Fruitadens haagarorum Butler et al., 2010

Sereno, Paul C., 2012, Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs, ZooKeys 226, pp. 1-225 : 26-29

Fruitadens haagarorum Butler et al., 2010 Fig. 9ATables 13

Fruitadens haagarorum Butler et al., 2010 - Callison and Quimby (1984: fig. 3B, C); Callison (1987, fig. 4); Kirkland (2006: fig. 22A); Galton (2006: fig. 2.7 A–G); Galton (2007: Figs 2.6 E–G, 2.7 A–G); Butler et al. (2010: Figs 1-3); Butler et al. (2012: Figs 1-7, 8A, B, 9-16)

Holotype.

LACM 115747, adult with partial maxillae and dentaries, cervical, dorsal, sacral and caudal vertebrae, proximal right femur, proximal and distal ends of the tibiae, and partial right metatarsal 1 ( Butler et al. 2010: fig. 2b, e, i; Butler et al. 2012: Figs 1, 2-4, 8A, 9-12, 13 A–E, 14G, H).

Referred material.

LACM 115727, adult partial postcranial skeleton with partial cervical, dorsal and caudal vertebrae, partial right and left femora, and an articulated distal left tibia and coossified astragalocalcaneum; LACM 120478, subadult with left humerus, distal left femur, and an articulated left tibia, fibula and coossified astragalocalcaneum; LACM 120602, distal caudal vertebra, left astragalocalcaneum, partial metatarsus and pes; LACM 128258, subadult with right premaxilla, partial left maxilla, partial left and right dentaries, and one dorsal and one distal caudal vertebra; LACM 128303, anterior left dentary ( Butler et al. 2010, 2012).

Type locality.

Fruita Paleontological Area, approximately 10 kms southwest of Fruita, Mesa County, west-central Colorado, USA; approximately N39°10', W108°48'.

Horizon.

Just above the "clay change" near the base of the Brushy Basin Member and about 100 m from the base of the Morrison Formation ( Kirkland et al. 2005: fig. 6; Kirkland 2006); Upper Jurassic (early Tithonian), ca. 153 Ma ( Kirkland 2006; Gradstein and Ogg 2009). The boundary between the Salt Wash and Brushy Basin Members has undergone revision in the Fruita Paleontological Area ( Kirkland 2006). Galton (2002) reported that the fossils were found in the Salt Wash Member but later correctly cited the overlying Brushy Basin Member as the unit of origin ( Galton 2007).

Revised diagnosis.

Heterodontosaurid with (1) a discordantly small dentary tooth immediately distal to the caniniform dentary tooth and (2) a prominent anteromedial flange on the distal end of the tibia.

Comments.

Butler et al. (2010: 376) listed nine features in the original diagnosis for Fruitadens haagarorum , indicating that some were primitive and others “autapomorphic” at different phylogenetic levels. Butler et al. (2012: 3) added one character, "small foramen on the anteroventral aspect of the medial dentary", for a total of ten features.

This suite of features constitutes a differential diagnosis-a unique combination of the features that describes a monospecific genus rather than a set of autapomorphies hypothesized to have arisen in the immediate ancestry of the taxon ( Sereno 1990). In the revised diagnosis above, a higher bar is applied that restricts listed features to those that are plausibly unique to Fruitadens or derived for Fruitadens within Heterodontosauridae . One of the features stands out as plausibly unique to Fruitadens and present in the holotype-the prominent anteromedial flange on the distal end of the tibia ( Butler et al. 2010: fig. 2i). Comparison to other heterodontosaurids, however, is limited to Heterodontosaurus , which does not exhibit the condition. The small medial dentary foramen, which was added by Butler et al. (2012) as an autapomorphy, is not present in one of the two dentaries that preserve the region ( Butler et al. 2012: fig. 5D).

One dental feature listed in the revised diagnosis may be an autapomorphy but is homoplasious among heterodontosaurids. The dentary tooth immediately distal to the caniniform tooth in Fruitadens is unusually small, its crown apparently somewhat smaller than the rudimentary first dentary tooth (Fig. 9A). Heterodontosaurus and a new taxon from southern Africa, Pegomastax gen. n. sp. n., are the only other heterodontosaurids with a discordantly small tooth immediately distal to the caniniform tooth.

