Naturwissenschaften (2007) 94:657–665
DOI 10.1007/s00114-007-0239-x

ORIGINAL PAPER

Earliest zygodactyl bird feet: evidence from Early
Cretaceous roadrunner-like tracks
Martin G. Lockley & Rihui Li & Jerald D. Harris &Masaki Matsukawa & Mingwei Liu

Received: 10 January 2007 / Revised: 26 February 2007 / Accepted: 28 February 2007 / Published online: 27 March 2007
# Springer-Verlag 2007

Abstract Fossil footprints are important in understanding
Cretaceous avian diversity because they constitute evidenceof paleodiversity and paleoecology that is not always
apparent from skeletal remains. Early Cretaceous bird tracks
have demonstrated the existence of wading birds in East
Asia, but some pedal morphotypes, such as zygodactyly,
common in modern and earlier Cenozoic birds (Neornithes)were unknown in the Cretaceous. We, herein, discuss theimplications of a recently reported, Early Cretaceous (120–110 million years old) trackway of a large, zygodactyl birdfrom China that predates skeletal evidence of this footmorphology by at least 50 million years and includes the
only known fossil zygodactyl footprints. The tracks demonstrate the existence of a Cretaceous bird not currently
represented in the body fossil record that occupied aroadrunner (Geococcyx)-like niche, indicating a previously
unknown degree of Cretaceous avian morphological andbehavioral diversity that presaged later Cenozoic patterns.

Communicated by G. Mayr

Keywords Zygodactyl . Cretaceous . Roadrunner . China .
Evolution . Diversity

Electronic supplementary material The online version of this article(doi:10.1007/s00114-007-0239-x) contains supplementary material,
which is available to authorized users.
M. G. Lockley (*)
Dinosaur Tracks Museum, University of Colorado at Denver,
Denver, CO 80217, USAe-mail: Martin.Lockley@cudenver.edu
R. Li
Qingdao Institute of Marine Geology, China Geological Survey,
Qingdao 266071, People’s Republic of Chinae-mail: rihuiliqd@yahoo.com.cn
J. D. Harris
Science Department, Dixie State College,
225 South 700 East,
St. George, UT 84770, USAe-mail: jharris@dixie.edu
M. Matsukawa
Department of Environmental Sciences,
Tokyo Gakugei University,
Koganei, Tokyo 184-8501, Japane-mail: matsukaw@u-gakugei.ac.jp
M. LiuFourth Geological and Mineral Resources Survey of Shandong,
Wei-fang 261021, People’s Republic of China

Introduction
Neornithean (modern) birds occupy and have occupied abroad diversity of ecological niches, many of which arefacilitated by unusual pedal adaptations, such as zygodactyly and heterodactyly, that are presently unknown in any
Mesozoic bird. Until quite recently, no extant bird clade
was thought to have arisen until after the end-Cretaceous(K/Pg) extinction event (Feduccia 1995), despite predictions
that Cretaceous neornitheans (terminology sensu Harris etal. 2006) must have been present based on phylogenetic
and evolutionary projections (Dyke and van Tuinen 2004;
Sibley and Ahlquist 1990). Definitive evidence of Mesozoic neornitheans has only recently emerged (Clarke et al.
2005; Kurochkin et al. 2002), but even these are basal(Galloanseres); no undisputed Cretaceous members of the
Neoaves (all neornitheans except Galloanseres and Ratitae)
are known. All Cretaceous birds for which the pedal
skeleton is known are either anisodactyl (digit I reversedwith respect to II–IV), the plesiomorphic state for all avians
more derived than Archaeopteryx (Mayr et al. 2005, 2007),

