Celericolius acriala

Ksepka, Daniel T. & Clarke, Julia A., 2010, New fossil mousebird (Aves: Coliiformes) with feather preservation provides insight into the ecological diversity of an Eocene North American avifauna, Zoological Journal of the Linnean Society 160 (4), pp. 685-706 : 689-694

CELERICOLIUS ACRIALA GEN. ET SP. NOV

Holotype specimen: FMNH PA 730 ( Figs 3–5 , Table 1), a nearly complete but poorly preserved skeleton with tracheal rings and carbonized feather traces, showing slight disarticulation of most skeletal elements, and missing some pedal phalanges. Both of the coracoids, the right humerus, and the left femur are not visible, but some or all of these elements are possibly hidden beneath other elements or matrix.

Etymology: Celericolius combines the Latin Celer (swift) plus colius referencing the agile flight style inferred for this mousebird species. The species epithet, acriala , combines the Latin root ac (sharp) and ala (wing) referencing the elongate, tapered wing.

Type locality and horizon: Lewis Ranch Site 1 (locality A of Grande & Buchheim, 1994), near Kemmerer, Wyoming, USA. Beds at this locality belong to the FBM of the Green River Formation, and are dated to the early Eocene (51.66 ± 0.09 Ma; Smith et al., 2008).

Diagnosis: Celericolius acriala exhibits two autapomorphies: (1) processus transversus vertebrae of the free caudal vertebrae distinctly anteroposteriorly broadened, and (2) ulna proportionately more elongate than that of all other Coliiformes , except Oligocolius brevitarsus (pronounced elongation of the ulna evolved independently in these two taxa based on their inferred phylogeny). The ulna: humerus ratio is greater than 1.10 in C. acriala whereas in other Coliiformes including Sandcoleidae this ratio is typically less than 1.00. Within Colii, only Palaeospiza bella , Oligocolius brevitarsus , and extant Urocolius exhibit an ulna that is longer than the humerus.

Celericolius acriala differs from all other stem Coliiformes , but resembles extant Coliidae , in that pedal phalanx II- 1 is subequal to phalanx II- 2 in length. In other stem Coliiformes preserving the foot, phalanx II- 1 is strongly abbreviated, measuring onehalf of the length ( Palaeospiza and Masillacolius ) or one-third of the length ( Sandcoleidae and Selmes ) of phalanx II- 2. Because pedal phalanges remain unknown for several fossil taxa, it is currently equally parsimonious to interpret the elongate phalanx II- 1 as a local autapomorphy of C. acriala acquired convergently in Coliidae , or as a synapomorphy of a clade uniting C. acriala and Coliidae , with a reversal to the abbreviated state in Palaeospiza bella .

Celericolius acriala can be diagnosed by the features listed above and can also be further differentiated from the comparatively closely related Palaeospiza bella and Oligocolius brevitarsus by a

Pectoral girdle and limb

Scapula, length (left) 21.3 Humerus, length (left) ~24.0 Humerus, deltoid crest length (left) ~7.3

Ulna, length (right) 27.6 Radius, length (left/right) 24.4/24.8 Carpometacarpus (left/right) 16.6/16.1 (length to distal end of metacarpal II)

Phalanx I-1, length (left/right) 5.6/5.4 Phalanx II-1, length (left/right) 7.4/7.6 Phalanx II-2, length (left) 6.4 Phalanx III-1, length (left) 4.6

Pelvic girdle and limb

Femur, length (right) 21.2 Tibiotarsus, length (left) 34.8 Tarsometatarsus, length (left/right) 21.9/~22.0 Phalanx I-1, length (left) 9.9 Phalanx II-2, length (left) 6.0 Phalanx II-3 (ungual), length (left) 4.9 Phalanx III-1, length (right) 2.5 Phalanx III-2, length (right) 2.9 Phalanx III-3, length (left/right) 6.1/6.8 Phalanx III-4 (ungual), length (right) 6.0 Phalanx IV-2, length (left) 1.8 Phalanx IV-3, length (left/right) 1.3/1.5 Phalanx IV-4 length (left/right) 5.7/5.7 Phalanx IV-5 (ungual), length (right) 6.0

small (versus strongly projected) processus intermetacarpalis, and by the subequal distal projection of metacarpals II and III (versus metacarpal III being significantly longer). Additionally, the tarsometatarsus of C. acriala is more elongate than in Oligocolius brevitarsus (tarsometatarsus: humerus ratio 0.93 versus 0.65). Celericolius acriala can be further differentiated from Masillacolius brevidactylus and Primocolius sigei by the more distally extensive crista deltopectoralis of the humerus in the new species, and from Selmes absurdipes by the straight (versus curved) humerus shaft.

