Johnbell hatcheri

Hitz, Ralph B., Flynn, John J. & Wyss, André R., 2006, New Basal Interatheriidae (Typotheria, Notoungulata, Mammalia) from the Paleogene of Central Chile, American Museum Novitates 3520 (1), pp. 1-32 : 3-13

Johnbell hatcheri , new taxon

Figures 2–5

HOLOTYPE: SGOPV 3106 : left maxilla, LI3-M3.

PARATYPE: SGOPV 2950 : mandibles preserving symphysis, Ri2-3, Li1-m3 .

REFERRED SPECIMENS: SGOPV 2910: Rp4- m2. SGOPV 2902: Rp1-m3. SGOPV 5001: a palate and mandibles, partially prepared,

LP2-M3 and Lp3-m3 exposed. SGOPV 3451: RP2-M2.

ETYMOLOGY: For John Bell Hatcher, a coal miner turned vertebrate paleontologist, known for his prodigious collecting and pioneering fieldwork in remote regions of Patagonia, where he secured valuable Miocene fossil mammal collections including many interatheriid specimens.

DIAGNOSIS: Johnbell hatcheri is a small typothere possessing upper molars that are longer than wide and have fossettes that disappear rapidly with wear, leaving a featureless crown save for a persistent lingual sulcus. Among typotheres, this combination of features is found only in one subgroup of interatheriids (unnamed clade, node 2 in fig. 6, this paper). The following distinctions exclude Johnbell hatcheri from other groups of small-bodied typotheres: archaeopithecids have upper molars that are equidimensional or wider than long, with long-lived fossettes, and archaeohyracids and hegetotheriids are much larger typotheres lacking a persistent lingual sulcus on upper molars. Assignment to the Mesotheriidae is ruled out by their larger size and pronounced trilobed (only trilobed early in wear in trachytheriines, persistently trilobed in mesotheriines), hypsodont upper molars.

Johnbell hatcheri is an early diverging member of the least inclusive clade (unnamed) including Interatheriinae (sensu Hitz et al., 2000, see fig. 6, node 4 this paper), Eopachyrucos , and Ignigena minisculus (new taxon, below). This clade (clade 2, fig. 6) is characterized ancestrally, and distinguished from all other interatheriids, by the following derived features: smooth posterior ectoloph on the upper premolars, and upper molars longer than wide. Interatheriids excluded from this unnamed clade ( Notopithecus , Transpithecus, Guiliemoscottia , Punapithecus , Antepithecus ) lack these features, readily distinguishing them from Johnbell hatcheri .

Conversely, Johnbell hatcheri is excluded from clade 3 of figure 6 (i.e., Interatheriinae — sensu Hitz et al., 2000 —plus Eopachyrucos ), in lacking many of the synapomorphic dental features diagnosing that clade (see phylogenetic analysis, below): a low or flat ectoloph on M1-3, and hypsodont or hypseledont cheek teeth (HI for Johnbell hatcheri SGOPV 3106 is 0.86 as measured on the M2). Johnbell hatcheri is one of the smallest interatheriids known (rivaled only by Ignigena minisculus and Punapithecus ). Its strikingly narrow premolars (in the transverse direction) and anteroposteriorly narrow internal sulcus on the upper molars nevertheless clearly differentiate it from these two other diminutive taxa. In addition, Johnbell hatcheri lacks the anterior cingulae on the upper cheekteeth present in Punapithecus .

TYPE LOCALITY: The holotype derives from Locality Set 3 ( Flynn et al., 2003) in the Abanico (5 Coya Machalı´) Formation, Río Tinguiririca valley, central Chile. Locality Set 3 (5 ‘‘Locality C’’ of Charrier et al., 1996: figs. 6–8) occurs in purplish volcaniclastic sediments north of the Río Tinguiririca, less than 5 km north of the other two main localities (Locality Sets 1 and 2, Flynn et al., 2003) yielding the Tinguiririca Fauna at Termas del Flaco (see Wyss et al., 1994; Charrier et al., 1996; Flynn et al., 2003).

