Systematic Palaeontology

Family Gastornithidae Fürbringer 1888

Gastornis sp.

Material. Canadian Museum of Nature (Ottawa, Canada) CMN 32412. The specimen is a nearly complete pedal phalanx from locality ELS76-85, Margaret Formation, Eureka Sound Group, central Ellesmere Island, Nunavut, Canada (early Eocene; late Wasatchian NALMA).

Description. CMN 32412 lacks the stouter and more trapezoidal outline of more distal phalanges of Gastornis24,25 and has the broadened articular end present in the most proximal phalanx. The proximal dorsal to dorsolateral portion of the specimen is missing and there is no evidence of a dorsoplantarly-oriented ridge on the articular surface that would have fit into the trochlear furrow. The proximal end is wider (28 mm, broken) than the distal end (24.8 mm). The width of the proximal end would have been greater than 44% of the length of the bone (62.2 mm; since it is damaged). The lateral edge of the bone is relatively straight (dorsal view); while the medial edge is slightly more concave. The plantar lip of the lateral collateral ligament fossa (the lateral plantar rim of the trochlea) is visible in dorsal view (extending lateral to the shaft) and continues proximally about one-third the length of the bone. In contrast, the equivalent medial part of the trochlea is not visible in dorsal view. The trochlea has a shallow trochlear furrow and the lateral trochlear ridge extends slightly proximal to the medial ridge on the plantar surface. The lateral collateral ligament pit is large and deepest distoplantarly and the medial pit is nearly circular in outline. In addition in lateral view, the plantar surface of the bone is concave, with the trochlea and condylar articulations being dorsoplantarly wider than the shaft.

Order Anseriformes Wagler 1831

Family Presbyornithidae Wetmore 1926

Presbyornis cf. P. pervetus Wetmore 1926

Material. CMNFV 53369. The specimen is the distal approximately one-third of a left humerus (Fig. 2), from locality ELS76-43 (=77H7-9-3), Margaret Formation, Eureka Sound Group, central Ellesmere Island, Nunavut, Canada (early Eocene; late Wasatchian NALMA).

Description. The bone is dark brown in color and it exhibits some sand-induced surface pitting. That pitting is more pronounced on the distal end of the bone. Despite that overprint, the surface of the bone appears to have been smooth, indicating that the specimen likely is from an osteologically adult individual26. The proximal end of the shaft is subcircular in cross-section, but slightly wider medio-laterally.

From the proximal end towards the distal end of the cranial surface, the shaft flattens from a distinctly convex surface proximally. There is a distinct brachial (muscle) fossa that is deepest ventrodistally and shallows dorsally. The fossa’s distal end has a distinct border. Overall, the fossa is ovoid in outline with its long axis oriented dorsoproximal to ventrodistal. The distal edge of the brachial fossa is at the same proximodistal level as the proximal end of the ventral collateral ligament attachment at the apex of the ventral supracondylar tubercle. Just ventral to the proximal end of the apex of the supracondylar tubercle is a poorly preserved muscle scar pit (attachment of pronator brevis of Howard27 that is equivalent to the origination of the m. flexor carpi ulnaris of Livezey28) that is separate from the ventral collateral ligament attachment. The ventral collateral ligament scar on the supracondylar tubercle is proximodistally elongate and its dorsal margin is relatively straight. The ventral condyle extends distal to the dorsal condyle. The area distal to the brachial fossa and proximal to the condyles is flat to slightly concave. The dorsal condyle extends proximally to the same level as the proximal end of the ventral collateral ligament scar. The long axis of the outline of the dorsal condyle is not parallel to the long axis of the humerus because the distal end is located dorsal to the proximal end.

The shaft is convex in caudal view. The olecranon fossa is relatively small and restricted to the area immediately caudal to the ventral condyle. The fossa is deepest ventrally. The distal end of the bone is worn and there is a slight hint of a shallow scapulotriceps groove at the most distal part of the caudal side. The dorsal epicondyle has a pair of muscular pits (present in other anseriforms), but they are poorly preserved because the division between the two pits cannot be discerned. The dorsal epicondyle extends further proximal than the supracondylar tubercle/collateral ligament scar. The ventral side of the specimen is poorly preserved and no muscle scars are visible.

Aves indet.

Material. CMNFV 53368, a phalanx (Fig. 2) from locality ELS76-42 (=76H7-10-10), Margaret Formation, Eureka Sound Group, central Ellesmere Island, Nunavut (Early Eocene; Wasatchian NALMA).