DO00239; No of Pages

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or pamprodactyl (pedal digit I un-reversed) and, in somecases, possibly totipalmate (all digits conjoined by webbing; Marsh 1880; Mayr et al. 2005). The sole reported
exception is the ostensibly heterodactyl enantiornithean
Dalingheornis liweii (Zhang et al. 2006), for which evidenceof heterodactyly is ambiguous at best: First, the distal endof the second metatarsal in this taxon does not appear to bestrongly rotated medially as it is in heterodactyl neornitheans, at least not significantly more so than in otherenantiornitheans (e.g., Chiappe 1993, 2002b; Zhou andZhang 2003a). Second, the ostensibly reversed second digit
is not in articulation with the tarsometatarsus, nor are its
phalanges articulated; its ungual appears to have been rotatedpostmortem but pre-burial, during diagenetic compaction of
the specimen, or both, a phenomenon occasionally seen in
the pedal digits of specimens of some other fossil bird as well[e.g., Sinornis (Sereno et al. 2002, Fig. 8.6)].
Early Cretaceous bird taxa far outnumber coeval avian
ichnotaxa particularly in central and eastern Asia, which
has been a locus of discovery and study of birds from thisepoch (You et al. 2006; Zhou and Zhang 2004; Chiappe
2007). Although the high frequency of Asian fossil bird
discoveries is partly attributable to exceptional preservation, the high incidence of bird track discoveries (Kim et al.
2006; Lockley et al. 1992, 2006a,b), in a variety of
sedimentary facies, cannot be dismissed as solely a functionof preservational bias. Moreover, the most numerous anddiverse Early Cretaceous birds are small (crow-sized or
smaller) basal pygostylians or members of the Enantiornithes, all taxa of which that are represented by remains that
include the foot skeleton are perching birds with anisodactyl feet (Chiappe et al. 1999; Zhou 2004), that are generally
inferred to have occupied many of the same niches as

extant passeriforms and coraciiforms. Among Early Cretaceous members of the Ornithuromorpha (Chiappe 2002a),the clade that includes the Neornithes and its immediate
outgroups, ostensibly basal taxa (Hou et al. 1996) weresimilarly perchers, but many more derived taxa display
evidence of specialization for aquatic/amphibious niches(You et al. 2006; Zhou and Zhang 2006). Some Late
Cretaceous ornithuromorphs occupied ground bird niches(Chiappe 2002b). The coeval bird track record is dominatedby morphologies reflecting feet similar to those of modern
shorebirds (Lockley et al. 1992), while footprints of grounddwelling taxa are comparatively rare. Thus, the combined
body and trace fossil record of Cretaceous birds iscumulative, not duplicative, stressing the often undervalued
utility of footprints.
Fossil footprints from the Lower Cretaceous (Aptian–
Albian, 120–110 million years old) Tianjialou Formation of
Shandong Province, China (Fig. 1), were described andnamed Shandongornipes muxiai (Li et al. 2005). Although
not recognized as such when first described, these tracks
are unique because they indicate the presence of an
unexpectedly large Early Cretaceous, zygodactyl, cursorial
bird that is unknown from body fossils and occupied aniche unknown for any bird of this time period. Shandongornipes is the only zygodactyl bird track type known andis twice as old as the oldest osteological evidence of avian
zygodactyly.

Fig. 1 Type locality of Shandongornipes muxiai in Shandong
Province, People’s Republic of China. a Map of China (white)showing the location (star) of the Tianjialou Formation outcrop that
produced the Shandongornipes holotype in Junan County, Shandong

Province, near the town of Lingquan. b Footprint-bearing Tianjialou
Formation outcrop in Junan. Shandongornipes tracks are circled(dashed line indicates damaged track); non-avian theropod (Th) andornithopod (Or) tracks are also present on several bedding planes

Materials and methods
The original description of Shandongornipes (Li et al.
2005), on the basis of natural impressions (negative or