DESCRIPTION AND COMPARISONS

In overall size, the holotype individual is approximately equal to the extant Urocolius indicus (Redfaced Mousebird) , a bird with a body mass of ~ 50 g ( Downs et al., 2000). The holotype specimen is largely articulated and appears to be nearly complete, except for damage to the skull and the loss of two pedal unguals. The coracoids, right humerus, and left femur are either not preserved or are hidden beneath the remains of the sternum and pelvis ( Fig. 4 ). The specimen has suffered postmortem degradation, and much of the skeleton is flattened and poorly preserved, with the exposed bone surface often lost, obscuring features such as muscle insertion scars. Some areas of the skull exhibit minor retouching. At the tip of the premaxilla and dorsal portions of the mandible, incomplete areas have been filled in by applying paint to the negative mold left in the slab by missing bone.

As in extant Coliidae , the skull roof is domed and rounded, and a large fonticulus orbitocranialis is present. Unfortunately, fine details of the skull are nearly entirely lacking because of poor preservation. As noted above, the beak tip has been retouched. Because of this modification the precise shape and total length of the beak must be considered uncertain. The mandible is slender and straight near its midpoint, but the caudal and rostral portions are not preserved.

A few tracheal rings are scattered on the slab. Tracheal rings are also preserved in at least one specimen of the sandcoleid Eoglaucidium pallas from Messel ( Mayr & Peters, 1998: fig. 3), but are not preserved in the articulated holotype skeleton of the sandcoleid Anneavis anneae , also from FBM deposits ( Houde & Olson, 1992). However, because the Anneavis anneae holotype was collected from a different quarry (Warfield Springs, locality K), representing a more near-shore environment than Lewis Ranch Site 1 ( Grande & Buchheim, 1994), it is possible that the lack of tracheal ring preservation in that specimen may be related to preservation in a distinct depositional environment.

The cervical and dorsal vertebrae are poorly preserved. In contrast, the caudal vertebrae and pygostyle are well-preserved and are exposed in ventral view. The processes transversus vertebrae of the caudal vertebrae are much wider anteroposteriorly than in extant Coliidae . Sandcoleus copiosus and a privately owned specimen referred to Chascacocolius ( Mayr, 2005a) also possess narrow processes transversus vertebrae, comparable with those in Coliidae . As in extant mousebirds, the discus pygostyli is greatly expanded, forming a distinct broad, rounded plate ( Fig. 5 ).

Much of the sternum is obscured by overlying elements, but several phylogenetically important features of the caudal margin can be observed. The incisura lateralis is shallow compared with extant Coliidae . In this respect, C. acriala shows a condition intermediate between that in Sandcoleidae , where the incisure lateralis is reduced such that the trabecula intermedia arises from the trabecula lateralis, and in Coliidae , where the incisura lateralis is very deep and the trabecula lateralis and trabecula intermedia are clearly separated. Distally, the trabecula lateralis shows slight mediolateral expansion, but does not approach the inverted T-shape developed in extant Urocolius . The scapula is shorter than the humerus, as in most stem mousebirds. In Coliidae and Oligocolius brevitarsus , the scapula exceeds the humerus in length. The corpus scapulae is fairly straight and maintains a nearly constant width, with no distal expansion.