Description. The specimen appears to be the proximal phalanx likely from digit III of the left side. The bone is off-white to gray in color and is deeply pitted all over its surface. The pitting is caused by sand grains that were embedded in the bone surface and does not indicate an early ontogenetic age (Fig. 2). The proximal end is mediolaterally crushed/flattened and the left side (dorsal view) of the distal end is broken off. The proximal articulation maintains a central ridge for articulation with the tarsometatarsus. In lateral view, the articulation is C-shaped. The distal end (lateral sides) has a deep collateral ligamental pit. The furrow for the articulation with the next more distal phalanx extends dorsally and plantarly to about the same proximal level and that level is proximal to the ligamental pits. The plantar side appears to have been flatter than the dorsal (convex) side, but the crushing of the specimen has distorted the shape.

Comparisons.

The length of the Ellesmere Gastornis specimen (62.2 mm) is less than the largest specimens of Gastornis from North America (68.6–80.1 mm) and that from Louvois, France (75.9 mm)25. The morphology of the Ellesmere specimen is similar to that reported and illustrated for the proximal phalanx of digit IV of Gastornis from Louvois25, Wyoming24, Monthelon29 and New Jersey30, but the plantar ridges mentioned in some Gastornis specimens appear to be absent in this specimen. The proximal widening of the articular region (in dorsal view) is similar across specimens. The Ellesmere specimen lacks the medially expanded plantar rim of the trochlea (plantar lip of the collateral ligament pit) that is visible in dorsal view in the Paleocene Louvois specimen25 and that state appears absent in other Gastornis specimens24,29. Furthermore, the plantar lip of the collateral ligament pit in CMN 32412 extends relatively more proximal than the state in the Louvois specimen25, but is similar to Gastornis from Wyoming24. The length of the phalanx and the ratio of the proximal width to the length of the bone (above) are within the ranges reported for Gastornis25.

The variation in the morphology of the humerus seems to be within that seen among the abundant Wyoming specimens of Presbyornis pervetus (Fig. 2). There are no apomorphies (versus other Paleogene anseriform taxa) in the distal humerus for Presbyornis, but there also are no character differences between the Ellesmere Island fossil and specimens of Presbyornis pervetus from Wyoming. The Ellesmere specimen was directly compared to casts of Presbyornis pervetus from Wyoming. The postcranial skeleton of Presbyornis is notoriously plesiomorphic31,32. The same shape, depth and overall morphology of the brachial fossa can be seen among Presbyornis specimens from more southerly locations (Fig. 2). However, the Ellesmere specimen is on the larger end of the size range present in Wyoming (Table 1) and the Ellesmere Island specimen also is within the known stratigraphic range of Presbyornis in Wyoming5,33 (i.e., Wasatchian NALMA). There are no characters that appear to separate the Ellesmere humerus from what is currently recognized as Presbyornis pervetus, but also no clear derived characters to unite them into a clade.

Table 1 Measurements (in mm) of the distal end of the humerus of Presbyornis specimens. Full size table

Various authors have suggested that the morphological variation exhibited among the Wyoming Presbyornis bone assemblages might represent multiple closely related species33,34, but at present no one has discerned a way to separate any potential species within the Presbyornis pervetus collections from North America. In contrast, an early Eocene species of Presbyornis, P. mongoliensis, has been described from Mongolia35 and it likely does represent a species different from the penecontemporaneous North American Presbyornis pervetus. That Mongolian material also is smaller than the Ellesmere specimen35 (Table 1). Thus, it is possible that the Ellesmere specimen represents a taxon separate from Presbyornis pervetus, but there is no way at present to determine that status. Furthermore, the slightly larger size of the Ellesmere specimen compared to individuals at lower latitudes could be the result of Bergman’s Rule and that cline has been proposed for the Arctic and mid-latitude mammal Heptodon from the Eocene21.

The indeterminate avian phalanx is consistent with Presbyornis (clearly lacking any autapomorphies), but it also easily could belong to another taxon of bird (given its somewhat more robust morphology), though its size and morphology is certainly inconsistent with that of the much larger Gastornis, which has stouter and more robust pedal phalanges (Fig. 2). In contrast to the Presbyornis specimen, the Gastornis phalanx is not larger than individuals known from further south in North America, but is within the known size variation for the group. The variation in size among Gastornis individuals has been attributed to both species differences and the possibility of sexual dimorphism25.