The other features listed in the diagnosis by Butler et al. (2010, 2012) are demonstrably primitive within Heterodontosauridae or may be artifacts of preservation. The lack of a maxillary caniniform tooth, for example, is the common condition; only Echinodon has a caniniform tooth in the maxillary series among heterodontosaurids. Denticles that extend along more than one-half of the crown occur in Tianyulong and in ornithischian outgroups such as Lesothosaurus . Other features have broader distributions within Heterodontosauridae , such as the small peglike first dentary tooth, which is present in Echinodon , Lycorhinus , and Abrictosaurus . The dentary caniniform tooth was said to be shorter than in some heterodontosaurids and more nearly equal in depth to noncaniniform dentary crowns. The relative depth of individual crowns, however, depends to a great extent on the stage of replacement, which can be difficult to estimate in the case of caniniform teeth. The dentary caniniform tooth in the Kayenta heterodontosaurid, for example, has a similar relative depth to that in Fruitadens , although it is clearly undergoing eruption and would be considerably larger when fully functional (Fig. 9A, B). Most of the caniniform tooth in question (LACM 128258), furthermore, seems to have broken away by the time the specimen was described by Butler et al. (2010: fig. 2d; compare Galton 2007: fig. 2.7B). The pair of foramina on the anterior aspect of the astragalus can be compared only in Heterodontosaurus among heterodontosaurids, and there is some evidence the condition is present (SAM-PK-K1332).

The separation of the ascending process of the astragalus as a separate ossification was listed among the autapomorphies. The suture separating the distal tip of the astragalar ascending process, however, seems to continue laterally as a fracture line across the distal shaft of the fibula. The ascending process had been viewed as a separate ossification in the theropod Dilophosaurus (Welles 1984); review of this specimen, however, suggests that it also appears to be a postmortem fracture passing through vascular foramina. Persistence of such a suture in Fruitadens , in addition, seems unlikely in a taxon distinguished by coossification in this particular region of the limb skeleton (e.g., the tibiotarsus).

Discovery.

The six available specimens of Fruitadens haagarorum were collected between 1975 and 1980 in the Fruita Paleontological Area from four separate localities above a distinctive horizon ("clay change") near the base of the Brushy Basin Member of the Morrison Formation ( Kirkland et al. 2005; Kirkland 2006). The localities were not discovered in a single horizon but rather within a zone perhaps 10-15 m in thickness above the "clay change" (G. Callison, pers. comm.). Four specimens were described in the initial description ( Butler et al. 2010); two additional fragmentary specimens were included more recently ( Butler et al. 2012; LACM 120602, 128303).

Fruitadens is a small-bodied heterodontosaurid, the adult specimens of which appear to be slightly larger than two other small-bodied heterodontosaurids, Echinodon and Tianyulong (Table 3). The sole specimen known of the Kayenta heterodontosaurid is the smallest heterodontosaurid on record (skull length estimate of 53 mm), but histologic evidence suggests that it is a subadult roughly the same size as half-grown individuals of Heterodontosaurus (e.g., AMNH 24000; Fig. 2C).

Association.

The holotype of Fruitadens and three referred specimens were described as associated individuals, the supportive evidence limited to the lack of duplicate bones and consistent state of preservation within each specimen ( Butler et al. 2010: suppl. info.; Butler et al. 2012). A first-hand account of the discovery of the holotype (LACM 115747) and a referred subadult specimen (LACM 120478) confirms their association as individuals. The first pieces of the holotype were surface-collected on a slope, which led to an in situ portion of the specimen that was recovered in a small field jacket (G. Callison, pers. comm.). The subadult specimen LACM 120478, which preserves the most complete long bone lengths, was also found in a confined space by quarrying at a nearby locality 5572 ("Main Callison Quarry"). A referred adult specimen was found at locality 5576 ( “George’s Coelurosaur site"), and there is no specific site information available for a referred subadult with the most complete set of jaws (LACM 128258), except that it comes from the same general area (Fruita Paleontological Area).

Jaws.

The dentary in Fruitadens has a vascularized buccal emargination, presumably as an aid to the processing plant materials within the oral cavity (Fig. 9A). The anterior end of the dentary in Fruitadens is subrectangular, whereas in as Echinodon the ventral side of the anterior end of the dentary is strongly beveled. Both Fruitadens and Echinodon have a well-demarcated vessel tract passing from the anterior dentary foramen toward the predentary.

