geosciences

Article Lower Eocene Footprints from Northwest Washington, USA. Part 1: Reptile Tracks

George E. Mustoe *

Geology Department, Western Washington University, Bellingham, WA 98225, USA



Received: 23 June 2019; Accepted: 19 July 2019; Published: 22 July 2019


Abstract: Lower Eocene fluvial strata in the Chuckanut Formation preserve abundant bird and mammal tracks. Reptile trace fossils include footprints from a small turtle (ichnogenus Chelonipus), and several Crocodylian trackways that consist of irregularly spaced footprints associated with linear tail drag marks. The latter trackways represent “punting” locomotion, where a submerged Crocodylian used intermittent substrate contacts to provide forward motion of their neutrally buoyant bodies. Two adjacent sandstone blocks preserve Crocodylian trace fossils that are named herein as a new ichnogenus and ichnospecies Anticusuchipes amnis. Two other Crocodylian trackways lack sufficient detail for ichnotaxonomic assignment.

Keywords: Chuckanut Formation; Crocodylian; footprints; ichnology; trace fossils; tracks; turtle; Washington

1. Introduction Birds and animals are represented by a diverse variety of footprints preserved on bedding planes in gently-dipping Chuckanut Formation strata in the Mount Baker foothills in Western Whatcom County, Washington, USA (Figure1, Table1). These locations are in a forested area that has been subject to extensive commercial logging, where road construction yields new bedrock exposures. Landslides also reveal fossiliferous bedrock. The discovery of these Eocene tracks is noteworthy because evidence of vertebrate life is scarce in Paleogene continental sedimentary rocks in the Pacific Northwest. This paper describes reptile tracks; avian and mammalian trace fossils will be described in subsequent reports. These discoveries are noteworthy given the scarcity of reports of Cenozoic reptile tracks. For perspective, a comprehensive bibliography of 584 references describing Cenozoic vertebrate tracks [1] comprise 1% Amphibia, 2% Reptilia, 32.7% Aves, and 64.3% Mammalia. The only known turtle tracks are two Eocene ichnospecies, Chelonipus chardronicus and C. parvus from Texas USA [2], and two Paleocene Crocodylian ichnotaxa, Albertasuchipes russellia from Alberta, Canada, [3], and Boreaosuchipes hanksi from North Dakota, USA [4]. Our report establishes a new Crocodylian ichnotaxon, Anticusuchipes amnis, and describes a turtle trackway as Chelonipus isp. For ichnotaxonomic purposes, the term crocodyle is used include members of three families: Crocodylidae (true crocodiles), Alligatoridae (alligators and caimen), and Gavailidae (gharail and false gharail).