Naturwissenschaften (2007) 94:657–665

concave epireliefs) and plaster replicas (casts or positive,
convex hyporeliefs), was made from a single trackway
preserved in situ at Houzuoshan Dinosaur Park. The entire
trackway configuration was recorded by tracing each track’s
outline on acetate film; tracks were enumerated sequentially inthe order that they were made (the direction of the trackmaker’s travel); track 2 was irreparably damaged, and track
3 was partly lost after the replica of it, mentioned below,
was made. Track number 4 (Fig. 2a,e,g) was replicated inplaster directly from the rock surface and designated as theholotype [LRH-DH01; abbreviations refer to the Li Rihui
collection (LRH) from Houzuoshan Dinosaur Park (DH)reposited in the Qingdao Institute of Marine Geology];
track number 3 (Fig. 2a,c) was replicated the same way
and designated as the only paratype (LRH-DH02; seebelow for more on specimen numbers). Field numbers(LRH dz66–70) were given to all five tracks, althoughnumber 2 was almost entirely missing, and numbers 1 and5 were not replicated.
In the present study, which involved additional fieldwork (by RL, ML, and MM), tracks 1, 3, 4, and 5 wereenumerated sequentially in the order in which they were
made, molded in latex, cast in plaster, and reposited in the
University of Colorado (CU) at Denver Dinosaur Tracks
Museum; the casts are designated as CU 214.116–119,
respectively. CU 214.117 and 214.118, respectively, represent copies of the “reverse” plaster casts of LRH dz68 anddz 67 made before the former was damaged. CU 214.122 isa replica of “un-reversed” track 4 (=CU 214.118), and CU
214.123 preserves a post-damage record of track 3. These
tracks are reposited with 13 other bird and non-avian
dinosaur track replicas (CU 214.103–115) from the same
locality. In the present study, all four Shandongornipes
tracks, including previously un-replicated tracks 1 and 5,
are considered part of the holotype trackway in theemended description in the main text. Track outlines weredrawn from a duplicate set of plaster replicas using low
angle light and a fine point graphite pencil. These tracings
were directly scanned and used as the basis of the revised

Fig. 2 Tracks of Shandongornipes muxiai. a Map of holotype
trackway (LRH-DH01). b–e Photographs and schematics of individual tracks in trackway (digit numbers labeled), including b photographof first (left foot, LRH-dz70), c outline drawing of third (left foot,LRH-dz68), d outline drawing of fifth (right foot, LRH-dz66), and ephotograph of fourth (left foot, LRH-dz67). f Segment of a trackway
made by extant roadrunner G. californianus near St. George, Utah;segment includes same left–right sequence preserved in LRH-DH01 (a)and shown at same scale. g Schematic of fourth Shandongornipes print(LRH-dz67). h Individual G. californianus right footprint [from Elbroch
and Marks (2001); used with permission] rotated with hallux inapproximately same orientation as Shandongornipes print in (g)659

illustrations (Fig. 2c,d,g). All measurements of footprint
lengths and widths and digit divarication angles were madeto the nearest 1.0 mm and 1.0° with transparent ruler andprotractor and checked on latex molds, replicas, andphotocopies.

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Description of specimens

Revised diagnosis

Ichnotaxonomy

Tetradactyl, paraxonic, zygodactyl footprints in narrow (6–7 cm wide) trackway with long (41–46 cm) steps. Elongate
hallux (digit I) oriented caudally, subparallel to trackway
axis, and in opposition to the hypex between craniallydirected digits II and III, which subtend a narrow mean
angle of about 32°. Digit IV rotated caudolaterally tosubtend a mildly acute mean angle of about 77° with thehallux. Phalangeal pad formula 2-3-3 for digits II, III, andIV, respectively (Fig. 2a–e).

