ARDEIDAE Leach, 1820

Stidham, TA, Tsubamoto, T, Zin-Maung-Maung-Thein, N, Thaung-Htike, N, Egi, N, Nishioka, Y, Maung-Maung, N & Takai, M, 2016, A night heron (Ciconiiformes, Ardeidae) and a stork (Ciconiidae) from the Pliocene of Myanmar (Burma), Palaeontologia Electronica 4 (8), pp. 1-12 : 4-6

Family ARDEIDAE Leach, 1820

Tribe cf. NYCTICORACINI Bock, 1956

Figure 2

Specimen. NMMP-KU-IR 0343 is a distal tarsometatarsus.

Description. The specimen is the distal end of a right tarsometatarus broken near where the facet for metatarsal I would be positioned ( Figure 2 ). The distal mediolateral width of the specimen is 10.1 mm, and the mediolateral width of trochlea III is 3.1 mm. The specimen is fractured through the shaft near its preserved midpoint, and the distal and lateral portions of trochlea IV are missing. The trochleae have a spongy appearance that could indicate a subadult ontogenetic age of this individual, or could be the result of wear or even acid etching (e.g., from the action of a carnivore’s digestive system).

Trochlea II extends distal to trochlea IV, and it is much wider (mediolaterally) than trochlea IV. The area where the dorsal opening of the distal foramen should be is obscured, but the plantar opening is small and near the same proximodistal level as the proximal end of the plantar ridge extending from the base of trochlea II. The furrow on trochlea III extends from the plantar proximal end onto the dorsal surface, but the furrow does not reach the dorsal proximal end. The furrow is restricted to the plantar surface in trochlea II. Proximal to the plantar base of trochlea II is a short ridge and groove that are oriented from distomedial to proximolateral ( Figure 2.5 ). The medial side of trochlea II is concave, and in dorsal view, the proximal end of trochlea II distinctly projects medial to the medial edge of the shaft. In distal view, trochlea III extends plantar to trochlea II, and the dorsal part of trochlea II is mediolaterally wider than its plantar portion. The lateral side of the shaft proximal to trochlea IV is relatively flat, and there is a slight ridge extending proximal from the lateral proximal base of the broken trochlea IV. The lateral intertrochlear incision is relatively shallow compared to the distal extension of the preserved part of trochlea IV. The proximal end of the specimen is near where the facet for metatarsal I would be, and it is not clear if the facet was present or not.

Comparisons. Scofield et al. (2010) and Worthy et al. (2013) utilized many tarsometatarsus characters in their analyses of fossil heron and bittern specimens. Several of these characters are present and observable in this fossil. Trochlea IV is damaged, but the preserved portion of the dorsal aspect does not preserve any evidence of a trochlear furrow ( Scofield et al., 2010; Worthy et al., 2013; character 1), although a furrow may have been present in the missing portion. The trochlear furrow of trochlea III is strong all the way to its proximal plantar end ( Scofield et al., 2010; Worthy et al., 2013; character 2 - state 2). That state does not occur in Ardea and Egretta ( Scofield et al., 2010) . The area where the dorsal opening of the distal foramen would be is obscured in the fossil ( Scofield et al. 2010; character 3). The dorsal opening of the foramen is either absent or very small (states 0 or 1) because it likely would be visible if it was large. The obscured dorsal opening (if present) of the distal foramen means that characters 3, 4, and 5 cannot be assessed. Trochlea III is oriented in parallel with the shaft of the tarsometatarsus ( Scofield et al., 2010; Worthy et al., 2013; character 6 - state 0). That state is present in Nycticorax , Nyctanassa , Pilherodius , and Tigrisoma , and absent in Ardea , Egretta , Botaurus , Ixobrychus , and Cochlearius ( Scofield et al., 2010) . Trochlea II protrudes abruptly (with the dorsomedial proximal edge extending distinctly medially) from the medial side of the shaft ( Scofield et al., 2010; Worthy et al., 2013; character 7 - state 2). That state is present in the night herons, Pilherodius , and Tigrisoma , and is absent in other herons ( Scofield et al., 2010). The plantar rim of trochlea IV is missing, and character 8 of Worthy et al. (2013) cannot be coded. Although broken, it does not appear that trochlea IV protrudes laterally beyond the shaft, or only protruded very slightly ( Scofield et al., 2010; character 8 - state 0; Worthy et al., 2013; character 9 - state 0 or 1). That morphology is present in Nycticorax , Nyctanassa , Ardea ibis , Pilherodius , Botaurus , Ixobrychus , and Cochlearius , and absent in Egretta , other species of Ardea , and Tigrisoma ( Scofield et al., 2010) . Worthy et al. (2013) consider their version of the character as variable within night herons (i.e., states 0 and 1). The length of trochlea IV is very short distal to the intertrochlear incision relative to the width of the lateral intertrochlear incision ( Worthy et al., 2013; character 10 - state 0). That state is present in night herons, Ardea , Botaurus , Egretta , and others ( Worthy et al., 2013). It is unclear if there was a facet for metatarsal I since the bone is broken near where that facet would occur ( Scofield et al., 2010; character 9; Worthy et al., 2013; character 11). The area between where the metatarsal I facet and the plantar base of trochlea II appears to be relatively flat, although the bone is broken through the shaft in that area (likely state 0 of character 12 of Worthy et al., 2013). Given the published phylogenetic distribution of those characters (above), the combination of features preserved in the Sulegon fossil are consistent only with Nycticorax , Nyctanassa , and Pilherodius . Those three taxa form a clade in Scofield and coauthor’s (2010) work and suggests that this fossil belongs to that clade (which includes the night herons). However, Worthy et al. (2013) did not recover a monophyletic grouping of the night herons. Instead, they form a paraphyletic stem at the base of Ardeidae ( Worthy et al., 2013) .

