CICONIIDAE Sundevall, 1836

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 : 7-9

Family CICONIIDAE Sundevall, 1836

Figure 3

Specimen. NMMP-KU-IR 0355 is a crushed left distal tibiotarsus.

Description. The specimen is crushed mediolaterally, but has a preserved mediolateral width of 18.3 mm (and a maximum preserved craniocaudal depth of 27.4 mm). The proximal end of the ossified supratendinal bridge is obscured, and the extensor groove is not clearly visible ( Figure 3 ). The lateral condyle extends proximal to the medial condyle. The intercondylar tubercle (= tuberculum ligamenti tibiometatarsale intercondylare of Smith et al. [2013] and tubercle for the attachment of ligamentum meniscotibiale intertarsi of Zinoviev [2013]) is larger and roughly in the shape of a rounded square in outline. It is clearly visible in distal view within the intercondylar incision, and the tubercle is at the same proximodistal level as the distal edge of the ossified supratendinal bridge. The tubercle projects far cranially and can be seen in lateral view extending cranial to the proximal end of the lateral condyle. The medial surface of the medial condyle is deeply concave, and there is a raised area (not quite an epicondyle) adjacent to the proximal end of the medial condyle. There is no evidence of a groove for m. fibularis on the craniallateral corner of the shaft, but there is a short proximally directed ridge proximal and slightly lateral to the intercondylar tubercle (presumably for attachment of the extensor retinaculum). The extensor groove opens distally into the intercondylar incision (fossa), and the distal edge of the intercondylar tubercle forms part of the proximal edge of that incision (fossa). The intercondylar fossa cuts into the lateral surface of the medial condyle and appears to indent the medial side of the lateral condyle just distal to the intercondylar tubercle. The result is that the proximal end of the medial condyle appears to narrow proximally (in cranial view), while the lateral condyle extends to join with the intercondylar tubercle. The lateral condyle is deeply concave on its lateral face, and there is a low craniocaudally elongate ridge that maybe equivalent to a lateral epicondyle in that concave area. Near the craniocaudal midpoint of the distal edge of the lateral condyle is a shallow notch that extends onto the distal face of the bone. The area where a corresponding notch would be (if present) on the medial condyle is broken with a chip of bone missing. The distal face of the specimen is fractured, and bone near the center is missing. The intercondylar incision appears to have extended to the distal face, but the preserved articulation for the tibial cartilage on the caudal face is relatively flat except for a slightly concave area preserved on the medial edge. Though broken, it appears that the proximal end of the articulation for the tibial cartilage extended medially and laterally well beyond the shaft of the tibiotarsus, and that articulation’s proximal end has a rounded outline. The caudal part of the preserved shaft is crushed, and it appears that some bone is also missing from the area since it is much narrower than the cranial portion (even given the crushing of the entire specimen).

Comparisons. The stork specimen shares the presence of a large centrally positioned intercondylar tubercle, large size, semi-rounded intercondylar fossa, and other features with extant (and extinct) storks. However in general, the Sulegon stork has more square profile to the intercondylar tubercle (than the more mediolaterally elongate state in many storks) and has a less pronounced medial epicondyle (than the state in many storks, see below for details). It might represent an undescribed species, but as the specimen is preserved, that cannot be definitively determined.

The stork Anastomus l. lamellaigerus (MVZ 133407) is much smaller than the fossil. In addition, the intercondylar tubercle is larger and more rounded than the state in the fossil. Ciconia abdimii (MVZ 133692) is much smaller than the fossil. Its medial epicondyle is much larger than the fossil, and its intercondylar tubercle is not as prominent and rounded as the state in the fossil. The lateral epicondylar fossa is deeper in the fossil than in Ciconia . Ephippiorhynchus senegalensis (MVZ 140361) is approximately the same size as the fossil. In cranial view, the medial condyle differs from the fossil since its proximal end is more triangular in shape/outline, and there is a pit on the proximal end of the lateral condyle that appears to be absent in the Sulegon specimen. In Jabiru mycteria (MVZ 133932), the intercondylar tubercle is more elongate, and the medial epicondyle is much larger than in the Myanmar fossil. In Leptoptilos crumeniferus (MVZ 134058), the intercondylar tubercle is more elongate, and the medial epicondyle is larger than the states in the fossil. Mycteria americana (MVZ 24920) is smaller than the Sulegon specimen, its intercondylar tubercle also is mediolaterally elongate, and the medial epicondyle is larger too.