Shandongornipodidae ichnofam. nov.
Type ichnogenus and ichnospecies
Shandongornipes muxiai Li, Lockley, et Liu 2005.
Description
Tetradactyl, paraxonic, zygodactyl tracks (i.e., with cranially directed digits II and III in opposition to caudally
directed digits I and IV). Trackway narrow (about 7.0 cm),
only marginally wider than foot width (about 6.0 cm), withlong step (about 44 cm) averaging about 5 times foot length(about 8.5 cm).
Holotype and paratype
In the initial description (Li et al. 2005), the holotype of
Shandongornipes was referred to as LRH-DH01, but thisdid not specify precisely what this specimen number
entailed. LRH-DH01 is a plaster cast of track number 4 (aright foot) in the trackway of five sequential tracks made bya single individual, which is equivalent to the original trackthat was given the registration or “field photo number”
LRH-dz67. The initial description (Li et al. 2005) also
individually numbered each track in the trackway (LRHdz66 through dz70). Thus, the term “holotype” was used to
specify both the replica (LRH-DH01) and the actual
uncollected track (LRH-dz67) in the sequence. The same
applies to the paratype (LRH-DP02), which corresponds tofield number LRH-dz68 (Li et al. 2005; see also “Materials
and methods”). The intention of the authors, however, wasto specify the entire trackway as the holotype. In tandem,
both the cited paratype and the holotype represent left andright tracks (numbers 3 and 4, respectively) in a sequence
made by the same individual, and together with theintervening step, they are representative of the entire
morphology and gait of the ichnotaxon.
Horizon and locality
Lower Cretaceous Tianjialou Formation (Dasheng Group),
DH (near the village of Houzuoshan), near Lingquan, Junan
County, Shandong Province, People’s Republic of China(Fig. 1).
Known distribution
Early Cretaceous of eastern Asia.

Revised description
The only known specimen is a single trackway of fiveconsecutive, tetradactyl, paraxonic, well-preserved, zygodactyl bird tracks (Figs. 1b and 2a). Shandongornipes
tracks have average track length (with hallux) of 8.5 cm(range 7.8–8.8 cm) and width (measured perpendicular to
trackway axis) of 5.3 cm (range 4.4–6.0 cm). All digit
plantar surface traces are separate from one another,indicating that their proximal connection to the metatarsus
was typically elevated above the substrate. All digit traces
end in narrow, acuminate claw marks. The hallux trace isshort, consisting of a single elongate claw mark averaging
1.4 cm long (range 1.0–1.8 cm) and separated proximallyfrom all other digit traces by at least 2.0 cm (Fig. 2b–e).
Digit II consists of two proximal pads (mean length 1.1 cm)
and a claw trace (mean total length 3.5 cm; range 3.4–
3.6 cm) directed craniomedially toward the trackway
midline at an acute angle of 10°–15°. Digit III is thelongest (mean 4.23 cm; range 4.1–4.4 cm) and comprisedof three proximal pads and one distal claw impression; thedigit is directed craniolaterally away from the trackway
midline at an acute angle of 10°–15°. The length of thedigit III impression demonstrates that the track maker was
somewhat larger than Sapeornis, the largest Early Cretaceous bird known from osteological remains (Zhou andZhang 2003a), which has a digit III length of roughly
3.9 cm. Digit IV is caudolaterally directed at a 10°–25°
angle caudal to a line perpendicular to the trackway axis;
the digit impression comprises three short pads (meanlength 0.6 cm) plus a claw trace (mean length 2.95 cm;
range 2.7–3.4 cm). The average divarication angles are
152.75° (range 137°–163°) for digits I and II, 32.25° (range
26°–36°) for II and III, 101.25° (range 89°–114°) for III
and IV, and 77.0° (range 68°–94°) for I–IV. The average
step and stride lengths of the trackway are 44 cm (range
41–46 cm) and 87 cm (range 86–88 cm), respectively. Pace
angulation values are high (175°).
Because there are no fossil footprints that bear any close
resemblance to Shandongornipes, we erect the newNaturwissenschaften (2007) 94:657–665

