The authors have declared that no competing interests exist.
Conceived and designed the experiments: NI DJV PCS. Performed the experiments: NI DJV PCS JAW. Analyzed the data: NI DJV PCS JAW DBD DMM LB SZ. Contributed reagents/materials/analysis tools: NI PCS DMM LB SZ. Wrote the paper: NI DJV PCS JAW DBD DMM LB SZ. Designed the initial versions of the figures for this paper: NI DJV.
We describe an extensive ichnofossil assemblage from the likely Cenomanian-age ‘lower’ and ‘upper’ units of the ‘Kem Kem beds’ in southeastern Morocco. In the lower unit, trace fossils include narrow vertical burrows in cross-bedded sandstones and borings in dinosaur bone, with the latter identified as the insect ichnotaxon
The likely Cenomanian-age Upper Cretaceous ‘Kem Kem beds’ are exposed along the face of a limestone-capped escarpment extending some 250 km in length in southeastern Morocco
A, Location of Morocco (left corner of Africa map) and the Kem Kem outcrops shown in red, modified from Sereno and Larsson (2009)
A, Simplified section through the Kem Kem sequence, modified from Martill et al. (2011). B, Debris-covered slope of Gara Sbaa, at the main collecting level for dinosaur footcasts. C, Main track horizon, marked by arrow. D, Ichnofossils collected on the same horizon (burrowing structures?) Scale bar equals 8 cm in D.
The Kem Kem beds rest unconformably on Paleozoic (Cambrian through Silurian) marine strata and are capped by a cliff-forming Cenomanian-Turonian-age marine limestone
Although radioisotopic dating of the Kem Kem beds has not been possible, a minimum relative age of Cenomanian (ca. 95 Mya) has been inferred from the elasmobranch taxa within the beds and from the ammonite
The trace fossils described below come from several horizons in the Kem Kem beds, with the majority found in the middle portion of the upper unit at the Gara Sbaa locality
The ichnofossil assemblage either was collected or photographed during expeditions to southeastern Morocco in 1995 from the University of Chicago (led by PCS) and in 2008 from University College Dublin (led by NI). For permits and support of our fieldwork we thank the Ministère de l'Energie, des Mines, de l'Eau et de l'Environnement (previously Ministère de l'Energie et des Mines) and the Faculté des Sciences Aïn Chock Casablanca in Morocco. Collected specimens are curated primarily at the University of Chicago (UCRC I 173, 252–264 and 1995, temporary collection, see
UCRC I # (only rows 1–15) | ML | MW | MDe | LD II | LD III | LD IV | DII-DIII | ° DII/IV |
250 | 310 | 187 | / | 230 | 310 | 200 | 167 | 50 |
251 | 374* | 275 | 90 | 300 | 370 | 280 | 272 | 48 |
252 | 225 | 178 | 80 | 165 | 225 | 150 | 162 | 63 |
253 | 310 | 161 | 95 | 185* | 310 | 180 | 148* | 53 |
254 | 190* | 175* | 110 | 155* | 190* | 135* | 161* | 57* |
255 | 245 | 272 | / | 180 | 245 | 180 | 260 | 88* |
256 | 370 | 255 | 80 | 255 | 365 | 250 | 240 | 54 |
257 | 335 | 225 | 178 | 185 | 335 | 180 | 190 | 60 |
258 | 430 | 301 | 120 | 240 | 390 | 240 | 260 | 57 |
259 | 285* | 265* | 95 | 185* | 285* | 165* | 230* | 79 |
260 | 377 | 309 | 60 | 235 | 345 | 230 | 250 | 59 |
261 | 370 | 250 | 85 | 220 | 340 | 215 | 230 | 58 |
262 | 318 | 245 | 80 | 165 | 240 | 165 | 225 | 80 |
263 | 178 | 187 | 40 | 130 | 178 | 130 | 183 | 77 |
264 | 291 | 204 | 65 | 160 | 260 | 150 | 160 | 41 |
FSAC-KK12 | 272 | 227 | / | 186 | 272 | 181 | 180 | 58 |
FS1 | 343 | 246 | / | 296 | 343 | 268 | 250 | 53 |
FS2 | 333 | 283 | / | 266 | 333 | 270 | 233 | 60 |
FS3 | 361 | 323 | / | 294 | 361 | 300 | 308 | 67 |
FS4 | 525 | 510 | 80 | 385 | 525 | 385 | 472 | 70 |
Abbreviations: De, depth, DII-IV, digits II to IV; FS, field specimen (i.e. not collected); FSAC-KK, Faculté des Sciences Aïn Chock Kem Kem collection; L, length; M, maximum; UCRC I, University of Chicago Research collection ichnofossil; W, width;° angle; * rough estimate because of substantial missing parts. Second column from the left records distance between digits II and III. Measurements in mm. Note that the poor definition and outline of the specimens mean that the values are estimates and approximate. See supporting information (
Relatively straight, cylindrical burrows with irregular cementation of their infilling and neighboring sand occur rarely in cross-stratified sandstones of the lower unit of the Kem Kem beds (
A, B, Vertical burrows within thickly-bedded cross-bedded (A) and planar-bedded (B) sandstones. C, D, Heavily bored dinosaur bone fragment. The outer cortex of this bone shows deep longitudinal cracks and a loss of the most external bone (C) indicative of stage 4 weathering of Behrensmeyer (1978)
Vertebrate bone fragments are common in the lower unit and lowermost portion of the upper unit. Large bone fragments that exceed 5 cm in length frequently exhibit two postmortem features: fracturing from subaerial weathering and insect borings (
Horizontal to subhorizontal, back-filled burrows are common in select horizons in the upper unit of the Kem Kem beds (
These burrows are preserved as convex hyporelief most commonly observed together on the undersides of weathered-out siltstone and sandstones slabs. Coverage can reach nearly 100% of the slabs. Relative abundances vary from slabs dominated by horizontal meniscate burrows (A), to more even mixes (B), and slabs dominated by subvertical burrows (C). D, Close-up of mixed assemblage showing the oval to asymmetric cross-sections of the subvertical burrows. Scale bars equal 10 cm in A–C and 4 cm in D.