KNOWN DISTRIBUTION: Tinguirirican SALMA ( Flynn et al., 2003; 5 Tinguiririca faunal interval or ‘‘Tinguirirican’’ of Flynn and Swisher, 1995). Known from the Tinguiririca Fauna type locality (Locality Set 1; SGOPV 2902, 2910, and 5001) and Locality Set 3 (the holotype and paratype, SGOPV 3106 and 2950) of Flynn et al. (2003) (Localities ‘‘A’’ and ‘‘C’’ of Charrier et al., 1996, figs. 6–8), respectively lying 2 km south and 3 km northwest of the town of Termas del Flaco and the Río Tinguiririca. The occurrence of Johnbell hatcheri at both Locality Sets 1 and 3 is important in correlating between the discontinuously exposed fossil-bearing horizons of the Abanico Formation from the north side of the Tinguiririca valley to strata south of the river. Additionally, a single specimen ( SGOPV 3451) is known from a locality (Cachapoal Locality and Fauna) within the Abanico Formation in the Río Cachapoal drainage, some 100 km north of Termas del Flaco ( Charrier et al., 1996 Flynn and Wyss, 2004). More detailed locality information is on file at the American Museum of Natural History. Radioisotopic determinations from Tinguiririca bracket the age of the fauna there between approximately 37.5–31.5 Ma, with the fossiliferous horizons themselves directly dated at, 31.5 Ma ( Wyss et al., 1994; Flynn et al., 2003) and the Tinguirirican SALMA likely spanning at least 31–33 Ma ( Flynn et al., 2003).

REMARKS: In a preliminary description of these specimens ( Hitz, 1997), SGOPV 2950 , a mandible, was designated as the principal specimen representing this new taxon, and SGOPV 3106 , a nearly complete maxilla, was designated as the equivalent of the paratype, although it was not formally named in that dissertation. Upon further consideration, we formally designate SGOPV 3106 the holotype and SGPOV 2950 the paratype, as maxillae tend to be more abundant in the notoungulate fossil record and preserve a greater number of diagnostic characters. Supporting the latter claim is the fact that in the phylogenetic analysis of Interatheriidae presented below, we recognize eighteen distinct characters for the upper dentition but only ten for the lower .

In addition to the dental features noted above, interatheriines (sensu Hitz et al., 2000) are diagnosed by two derived cranial features, an anteriorly projecting sliver of the frontal on the superoanterior orbit rim, and a posterior portion of the auditory bulla that laps onto the paraoccipital process. Given the scarcity of known cranial material, these features cannot be scored for most early interatheriids, including Eopachyrucos and Johnbell hatcheri . Nevertheless, the known distribution of these features argues that they diagnose a clade at least as inclusive as Interatheriinae , but less inclusive than Interatheriidae .

DESCRIPTION

Johnbell hatcheri is known from nearly complete maxillary and mandibular dentitions (figs. 2–5). Only SGOPV 5001 shows an articulated upper and lower dentition, but the teeth of this specimen are tightly clenched and embedded within an extremely hard and brittle volcaniclastic matrix, making the crowns of the lower dentition inaccessible to preparation (without risking destruction of the teeth). The remaining suite of upper and lower dentitions is allocated to the same taxon based on similarities of the labial faces of the cheekteeth (visible in 5001), common morphological features of the occlusal surfaces among those specimens with crowns prepared, similarity in size, and the presence of features identifying all specimens as basal interatheriids. Mensural information is presented in table 1.

UPPER DENTITION

The description of the upper dentition is based principally on the holotype SGOPV 3106, the specimen with the best preserved and most completely exposed upper dentition.

Only I3 is preserved among the upper incisors. It is incisiform; a shallow, vertically oriented groove divides the tooth into anterior and posterior portions labially, a division that is less apparent lingually. The upper canine is incisiform and similar in size to I3. The anteroexternal swelling (anterior portion of tooth) is better developed on the canine than on I3.

The first upper premolar bears a distinct anterolabial swelling on the ‘‘ectoloph’’ and a posterolingual shelf heel, the latter of which probably represents a diminutive protocone. Otherwise the tooth is relatively narrow.