ichnofamily diagnosed above1. The initial description (Li etal. 2005) interpreted the trackway as if it were ananisodactyl form tending toward heterodactyly and, thus,interpreted digit II as IV. Our detailed analysis anddescription indicate a zygodactyl, paraxonic form in which
the cranially oriented digits II and III are bisected by theparasagittal plane of the trackway axis, and lateral digit IV
is easily distinguished by its short pads compared to thoseof digit II. Most claw traces are widely separated from theirassociated distal phalangeal pads by distances of up to
1.0 cm, suggesting that the claws were strongly curved suchthat the main body of each was elevated; therefore, only its
plantarily directed claw tip penetrated the substrate.
Shandongornipes is unique among fossil bird trackways
in showing almost perfect and highly consistent preservation of all digital pad and claw traces in four out of fiveconsecutive footprints. This may be because, in contrast to
most bird tracks of the shorebird variety, its digits were
wider, more robust, and thus, more easily registered clear
impressions.
The relative lengths of the digit impressions (III longer
than IV; both longer than II) are remarkably similar to thoseof tracks made by the greater roadrunner (Cuculidae:
Neomorphinae: Geococcyx californianus), which are also
very similar in track size (typical foot lengths 7–9 cm) andstep length (reaching 37–48 cm; Elbroch and Marks 2001;
Figs. 2f,h and 3e,f). As direct relationship is unlikely—
Geococcyx is only known from the Late Pleistocene
through Recent and only from North America (Brodkorb
1971; Carpenter and Mead 2003)—it is a remarkable
example of convergence in only distantly related andtemporally widely separated avian clades.
Shandongornipes is associated with abundant trackwaysof bipedal non-avian theropod dinosaurs ranging in footlength from about 6.0–30.0 cm, suggesting that its trackmaker occupied a niche in which carnivorous cursorial
bipeds were common and where cursorial ability would
have an obvious advantage. Based on the formula:n ¼ 0:25g0:5 SL1:67 h1:17

1

Unlike phylogenetic systematics but like other parataxonomic
systems, ichnotaxonomy, by convention, does not parallel evolutionary phylogeny, utilizing instead artificial groupings that represent
particular morphologies and/or behavioral modes of, phylogenetically,
often disparate organisms—widely disparate organisms can produce
similar ichnites that indicate similar ecologies and/or behaviors. Thus,
an ichnofamily is not a Linnean entity per se, and ichnofamilies can bemonotaxic because they are intended only to indicate a high degree ofdifference from other ichnological morphologies and/or behaviors.
Given the multiple occurrences of zygodactyly in birds and, indeed,some other tetrapods, other fossil zygodactyl tracks should eventually
be found and also reside in this ichnofamily. See Bertling et al. (2006)
and Lockley (2007) for further details.

661

where v = velocity, g = acceleration due to gravity, SL =stride length, and h = hip height (estimated as 4 × footprint
length; Alexander 1976), we derived a speed estimate of
2.23 m/s, or 8.03 km/h, for the Shandongornipes trackway.
This likely represents an accelerated pace: The large SL
values are typical of higher speed gaits; the alternative
explanation, a particularly long hip height for such a shortfoot length, would be highly unusual and unparsimonious.
This supports the conclusion that the track maker was a
relatively fast-moving cursorial biped, with cursorial ability,
similar to Geococcyx.

Discussion
Shandongornipes tracks were most parsimoniously madeby a bird rather than a non-avian theropod. Most prominently, among Mesozoic tetrapods, bipedality is only
known in the Dinosauria, particularly in some ornithischians and in theropods. Of these, anisodactyly is seen
only within Aves; it is not present in Archaeopteryx (Mayret al. 2005, 2007), although it is present in some other nonpygostylians such as Jeholornis (Zhou and Zhang 2003b)
and Dalianraptor (Gao and Liu 2005). The small overall
track size, slenderness of the digit impressions, and wide
divarication angles between the outer digits are also characteristic of (though not unique to) bird footprints. Small bird
tracks generally exhibit only indistinct pads, however, and, as
above, Shandongornipes tracks are unique in this respect.
Shandongornipes tracks were originally assumed topertain to a shorebird-like taxon (Li et al. 2005), but the
tracks are unique in possessing impressions of not only theplesiomorphically reversed hallux but an additional reversed or highly divergent digit IV as well. Among extant
cursorial birds, zygodactyly is best known in the greater
roadrunner (Engels 1938), but evolved convergently tovarying degrees in numerous arboreal, exclusively neoavian
taxa, including parrots (Psittaciformes; Fig. 3g), woodpeckers (Picidae; Fig. 3h), and owls (Strigiformes; Fig. 3i;see also Supplementary table 1 for a list of zygodactyl
neoavians and their fossil records). Geococcyx itself
plesiomorphically retains zygodactyly from its cuculid
ancestors. Shandongornipes thus could have been madeby a Cretaceous bird occupying a Geococcyx-like niche.
Given the predominant association of zygodactyly witharboreality among neornitheans (Fig. 3f–i; see also Supplementary table 1), a cursorial owl-, parrot-, or woodpecker-like Shandongornipes track maker with unusually long
steps appears unlikely. The holotype trackway forms acomponent of a larger ichnofauna that includes ornithopod,
deinonychosaur, and other non-avian theropod tracks(Fig. 1b). Deinonychosaur tracks in particular are extremelyrare in the palichnological record. The Tianjialou Formation