A jaw fragment housing three teeth was identified as a right premaxilla ( Butler et al. 2010: fig. 2a). Only the middle tooth preserves the entire crown; only the base of the distal crown is preserved, and the first crown in the series appears to have been lost. Butler et al. (2010: 376) noted that all of these teeth have waisted, subtriangular crowns; there is no development of a caniniform crown. Butler et al. (2012), further, suggested that a small portion of the left premaxilla might be attached across the median palatal suture. A portion of an ascending ramus has been shown as preserved on the premaxilla (Fig. 9A), but no part of this ramus appears to be preserved on the actual specimen ( Butler et al. 2012: fig. 7A). In other heterodontosaurids, the narial fossa extends close to the ventral margin of the premaxilla, whereas in Fruitadens the narial fossa is not evident in lateral view. The identification of this jaw fragment as a portion of the right premaxilla may be correct, but it exhibits several unusual features as compared to more completely known premaxillae in Echinodon , Tianyulong , Lycorhinus , and Heterodontosaurus .

The maxilla in Fruitadens has a deep margin between the antorbital fenestra and the maxillary tooth row, although this depth can appear greater with postmortem compression (Fig. 9A). As preserved it most closely resembles the condition in Echinodon and Lycorhinus . The buccal emargination is approximately one-half as deep in Tianyulong , Abrictosaurus , and Heterodontosaurus . Butler et al. (2010: 376, fig. 2c) correctly identified a partial left maxilla in a subadult (LACM 128258), whereas Galton (2007: Figs 2.6E, 2.7A) identified the same specimen as a right maxilla.

Dentition.

The location, size and shape of the anterior dentary teeth differ between Fruitadens and Echinodon . In Fruitadens (Fig. 9A) the first dentary tooth is closer to the anterior end of the dentary than in Echinodon , in which a short edentulous margin precedes the first dentary tooth ( Galton 2007: fig. 2.7B; Butler et al. 2010: fig. 2d). Both Fruitadens (LACM 115747) and Echinodon have a small peg-shaped first dentary tooth. Although described as unique to Fruitadens ( Butler et al. 2010: 376, 378), a rudimentary first dentary tooth (or tiny alveolus) is also known in Echinodon , Lycorhinus and Abrictosaurus . In some heterodontosaurids, including the Kayenta heterodontosaurid, Pegomastax gen n. sp. n., and Heterodontosaurus , there are no teeth mesial to the caniniform tooth.

The third dentary tooth in Fruitadens , or the first “cheek” tooth, is the smallest tooth in the dentary series (Fig. 9A). Although described as present in "other heterodontosaurids" ( Butler et al. 2010: 378), such a diminutive tooth is known only in Pegomastax gen n. sp. n. and possibly in Heterodontosaurus . In other heterodontosaurids such as Echinodon , Lycorhinus , and Abrictosaurus , the first cheek tooth is subequal to successive crowns at the anterior end of the dentary tooth series.

The premaxillary teeth described by Butler et al. (2010) would be unusual in form for heterodontosaurids, but identification of the jaw fragment as a right premaxilla may be problematic as discussed above. Galton (2007: 28) reported the presence of five premaxillary teeth in Fruitadens , but there appears to be no evidence in support of this statement.

The largest maxillary crowns in the distal portion of the series have a bulbous cingulum with well-defined basal and apical edges ( Butler et al. 2010: fig. 2b: Galton 2007: fig. 2.7C, F, G). The apical edge of the cingulum is maintained in the center of the crown base, where the median eminence joins the base of the crown (Fig. 9A). This well-defined cingulum is present only in the largest crowns and resembles the condition in the cheek teeth of thyreophoran ornithischians. A similar condition is present in Tianyulong . The apical edge of the cingulum merges with the crowns face in Echinodon and most other heterodontosaurids.

Computed-tomographic scans show active tooth replacement in Fruitadens ( Butler et al. 2010: Figs 2f, 3d, 3a-c; Butler et al. 2012: Figs 3, 4). Wear facets from tooth - to-tooth occlusion, however, have not been identified. The presence or absence of wear facets is difficult to determine with so few available crowns. Some of these specimens, in addition, are from younger individuals, which may not show wear typical of adults. The low-angle wear facets in Echinodon are less obvious and sometimes absent on newly erupted crowns.