The succeeding premolars are progressively larger but otherwise share the same distinctive morphology seen in the premolars of other interatheriids. A distinct parastyle and prominent paracone occur on P2-4, producing a fold or groove between the two (although it does not penetrate the crown deeply). A small protocone occupies a position near the metacone. The lingual premolar margins are smooth and straight, being oriented nearly anteroposteriorly. There is no hypocone, thereby making the three posterior upper premolars rather narrow. Anterior cingulae are lacking (but a small emargination may be present on P3-4). A high posterior cingulum may have occurred on P3-4, as one is present on P2, but it has since merged with the remainder of the crown through wear. A strong undulation on the ectoloph corresponds to the parastyle and paracone, but the remainder of the ectoloph is nearly straight posteriorly.

The first two upper molars are very similar in morphology, the most significant difference being the greater length of M2. These teeth are expanded anteroexternally, lending this portion of the teeth a wedge shape. A small parastyle is present, as is a shallow parastyle/ paracone inflection. The ectoloph shows mild undulations; the parastyle, paracone, and metacone have approximately an equal amount of relief on M1, but on M2 the metacone shows decidedly less relief compared to the parastyle and paracone. The protoloph extends posterolingually from the parastyle; it does not bear a cingulum (or if it did, this feature coalesced with the remainder of the crown early in wear). The protocone and hypocone are separated by a deep sulcus, which extends nearly to the roots. The posterior border is oriented transversely and is marked by a high cingulum, which appears to merge with the remainder of the crown early in wear. Anterior and posterior fossettes are seen on M1, but wear has nearly erased them. A posterior fossette is visible on M2, but also is nearly obliterated by wear. A small irregular feature is visible on the anteroexternal quadrant of the crown, which may be the remnant of an anterior fossette, but this structure is faint and we refrain from identifying it conclusively as a fossette. Given the faintness of the fossettes in SGOPV 3106 (the holotype, which exhibits an early wear stage), we conclude that even modest wear in this taxon would likely have rendered the molar occlusal surfaces featureless. The lingual sulcus on both molars is deep and probably persisted until wear neared the roots.

The third upper molar is a smaller version of M1-2. The main morphological contrast between it and the two preceding teeth is in the posterior margin. The hypocone is less prominent on M3, making the posterior margin angle anterointernally from the posterior margin of the ectoloph, rather than transversely as in M1-2. Fossettes are not visible on M3, and were probably absent entirely, given the early wear stage of this tooth on SGOPV 3106. The lingual sulcus appears to be deep and probably persisted into late wear stages.

Teeth of the upper dentition are rooted and may be described as brachydont, but the crowns of the cheekteeth are heightened somewhat. The hypsodonty index (HI; greatest ectoloph height divided by greatest ectoloph length) for M2 on Johnbell hatcheri (SGOPV 3106) is 0.85. For comparison, HI values of the M2 of other small basal interatheriids are: Notopithecus adapinus (specimen not identified), 0.81 ( Simpson, 1967); Antepithecus adapinus (specimen not identified), 0.72 ( Simpson, 1967); SGOPV 3168 (new taxon, Ignigena minisculus , below), 0.79; Punapithecus minor (MLP-V-10-1), 0.61; Antepithecus brachystephanus (SGOPV 3604, below), 0.45. All values were calculated based on teeth with trace to slight wear.

LOWER DENTITION

This description is based on the paratype (SGOPV 2950), SGOPV 2910, and SGOPV 5001, all of which possess well-preserved lower teeth.

The first two lower incisors are both small, each having a faint, vertically oriented groove lingually. The i3, which is slightly larger than i1-2, bears a similar groove. The lower canine is very poorly preserved and provides little information.

The p1 is transversely narrow; a deep, vertically oriented groove opens lingually, giving the crown a V-shaped appearance. The posterior arm of the V is slightly larger than the anterior arm and bears a small posterior heel set off from the remainder of the crown by a shallow vertical groove labially.

The p2 closely resembles p1, except that the posterior heel is larger and more distinct. This heel is accentuated by a shallow groove on the labial face of the posterior crest, as well as a vertical groove lingually.