A variety of organisms including arthropods and priapulid, sipunculid, and oligochaete worms can produce back-filled burrows in marine, non-marine aquatic, and subaerial settings
These are short, relatively straight, cylindrical burrows that taper to rounded, blunt ends in a subvertical orientation (
The subvertical burrows have an average diameter (∼40 mm) considerably greater than that typical for the vertical dwelling burrows attributed to the ichnogenus
Traces with a maximum diameter ranging from 8–15 cm occur within thin- or medium-bedded siltstones in the upper unit of the Kem Kem beds (
A–C, Lateral (A, B) and bedding plane (C) views of
A few large-diameter burrows are exposed on the upper surface of siltstone beds in association with ripple marks and gastropod trails (
The stacked laminae of increasing diameter suggest that these large-diameter traces were occupied by a relatively sessile, radially symmetrical, growing organism keeping apace with sediment aggradation (
Subcircular burrows on bedding surfaces that lack laminae could represent lungfish burrows
C
At least two varieties of crawling traces (repichnia) occur as rare traces in the upper unit of the Kem Kem beds. The first is known from a single specimen and includes a cylindrical burrow 1 cm in diameter connected to a trail 0.8 cm wide and 14 cm long (
The second track, which occurs with greater frequency, is developed as concave epirelief on fine sandstones and siltstone beds (
A, UCRC PI714. B, coprolite FSAC-KK 935. C, FSAC-KK 936. D, FSAC-KK 2871 and E, FSAC-KK 2838. See
Coprolites occur rarely in the middle of the section of the Kem Kem beds—the upper one-half of the lower unit and the lower one-half of the upper unit. They range in size from 2–5 cm to larger specimens over 10 cm in length (
In 1996, Sereno et al.
At least three track-bearing horizons occur in the upper unit of the Kem Kem beds, as best documented in a section near the village of Er Remlia, where footprints were discovered approximately 90, 60, and 10 m below the Cenomanian-Turonian limestone that overlies the Kem Kem beds (
A, B, Natural casts of theropod tracks (not collected). C, True track of a theropod (not collected). D, Superimposed natural casts of theropod tracks (not collected). Scale bars equal 8 cm in B–D.
A, Large isolated natural cast of theropod track (not collected). B, Relatively poorly defined natural cast of a theropod track (not collected). Scale bar equals 10 cm in B.
Most of the footprints are preserved as natural sandstone casts infilling footprint impressions in an underlying soft mudstone (
A, Removal of a plaster cast left behind by the 1995 University of Chicago expedition. B, Appearance of the track after removal of the plaster. C, Plaster cast, now catalogued as FSAC-KK 12. Scale bar equals 8 cm in B.
A, UCRC I 160. B, UCRC I 252. C, UCRC I 260. D, UCRC I 258. E, UCRC I 253. F, UCRC I 259. G, Image of the block that contained UCRC I 260 identified during 2008 UCD/UoP/FSAC expedition. H, Image of the block that contained UCRC I 258, identified during 2008 UCD/UoP/FSAC expedition. Scale bars equal 10 cm in A-F and 8 cm in G and H.
A, UCRC I 250. B, UCRC I 251. C, UCRC I 256. D, UCRC I 261. E, UCRC I 262. F, UCRC I 264. G, H, UCRC I 255 in (G) ventral view and (H) dorsal view. Scale bar equals 20 cm.
A, UCRC I 252. Track photo shown in
Sandstone casts that have infilled crisscrossed mudcracks suggest that the dinosaurs were walking across muddy substrates, which may have partially dried before or after passage of the trackmakers. Occasional footprint impressions were found in the sandstone layer itself, suggesting that conditions after initial deposition of sands over the mudstone were also favorable for the preservation of footprints.