Louchart et al. (2005) provides a thorough review of the fossil record of large-bodied storks, but that review does not illuminate any individual characters that would help to narrow the identification of this fossil specimen to species level. Furthermore, the recent phylogenetic analysis of storks by de Pietri and Mayr (2014) includes a single tibiotarsus character (the presence or absence of the intercondylar tubercle), and as a result does not aid in the taxonomic or phylogenetic placement of the Myanmar specimen within Ciconiidae .

The Myanmar specimen lacks the medial projection (cranial view) just proximal to the medial condyle that is illustrated for specimens referred to Leptoptilos falconeri from the Pliocene ( Louchart et al., 2005, figure 1). That morphology results in a concave medial margin of distal end of the bone in L. falconeri , and there is a flat (or flatter) margin in the Myanmar specimen and L. dubius . Specimens of L. robustus do not preserve that region of the bone ( Meijer and Due, 2010). In addition, with a larger sample size of the extant species, that character might be found to be variable, as so many other characters within stork species are variable (e.g., Louchart et al., 2005). It appears that the intercondylar tubercle is a bit more laterally positioned in the Myanmar specimen as compared to the more central position in fossil Leptoptilos ( Louchart et al., 2005; Meijer and Due, 2010), but that could be the result of the mediolateral crushing that the Sulegon specimen experienced.

The (uncrushed) distal craniocaudal depth of the distal tibiotarsus is smaller in the Myanmar specimen (27.4 mm) than the specimens referred to L. falconeri (larger than 30 mm; Louchart et al., 2005, figure 5). Overall, the Myanmar specimen is similar in size (distal depth 27.4 mm and distal crushed mediolateral minimum width of 18.3 mm) with the extant species in Ephippiorhnychus, Leptoptilos , and Jabiru ( Louchart et al., 2005, table 2). Given that the new fossil is mediolaterally crushed, the ratio of the distal depth to distal width cannot be calculated accurately (as preserved it is 1.49), and thus the ratio cannot be compared directly to that of Ephippiorhynchus and Leptoptilos (e.g., Louchart et al., 2005). Given the distal depth of the bone and a likely increase in its distal width (by 2 mm or so) when uncrushed, the fossil probably would fall into the Ephippiorhnychus and Leptoptilos dubius portion of the graph provided by Louchart et al. (2005, figure 5), but not into the region occupied by the ‘giant’ extinct storks. The Myanmar fossil is relatively close in size with the holotype of Cryptociconia indica ( Harrison, 1974) that was reassigned as Leptoptilos dubius / falconeri by Louchart et al. (2005), and correspondingly appears to fall outside of the size range for the very large or ‘giant’ storks of the Neogene and Pleistocene (e.g., L. falconeri , L. titan , and L. robustus ). There are no hindlimb limb bones known from L. lüi, and thus no comparisons can be made to the Sulegon specimen. However, given the very large size of the distal humerus in L. lüi ( Zhang et al., 2012), it is very unlikely that this smaller leg bone belongs to the same species. In summary, there are no features preserved that distinctly suggest a phylogenetic position of this specimen with a particular genus group (i.e., Ephippiorhynchus or Leptoptilos ), but its large size is suggestive that it belongs among the taxa placed in Leptoptilini . In addition given its smaller overall size (compared to the ‘giant’ storks), it is possible that the Myanmar specimen is referable or related to extant species that occur in Asia today.

Figures

FIGURE 3. Stereophotograph pairs of the stork distal tibiotatarsus (NMMP-KU-IR 0355) from the SLG1 locality, Myanmar. 1, caudal view; 2, cranial view; 3, distal view; 4, medial view; 5, lateral view. Abbreviations: it – intercondylar tubercle; le – lateral epicondyle; n – notch; r – ridge; sb – supratendinal bridge.