The p3 consists of a distinct trigonid and talonid. The trigonid is dominated by a gently convex protolophid (opening lingually) and a small metalophid. This convexity produces a shallow internal vertical trough between the metaconid and the anterior portion of the protolophid. A small talonid attaches directly to the metaconid and is demarcated from the trigonid by a strong labial vertical groove and a mild lingual one.

The p4 is larger than the p3 and bears a strongly curved protolophid. The anterior margin of the protolophid is oriented transversely and, given its size, may be termed a paralophid. The metalophid is also oriented transversely; a deep vertical groove lies between it and the paralophid. A small talonid arises from the metaconid and is separated from the trigonid by a deep labial groove and a shallower, more posterior lingual groove.

The trigonids of m1-3 are similar, consisting of a diagonal paralophid (the lingual margin of which is posteriorly positioned relative to the labial margin), a longitudinal protolophid, and a diagonal metalophid (parallel to the paralophid) displaying a prominent metaconid. The vertical groove between the paralophid and the metalophid is very shallow, reaching midway down the crown toward the roots. The molar talonids are demarcated from the trigonids by deep labial and lingual vertical grooves, the former being slightly more posteriorly situated than the latter. These grooves extend nearly to the base of the crown. No hypoconulid is seen on m1 or m2 and the talonids are essentially ovate in crown view, with the long axis oriented transversely. The m1 talonid attaches to the trigonid just labial to the metaconid. The m2-3 talonids attach more labially, nearer the center of the metalophid. The m3 talonid is larger than those of the preceding two molars and displays a prominent hypoconulid, which produces a slight groove on the tooth’s lingual surface.

MANDIBLE

SGOPV 5001 , the paratype SGOPV 2950 , and SGOPV 2910 preserve partial mandibles . SGOPV 2910 displays a mental foramen near the ventral margin of the horizontal ramus, positioned directly below p4. The other two specimens do not clearly preserve mental foramina, although a possible trace of one is present below p4 on SGOPV 5001 . The mandible is 7.47 mm and 7.90 mm deep below m2 on SGOPV 5001 and SGOPV 2910 respectively .

DISCUSSION

SGOPV 3451, consisting of a portion of a right maxilla containing P2-M2 (fig. 5), is remarkably similar to specimens of Johnbell hatcheri from the Río Tinguiririca valley (type area of the Tinguiririca Fauna) in size and morphology, and the anatomical description provided above applies to this specimen as closely as it does to specimens from the Tinguiririca region. SGOPV 3451 derives from an as yet little studied locality (yielding the Cachapoal Fauna; Charrier et al., 1996; Flynn and Wyss, 2004) in the Río Cachapoal drainage, about 5 km NW of the Río Las Leñas fossiliferous sites (Flynn et al., 1995) and, 100 km N of the fossiliferous strata cropping out near Termas del Flaco that bear the Tinguiririca and Tapado faunas.

The Cachapoal specimen, SGOPV 3451, shares a key diagnostic feature with Johnbell hatcheri (transversely narrow upper premolars), and the specimen otherwise displays no taxonomically significant features or size differences differentiating it from all other specimens here referred to Johnbell hatcheri , substantiating inclusion of this specimen in the hypodigm. This is the first Andean specimen from outside the stratotype sequence of the Tinguiririca Fauna (upper Río Tinguiririca valley) referable to a species from that fauna, demonstrating, among other things, the utility of Johnbell hatcheri for long distance biochronologic correlation.

SGOPV 3451 is in an earlier ontogenetic wear state than is the holotype of Johnbell hatcheri ( SGOPV 3106 ). The posterior cingulum on the M2 of SGOPV 3451 , for example, has not yet completely merged with the crown, as it has on SGOPV 3106 . The M1 and M2 of SGOPV 3451 both appear to display a faint anterior fossette, and the M2 also has a posterior fossette. The lack of a posterior fossette on M 1 in SGOPV 3451 is in contrast to the condition in SGOPV 3106 , which still retains a posterior fossette on this tooth despite greater wear. This variation seems taxonomically inconsequential, likely reflecting only the extremely ephemeral nature of the molar fossettes in this taxon .