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thus appears to preserve an unusual faunal record, possibly
a paleoenvironment typically frequented during the Early
Cretaceous by both deinonychosaurs and large, cursorial
ground birds that is not represented at most other trackbearing sites.
Two pedal morphologies could potentially produce
tracks with two reversed digits: zygodactyly (digit IV
reversed) and heterodactyly (digit II reversed), both ofwhich are perching/grasping adaptations that presumably
evolved from a plesiomorphic anisodactyl morphology(Bock and Miller 1959). Indeed, heterodactyl and zygodactyl foot structures are only distinguishable via the morphology of the tarsometatarsus. Two important factors argue infavor of a zygodactyl track maker for Shandongornipes.
First, heterodactyly is rare, known exclusively in the
Trogoniformes, for which direct fossil evidence of aheterodactyl pes dates back to the Middle Eocene (Mayr
2005), although a trogon is represented by isolated cranialmaterial from the Latest Paleocene–Earliest Eocene of
Denmark (Kristoffersen 2002), and a heterodactyl tarsometatarsus of uncertain affiliations is known from theLower Eocene London Clay (Mayr 1999). Extant trogons
are arboreal and have small, comparatively weak legs thatare poorly suited to a terrestrial lifestyle (Collar 2001).
Obligate and facultative zygodactyly, in contrast, arerelatively common in extant birds, having evolved convergently numerous times (Gill 1995; Fig. 3f–i; see also
Supplementary table 1). Second, the digital pad formula in
Shandongornipes is well preserved and suggests a 2-3-3
arthral formula for the plantar portions of digits II, III, andIV, respectively, with proximodistally long phalangeal pads
in digit II and short pads in digit IV. This conforms closely
to the most common avian pattern (2-3-4), although it is
known that digital pads can be both arthral and mesarthral
and, thus, do not necessarily reflect the underlying
phalangeal formulae in footprints (Olsen et al. 1998). Thus,
unparsimonious assumptions are required to assume that
Shandongornipes represents a heterodactyl foot.
The holotype trackway was initially interpreted in reverse(Li et al. 2005), as if it were an exaggerated anisodactyl track
tending toward heterodactyly. However, Shandongornipes
prints exhibit zygodactyly, with a tendency toward ectropodactyly. The latter, a trunk-climbing adaptation that typifies
most modern woodpeckers (Picidae; Bock and Miller 1959),
is characterized both by rotation of digit IV to a position
roughly 90° to digits II and III and decreased retroversion ofdigit I. Shandongornipes prints retain fully reversed halluces,
however, and thus cannot be considered truly ectropodactyl;
they also lack any indication of the conjoined bases of digits
II and III that are common among picids.
Developmental derivation of the zygodactyl from theanisodactyl condition requires digit IV to rotate caudolaterally,
increasing the hypex between digits III and IV and decreasing