The crista deltopectoralis of the humerus is strongly projected, as in other mousebirds. This crest extends for approximately one-third of the proximodistal length of the humerus, whereas it extends approximately one-fourth of the length of the humerus in Sandcoleidae and Coliidae . The humeral shaft is straight, more closely resembling the condition in extant Coliidae than the curved shaft of Sandcoleidae . Preservation at the distal end is poor, so it is not possible to determine whether C. acriala possessed the crescent-shaped depression proximal to the condylus dorsalis seen in some Colii. The ulna: humerus length ratio ( Fig. 6 ) is higher than in any other representative of Coliiformes , with the possible exception of Oligocolius brevitarsus ( Mayr, 2000c) . Accounting for the slight damage to the proximal humerus, is it not possible to say with certainty whether C. acriala exceeded the ratio of 1.11 in Oligocolius brevitarsus . Few details of the radius and ulna can be ascertained, although it is clear that both elements are robust, as in other Coliiformes . The olecranon appears blunt and weakly differentiated, as visible in the right ulna. The left carpometacarpus is exposed in ventral view, and the right carpometacarpus is exposed in dorsal view. Regions of the ventral bone surface of the left element are missing, exposing the internal surface of the shaft for much of its length, but fortunately the bone is intact at the proximal end. A small processus intermetacarpalis, which does not contact metacarpal III, is preserved ( Fig. 5 ). An impression of the processus extensorius of the left carpometacarpus indicates that this process was strongly projected. The spatum intermetacarpale is wide, although metacarpal III is less strongly bowed than in extant Coliidae . Although the distal end of metacarpal III has been lost from the slab on the right carpometacarpus, its impression indicates metacarpals II and III extended to the same level. The alular phalanx is fairly long. Distally, phalanx II-1 expands to a width equal to that of the carpometacarpus, giving the phalanx a hatchet-like shape. Phalanx II-2, although incomplete on the right side, was clearly elongate and straight, as indicated by the preserved portion. Phalanx III-1 bears a prominent, proximally positioned processus flexorius, and is slender and tapers distally, closely resembling the morphology in extant Urocolius .

Parts of the pelvis are exposed in ventral view, including the posterior portions of the right ischium and pubis. At the caudal portion of the synsacrum, distinct processus costales can be observed, a primitive feature for Coliiformes . As in extant mousebirds, the pelvis is mediolaterally wide. The foramen ilioischiadicum is elongate and narrow. The pubis is thin and rod-like, and does not approach or contact the ischium distally.

Although both legs are detached from the pelvis, the major hindlimb elements remain largely articulated. The hindlimb is relatively slender and elongate, but few details of the femur and tibiotarsus are discernable. The tarsometatarsus approaches the length of the humerus and is similar in proportions to the slender tarsometatarsus of Coliidae , as opposed to the stouter tarsometatarsus of Sandcoleidae . The cristae hypotarsi are proximodistally short. Unfortunately, no details of the foramina vascularia proximalia or canales hypotarsi can be discerned.

On the right foot, digit III is preserved overlying the other digits. Portions of phalanx IV-2 and all of phalanges IV-3, IV-4, and IV-5 are visible, whereas digits I and II are largely obscured. On the left foot, digits I and II are well exposed. Digit III is visible, but lacks the ungual, and digit IV is complete but largely obscured by overlying elements. Thus, digital proportions can be reconstructed by combining observations from the two feet. The ungual of digit I is oriented in the same direction as the remaining unguals on the left side, corresponding to the apparently pamprodactyl configuration of the digits. However, because phalanx I-1 is not in articulation with metatarsal I, this orientation is possibly an artifact of preservation. Phalanges II-1 and II-2 are elongate and subequal in size. In contrast, the proximal three phalanges of digits III and IV are all strongly abbreviated compared with the penultimate phalanx. All pedal unguals are long and strongly curved. In the proportions of the pedal phalanges, C. acriala shares the derived pronounced abbreviation of the proximal three phalanges of digits III and IV with all other Coliiformes . The unabbreviated phalanx II- 1 in C. acriala agrees with the condition in crown Coliidae . In contrast to C. acriala , other stem Coliiformes that preserve an intact foot show some degree of shortening in phalanx II-1, with the size of this element ranging from approximately one-third to onehalf of the length of phalanx II-2. As mentioned above, whether the phalangeal proportions in C. acriala represent convergence with Coliidae , or a synapomorphy of a larger clade including C. acriala and Coliidae , that is reversed in Palaeospiza bella , remains uncertain because of homoplasy and missing data.