Ignigena minisculus , new taxon

Figures 7–10

HOLOTYPE: SGOPV 3168 : left maxilla, including partial zygoma and C-M3.

REFERRED: SPECIMENS SGOPV 3167: left mandible with p1-p3, m1.

ETYMOLOGY: Ignigena , meaning ‘‘born of fire’’, in reference to the volcanoclastic deposits from which the specimens derive; and minisculus , meaning ‘‘small’’, in reference to the diminutive size of the specimens.

DIAGNOSIS: The same criteria listed in the previous section marking Johnbell hatcheri as a basal interatheriid, and distinguishing it from all known interatheriines, apply to this taxon as well. Fundamental differences in tooth characters differentiate Ignigena minisculus from other groups of small bodied typotheres: archaeopithecids have equidimensional upper molars with long-lived fossettes, and molars wider than long; archaeohyracids and hegetotheriids are much larger typotheres, and lack a persistent lingual sulcus on upper molars; even small mesotheriids are clearly larger and have pronounced trilobed (except for late wear stages in trachytheriines, where these teeth become bilobed) hypsodont upper molars.

Ignigena minisculus belongs to the least inclusive clade (unnamed) containing Interatheriinae (sensu Hitz et al., 2000, see fig. 6, node 4 this paper) plus Eopachyrucos and Johnbell hatcheri , characterized ancestrally, and distinguished from all other interatheriids, by the following derived features: smooth posterior ectoloph on the upper premolars and upper molars longer than wide.

Ignigena minisculus lacks the suite of synapomorphic dental features that diagnose the clade of Interatheriinae (sensu Hitz et al., 2000) plus Eopachyrucos : a low or flat ectoloph on M1-3; hypsodont or hypseledont cheekteeth (HI for Ignigena minisculus is 0.81, as measured on the M2).

Ignigena minisculus differs from Johnbell hatcheri in having upper premolars with stouter protocones, resulting in broader teeth (most apparent on P3-4). The lingual sulcus on M1-2 also is wider than on Johnbell hatcheri . Ignigena minisculus is distinguished from Punapithecus (besides Johnbell hatcheri , the only other diminutive basal interatheriid known) by the former’s larger size, lack of anterior cingulae on the upper premolars and molars, and upper molars that are longer than wide.

TYPE LOCALITY: Tapado (‘‘Main’’) Locality, Abanico Formation, Río Tinguiririca valley, east central Chile ( Wyss et al., 1994). The Tapado (‘‘Main’’) Locality is located 14 km northwest of the Tinguiririca Fauna localities (see above; Wyss et al., 1994; Charrier et al., 1996; Flynn et al., 2003). Although fossils potentially referable to the Tapado Fauna have been recovered from exposures on both the north and south sides of the Tinguiririca River (over a N-S distance of more than 3 km), most specimens have been recovered from sites south of the river. Casual observation, and even previous mapping (e.g., Klohn, 1957, map cross-section B) of the nearly continuous exposures of the Abanico Formation in the Tinguiririca valley give the impression that the western horizons producing the Tapado Fauna are substantially higher stratigraphically (many hundreds of meters) than the eastern deposits bearing the Tinguiririca Fauna. Nevertheless, the Tapado Fauna is undoubtedly older (probably Casamayoran SALMA) than the Tinguiririca Fauna, demonstrating that low-angle faulting has thrust (albeit cryptically) older strata over younger within the Tinguiririca valley. Evidence suggesting a Casamayoran age for the Tapado Fauna includes the didolodontid Ernestokokenia , Notostylops , and possibly Eohyrax isotemnoides ( Wyss et al., 1996; Flynn et al., 2005). A more definitive age assignment awaits ongoing taxonomic and radioisotopic work.

KNOWN DISTRIBUTION: Known only from the Tapado Fauna, from the Tapado (‘‘Main’’) Locality, south of the Río Tinguiririca; likely Casamayoran in age.

DESCRIPTION

Mensural information for the two interatheriid specimens currently known from the Tapado Fauna is presented in table 2.

