eurasia from Africa, most consistently following a—remarkable—early or middle Eocene trans-Tethyan dispersal event.

https://www.cell.com/current-biology/fulltext/S0960-9822(18)30668-7
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