Carbonized material adjacent to the wing bones and pygostyle is interpreted as integumentary traces. Contrast between these areas and the matrix is greater under ultraviolet light than under natural light; however, the carbonized material does not noticeably fluoresce. No structural details of the feathers (e.g. rachi and barbs) can be detected. Preservation is best along the leading edge of what is inferred as the tenth primary, and lessens in quality towards the trailing edge, so that it is unclear whether all primaries are represented. There is no indication of secondary feather preservation adjacent to the ulna. Traces of the leading primary extend 124.6 mm from the tip of manual phalanx II-2. This corresponds to approximately 60% of overall wing length (length of wing skeleton plus projecting feathers). Inferred wing shape is elongate and tapering, contrasting markedly with the short, rounded wing of extant mousebirds ( Fig. 7 ). A short, rounded wing shape has also been reported in several stem Coliiformes with preserved feathering traces, including Anneavis anneae ( Houde & Olson, 1992) and an unnamed sandcoleid from the Eocene of Messel (WDC-C-MG 148+149, Mayr, 2000c, 2009: fig. 16.3). The holotype of Palaeospiza bella preserves traces of wing feathering, but because the wings are tightly folded in this specimen it is difficult to reconstruct the wing shape.

Traces of the tail feathering extend for 187.1 mm from the caudal tip of the pygostyle. The elongate tail is consistent with that expected given the known phylogenetic distribution of tail length in Coliiformes . An elongate tail is present in all extant Coliidae , and has also been documented in the sandcoleids Anneavis anneae and Eoglaucidium pallas , as well as in the unnamed Messel sandcoleid (WDC-C-MG 148+149) ( Houde & Olson, 1992; Mayr & Peters, 1998; Mayr, 2000c).

Figures

Figure 3. Celericolius acriala holotype specimen (FMNH PA 730). Arrows indicate the extent of the carbonized feather traces.

Figure 4. Photograph and line drawing of Celericolius acriala (FMNH PA 730). Abbreviations: dI, pedal digit I; dII, pedal digit II; dIII, pedal digit III; dIV, pedal digit IV; f, furcula; lcmc, left carpometacarpus; lhu, left humerus; lra, left radius; ltbt, left tibiotarsus; ltmt, left tarsometatarsus; lul, left ulna; mpI-1, manual phalanx I-1; mpII-1, manual phalanx II-1; mpII-2, manual phalanx II-2; mpIII-1, manual phalanx III-1; py, pygostyle; rad, radiale; rcmc, right carpometacarpus; rest, artificially restored tip of the beak; rf, right femur; rra, right radius; rtbt, right tibiotarsus; rtmt, right tarsometatarsus; ruln, right ulna; sc, scapula; st, sternum; ul, ulnare; uln, ulna.

Figure 5. Details of the skeletal anatomy of Celericolius acriala. A, distal elements of left wing. B, pelvis, pygostyle, and left hindlimb. C, right foot. Abbreviations: cmc, carpometacarpus, dI, pedal digit I; dII, pedal digit II; dIII, pedal digit III; dIV, pedal digit IV; ext, impression of processus extensorius; hyp, cristae hypotarsi; isch, ischium; mpI-1, manual phalanx I-1; mpII-1, manual phalanx II-1; mpIII-1, manual phalanx III-1; pc, processus costales; pi, processus intermetacarpalis; pt, processus transversus vertebrae; pu, pubis; py, pygostyle; rad, radiale; tmt, tarsometatarsus; uln, ulnare. Scale bars: 5 mm.

Figure 6. Reconstructions of the wing bones of Green River Formation fossil mousebirds (Celericolius acriala and Anneavis anneae) and extant mousebirds (Urocolius indicus and Colius striatus), rescaled to equal lengths to show proportional differences.

Figure 7. A, spread wing of Urocolius indicus (Coliidae: NCSM 19741) in ventral view. B, spread wing of Hirundo rustica (Hirundinidae: PSU 11196a) in ventral view. C, wing of Celericolius acriala (Coliiformes: FMNH PA 730), with arrows indicating the leading edge of the wing.