Naturwissenschaften (2007) 94:657–665
Fig. 3 a–e Polarities in the symmetry and configuration of non-avian
theropod and avian pedal digits. All tracks shown are right feet. a, b
Polarity between caudalization of median (II and III) hypex (blackarrowhead) and lateral (III and IV) hypex (white arrowhead) in the
Cretaceous non-avian theropod tracks a Siamopodus (Lockley et al.
2006c) and b Saurexallopus (Lockley et al. 2004). c The nearly
symmetrical anisodactyl condition exhibited by Ardea (after Elbroch
and Marks 2001). d The heterodactyl condition seen only in trogons(track is hypothetical because no trogon footprints have been
described). e Zygodactyly exhibited by Geococcyx (after Elbroch
and Marks 2001). Note the strong respective senses of counterclockwise and clockwise rotation that differentiate d and e. f–k Examplesof track morphologies made by zygodactyl birds from various extant
neoavian clades. All tracks are of left feet shown to same scale (5 cm).
f Geococcyx (after Elbroch and Marks 2001). g Parrot (after specimenin CU at Denver collections). h Colaptes (after Murie 1954). i Athene(after Jaeger 1948). j Picoides (after Jaeger 1948). k Bubo (after Stuart
and Stuart 2000)

its counterpart between I and IV. Conversely, heterodactyly
derives from caudomedial rotation of digit II, increasing the II
and III and reducing the I and II hypexes. In either case, theparasagittal orientation of digits I and III remain relatively fixed.
In the former, the lateral hypex (III and IV) migrates caudally
relative to the medial hypex (II and III), and vice versa in thelatter case. Both hypex orientation polarities exist in non-avian
theropod tracks (Fig. 3; Lockley 1998), but tendency toward
ectropodactyly/zygodactyly (i.e., caudalization of hypex III
and IV) is far more common than toward heterodactyly,
foreshadowing the situation in extant birds.
Shandongornipes prints demonstrate variability in the
degree of zygodactyly (Fig. 2b–e). The most pronounced
retroversion of digit IV is in tracks 3 and 5 (left footprints),
whereas left track 1 and especially right track 4 exhibit
more subtly “ectropodactyl” configurations. This may
reflect facultative, rather than obligate, zygodactyly in the
track maker. Shandongornipes prints do not exhibit theextreme degree of zygodactyly seen in many extant bird
feet, but the degree of zygodactyly varies markedly in thelatter, as well (Fig. 3f–i). Variations in the rotation, digit
divarication, or step lengths (limping) between consecutive
left and right footprints in trackway sequences are known
elsewhere in the track record (Lockley et al. 1994) and do
not necessarily indicate an unusual or pathological condition in the Shandongornipes track maker.
Zygodactyl (and heterodactyl) footprints are completely
unknown in the fossil record. Of the neornithean taxa thatare characterized in part by zygodactyl feet, the fossil
records of some extend back to the Early or Middle Eocene;
others extend to the Paleocene (see also Supplementarytable 1), rendering skeletal evidence of zygodactyly amaximum of 65–56 million years old (Gradstein et al.
2004). Shandongornipes, therefore, effectively doubles the
known temporal range of this morphology and represents
an avian morphology and paleoecology unknown from any
body fossil and, hitherto, unknown before the Cenozoic.

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The Shandongornipes track maker appears to have been acursorial bird with a long step, functionally convergent withthe extant roadrunner Geococcyx (Fig. 2f). The phylogenetic
relationships of the Shandongornipes track maker cannot beestablished on the basis of tracks alone, but the restriction of
zygodactyl neoavian clades to the Cenozoic render aneornithean track maker highly unlikely. Shandongornipes,
thus, represents a striking example of evolutionary convergence of an Early Cretaceous bird with the locomotory habits
and possibly the ecology of the modern roadrunner.
Acknowledgments Earlier drafts of this manuscript were greatly
improved by discussions with and comments and reviews by L.
Chiappe, M. Lamanna, G. Mayr, A. R. C. Milner, J. O’Connor, H.-L.
You, and an anonymous reviewer. This study was supported
financially, in part, by National Natural Science Foundation of China
grant 40572011 to RL. Additional support was provided by the
University and Society Collaboration of the Japanese Ministry of
Education, Science, Sports and Culture grant-in-aid 1183303, 1999–
2001 to M. M. and by the CU at Denver Dinosaur Tracks Museum to
ML. The authors declare that all material was collected, examined,
and replicated in full compliance with the laws of their countries.

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