UPPER DENTITION

The interatheriid occurring in the Tapado Fauna compares closely with Johnbell hatcheri from the Tinguiririca Fauna, and the morphological features of the dentition presented above for Johnbell hatcheri also pertain to Ignigena minisculus , except where noted below. The two forms differ significantly only in the points noted in the diagnosis of Ignigena minisculus . Also in contrast to Johnbell hatcheri , the P4 of Ignigena minisculus shows a minor posterior cingulum high on the crown. A similar but more distinct cingulum occurs on M1 and M2. (However, the holotype of Johnbell hatcheri [SGOPV 3106] shows greater wear than specimens of Ignigena minisculus ; thus, the difference in posterior cingulum expression may reflect wear rather than discrete, taxonomically informative character differences.) The third upper molar in SGOPV 3168 is in the process of erupting, with only the tips of the paracone and protocone having emerged from the tooth crypt.

CRANIAL MATERIAL

The anterior root of the zygoma extends from the posterior half of M2 to the middle of P4 and is thus is not remarkably broad. The zygoma is marked by a small descending process. The palatine/maxillary suture of the palate is curved (concave posteriorly); its anterior end is even with P4.

LOWER DENTITION

Reference of lower dental material ( SGPOV 3167 ) to Ignigena minisculus is based on size (relative to the holotype SGPOV 3168 ) and similar suites of upper and lower dental features (as present in other basal interatheriids) .

SGOPV 3167 preserves fragments of an unworn lower dentition, the anterior premolar portion of which displays some unusual morphology for an interatheriid. The first two premolars are unworn. The occlusal outline of each of these teeth, which are narrow, forms a broad ‘‘V’’, opening lingually. In lateral view, the two wings of the ‘‘V’’, each of which is somewhat rounded, are separated by a cleft at their juncture, giving the teeth the appearance of ‘‘Mickey Mouse ears’’. This shape is more emphasized on p1 than on p2. The ‘‘V’’-shaped p1–2 seem to resemble the morphology of these teeth in Notopithecus and Johnbell hatcheri , but the latter two taxa are not known from unworn specimens, complicating direct comparison with SGOPV 3167.

Only the tip of the crown of p3 is preserved, but this tooth appears to have formed a thin crescent, convex labially. Other morphological details of this tooth are obscured by breakage.

The m1 is partly exposed labially, showing a convex (labially) protolophid, a transversely oriented metalophid, and a deep labial sulcus between the trigonid and talonid. The talonid is ovate in outline with a flattened posterior margin. The well-defined labial sulcus on m1 is similar to that observed in other basal interatheriids, such as Notopithecus and Johnbell hatcheri (although some other small non-interatheriid typotheres also have this feature).

Figures

Fig. 2. Johnbell hatcheri, new taxon, SGOPV 3106 (holotype), left maxilla, LI3-M3, occlusal view. Tinguiririca Fauna.

Fig. 3. Johnbell hatcheri, new taxon, SGOPV 3106 (holotype), left maxilla, LI3-M3, buccal view. Tinguiririca Fauna.

Fig. 4. Johnbell hatcheri, new taxon, SGOPV 2950 (paratype), mandibular symphysis, Ri2-3, Li1- m3, occlusal view. Tinguiririca Fauna.

Fig. 5. Johnbell hatcheri, new taxon, SGOPV 3451 (referred specimen), partial maxillary with RP2-M2, occlusal view. Cachapoal Fauna.

Fig. 7. Ignigena minisculus, new taxon, SGOPV 3168 (holotype), left maxillary fragment, partial zygoma, C-M3, occlusal view. Tapado Fauna.

Fig. 8. Ignigena minisculus, new taxon, SGOPV 3168 (holotype), left maxillary fragment, partial zygoma, C-M3, left buccal view. Tapado Fauna.

Fig. 9. Ignigena minisculus, new taxon, SGOPV 3167 (referred specimen), left mandible, p1-p3, m1, occlusal view. Tapado Fauna.

Fig. 10. Ignigena minisculus, new taxon, SGOPV 3167 (referred specimen), left mandible, p1–p3, m1, left buccal view. Tapado Fauna.