The great majority of footprints collected and documented in the footprint zone of the upper unit are tridactyl and can be attributed to theropod dinosaurs on the basis of the pointed or narrow shape of the ungual impressions
The footprint impressions show considerable relief (4.0–17.8 cm), indicating that the feet of the trackmakers sank deeply into a wet substrate. A wide range of preservation is apparent. Some tracks are poorly defined, with individual digital impressions indistinct (
Pedal digit III is longer than digits II and IV (
Measurements in mm.
Among Kem Kem theropods (
Sauropod footprints are extremely rare in the footprint zone, despite the occurrence of rebbachisaurid sauropod bone fragments among body fossils recovered in the Kem Kem beds. Two sauropod footprint casts have been identified
A right manual footprint cast (UCRC I 1995) that is 23 cm in depth was found about 8 km northeast of Gara Sbaa, measuring 53 cm in width and 31 cm in length (
Natural cast of sauropod left manus track in dorsal (A), anterior (B), and ventral views (C). Note the straight grooved sides of the track in anterior view (B), the five lobes corresponding to the tightly bound digits I–V, and the absence of any divergent phalanges or unguals. Scale bar equals 10 cm.
Although this Kem Kem track shares a crescentic outline somewhat similar to those of
The natural cast of a sauropod pedal footprint (UCRC I 173) measures 81 cm and 68 cm for major and minor axes (
A, Vental view. B, Anterior view, arrows showing claw marks. Scale bar equals 10
Ornithopod pes tracks are rare in the Kem Kem. Thus far, only one definitive, and one possible ornithopod print have been found (FS4, see
A, Definitive ornithopod track (not collected). B, Possible ornithopod track (UCRC I 263). Measurements of the track in (A) are provided in the text. Scale bar equals 10 cm in B.
Belvedere et al. (
The Kem Kem beds record the transition from full subaerial exposure and erosion of the underlying Paleozoic rocks to fully marine conditions in the overlying echinoid-rich limestone. Traces are uncommon within the coarse, sandstone-dominated lower unit. Besides rare vertical burrows within cross-bedded sandstones, the most abundant traces are those of osteophagous invertebrates on subaerially exposed bones deposited within channel lags.
The upper Kem Kem unit exhibits an overall fining upward sequence likely reflecting a combination of lowered overall gradient, decrease in clastic input, and a rise in eustatic sea level. Marine influence is increasingly represented up section by the presence of mud-drapes, “flaser bedding”, and inclined heterolithic bedding. The suite of traces reflects the decreasing energy of the system as well as the increasing marine influence. The invertebrate traces of the upper Kem Kem include
The prevalence of ichnofossils in the upper unit of the Kem Kem beds may be a consequence of its finer-grained, more heterogeneous sedimentology (interbedded mudstones, siltstones, fine sandstones) that may have facilitated preservation. The footprint horizons, which vary from one to three in sections of the upper unit and are not laterally continuous for kilometers, represent local conditions conducive to preservation of traces fossils.
Tracks other than those pertaining to non-avian dinosaurs are rare. We question the recent identification of pterosaur and crocodyliform tracks. Belvedere et al. reported “probable pterosaur” pedal tracks with “up to four short digit impressions” (
Belvedere et al. reported turtle tracks from the upper unit (
The single large ornithopod footprint (FS4,
Most of the Kem Kem tracks are small- to medium-sized theropod tracks (see
Belvedere et al. (
The trace fossil record, to the contrary, provides an independent source of information on the presence and abundance of Kem Kem vertebrates from that obtained from body fossils. It consists almost exclusively of dinosaur tracks that are dominated by small- to medium-sized theropods
Although the majority of Cretaceous track sites are pre-Cenomanian in age
The Kem Kem, which so far consists almost exclusively of theropod tracks, appears to be less diverse than some other assemblages and most similar to the theropod-dominated track assemblages from the Cenomanian of Brazil (see above). The small sample size and lack of associated tracks in the Kem Kem precludes meaningful comparisons; other than that the sequence appears to be largely dominated by theropod footprints, a trend also observed in the body fossil record (
The re-occurrence of this pattern in geographically and temporally widespread localities suggests potentially an underlying biotic or taphonomic origin. In the case of the Kem Kem ichnological assemblage, the preponderance of theropods may correspond to the shift to a marginal marine environment that favors the presence of scavengers and other carnivores over large, terrestrial herbivores. Factors may include theropod foraging behavior, the scarcity of suitable vegetation for herbivores, and the absence of suitably hard substrates for large herbivores.
(TIF)
We thank C. Abraczinskas and K. Hauk for assistance with the figures, C. Abraczinskas, M. Comier, R. Hing, J. Hopson, R. Loveridge, A. Mihi, D. Naish, H.-D. Sues, and D. Unwin for helpful discussions on aspects of this research, and the Ministère de l'Energie, des Mines, de l'Eau et de l'Environnement (previously Ministère de l'Energie et des Mines) and the Faculté des Sciences Aïn Chock Casablanca in Morocco for supporting the fieldwork. This paper is based in part on research originally carried out for the doctoral dissertation of NI at University College Dublin.