The Myanmar fossil is very similar to Nycticorax caledonicus pelewonsis and Gorsachius melanolophus . The distal mediolateral width of N. caledonicus pelewonsis (MVZ 95061) is 10.5/ 10.7 mm (left/right side) with a trochlea III width of 3.0 mm (left and right sides). That is close to the 10.1 and 3.1 mm equivalent measurements in the Sulegon fossil. There are no recognizable morphological differences with Gorsachius melanolophus (USNM 488330), except that the fossil is very slightly larger than the extant specimen (distal mediolateral width is 9.7/ 9.6 mm and trochlea III mediolateral width 5.2/ 5.3 mm). With those measurements, the proportions of the trochlea III as compared to the width of the distal end appear more consistent between the Myanmar specimen and Nycticorax rather than that in Gorsachius . Nycticorax nycticorax hoatctli has a plantar end of trochlea III wider than the fossil, and the dorsal proximal end is pointed (unlike the fossil). The early Pliocene purported night heron, Nyctanassa kobdoena from Mongolia, has deeper and wider intercondylar incisions than the states in the Sulegon specimen, and its generic assignment is in question ( Zelenkov, 2013). The Myanmar fossil is much bigger than Ixobrychus spp. , and the second trochlea in those species is relatively smaller than the state in the fossil. Trigrisoma mexicanum (MVZ 85515) has a fossa around the distal foramen (dorsal side) that is much deeper than that in the fossil (no depression visible). Tigrisoma also has a ridge on the plantar base of trochlea II that is more elongate, and overall the species is larger than the fossil. The ridge proximal to the plantar side of trochlea II is more robust in Egretta i. intermedia (MVZ 124052) and E. novaehollandiae (MVZ 143281). In Botaurus lentiginosus (MVZ 151604), the proximal plantar end of trochlea III is very wide (vs. the more narrow condition in the fossil), and trochlea IV is projected more laterally than the fossil (also in Bulbuculus i. ibis MVZ 164538). The proximal end of trochlea III in Bulbulcus i. ibis is more pointed than the condition in the fossil. Butorides anthonyi (MVZ 74758) is similar in morphology to the fossil but is smaller (distal mediolateral width 7.0 mm).

Figures

FIGURE 2. Stereophotograph pairs of the night herson distal tarsometatarsus (NMMP-KU-IR 0343) from the SLG1 locality, Myanmar. 1, distal view; 2, medial view; 3, lateral view; 4, dorsal view; 5, plantar view. Abbreviations: df – distal foramen; r – ridge.