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Figure 1. Chuckanut Formation fossil track locations. Reptile trace fossils are known only from Western FigureWashington 1. Chuckanut University Formation (WWU)-KC-1 fossil turtle track trackway) locations. and Reptile WWU-RCS-1 trace fossils (Crocodylian are knowntracks) only sites. from Western Washington University (WWU)-KC-1 turtle trackway) and WWU-RCS-1 (Crocodylian tracks) sites. Table 1. Reptile track locations. GPS locations are from WGS 84 datum. Locality Number LocationTable 1. Reptile track locations. GPS loLatitudecations are fromLongitude WGS 84 datum.Elevation (Meters) WWU * UWBM ** Locality Number KenneyLocation Creek KC-1 C0644Latitude 48.841 ◦ NLongitude 122.107 ◦ WElevation (Meters) 627 WWU * UWBM ** Racehorse Creek RCS-1 C1481 48.871◦ N 122.118◦ W 541 Kenney Creek KC-1 C0644 48.841° N 122.107° W 627 * Western Washington University. ** University of Washington Burke Museum of Natural History and Culture. Racehorse Creek RCS-1 C1481 48.871° N 122.118° W 541 * Western Washington University. ** University of Washington Burke Museum of Natural History and Culture. 1.1. Geologic Setting 1.1. GeologicThe Chuckanut Setting Formation consists of beds of conglomerate, arkosic sandstone, siltstone, and coalThe that Chuckanut unconformably Formation overlie consists Paleozoic of and beds Mesozoic of conglomerate, metamorphic arkosic basement sandstone, rocks. siltstone, These fluvial and coalsediments that unconformably were deposited overlie on a broad Paleozoic floodplain and Mesozoic that existed metamorphic prior to the mid-Tertiarybasement rocks. uplift These of the fluvial North sedimentsCascade Range were [deposited5,6]. Isolated on exposuresa broad floodplain extend along that faultexisted zones prior to connectto the mid-Tertiary the main outcrop uplift beltof the on Norththe west Cascade side of Range the Cascade [5,6]. Isolated Range exposures with the Swauk extend Formation along fault in zones Central to connect Washington. the main Correlative outcrop beltstrata on also the extendwest side north of into the BritishCascade Columbia Range wi whereth the they Swauk are called Formation the Huntingdon in Central FormationWashington. [7]. CorrelativeMustoe and strata Gannaway also extend [8] suggested north into that British this far-flung Columbia outcrop where distribution they are called is evidence the Huntingdon of a large Formationdepositional [7]. basin Mustoe that wasand dissectedGannaway by strike-slip[8] suggested faulting. that this far-flung outcrop distribution is evidenceThe of Chuckanut a large depositional Formation basin is one th ofat thewas thickest dissected nonmarine by strike-slip sedimentary faulting. formations in North America,The Chuckanut but extensive Formation forest coveris one restrictsof the thickest measurement nonmarine of continuoussedimentary stratigraphic formations in sections. North America,Estimates but of the extensive total thickness forest of cover the formation restricts inme theasurement main outcrop of continuous belt in northwest stratigraphic Washington sections. range Estimatesfrom 3000 of m the [9] tototal 8300 thickness m [10]. Recentof the formation mapping suggestsin the main a minimum outcrop belt thickness in northwest of 4000–5000 Washington m [11]. rangeSeveral from 3000 schemes m [9] haveto 8300 been m proposed[10]. Recent for ma dividingpping suggests the Chuckanut a minimum Formation thickness into of stratigraphic 4000–5000 mmembers [11]. [5,8,11]. The basic architecture is a three-component system composed of the basal Late Paleocene-EarlySeveral schemes Eocene have Bellingham been proposed Bay Member, for dividi overlainng the inChuckanut the Mount Formation Baker foothills into stratigraphic by the Early membersEocene Slide [5,8,11]. Member The basic (Figure architecture2). The third is a component three-component is the middle system Eocene-Latecomposed of Eocene the basal Padden Late Paleocene-EarlyMember, which Eocene is perhaps Bellingham separated Bay from Member, underlying overlain strata in the by anMount unconformity. Baker foothills The stratigraphicby the Early Eocenepositions Slide of two Member minor (Figure members 2). remainThe third uncertain component [11]. is the middle Eocene-Late Eocene Padden Member,All trackwhich fossils is perhaps that have separated so far from been underlying discovered strata have comeby an fromunconformity. Slide Member The stratigraphic strata in the positionsMount Baker of two foothills. minor memb Theseers discoveries remain uncertain are related [11]. to the gentle dip of Chuckanut strata in this part of the main outcrop belt, where outcrops commonly expose bedding plane surfaces where tracks are

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Geosciences 2019, 9, 321 3 of 19 likely to be preserved. In contrast, other stratigraphic members largely comprise steeply-dipping Geosciences 2019, 9, x FOR PEER REVIEW 3 of 18 strata, where bedrock exposures show bedding edges rather than bedding planes.

Figure 2. Stratigraphy of the Chuckanut Formation.

All track fossils that have so far been discovered have come from Slide Member strata in the Mount Baker foothills. These discoveries are related to the gentle dip of Chuckanut strata in this part of the main outcrop belt, where outcrops commonly expose bedding plane surfaces where tracks are likely to be preserved. In Figurecontrast, 2. Stratigraphyother strati ofgraphic the Chuckanut members Formation. largely comprise steeply-dipping strata, where bedrock exposuresFigure show 2. Stratigraphy bedding ofedges the Chuckanut rather than Formation bedding. planes. Locations areare shown shown in Tablein Table1. Reptile 1. Reptile trace fossilstrace havefossils found have only found at two only locations, at two WWU-KC-1 locations, (turtleWWU-KC-1All trackway) track (turtle fossils and trackway) that WWU-RCS-1 have and so WWU-RCS-1far (Crocodylian been discover tracks).(Crocodylianed have The WWU-KC-1cometracks). from The Slide WWU-KC-1 site hasMember been site describedstrata has inbeen the in describeddetailMount [12 Baker]. in The detail foothills. WWU-RCS-1 [12]. Thes The eWWU-RCS-1 site discoveries is at a major are site landsliderelated is at a majorto thatthe occurredgentlelandslide dip 9 that of January Chuckanut occurred 2009 in9 strata January the headwaters in this 2009 part in theofof Racehorsethe headwaters main outcrop Creek. of Racehorse Thebelt, landslide where Creek. outcrops (Figure The3 landslidecommonly) extends over(F exposeigure 1 km 3) bedding inextends length, plane over with surfaces1 an km estimated in wherelength, 5 tracks with105 marean3 × estimatedoflikely displaced to be5 × rockpreserved. 105 m distributed3 of displaced In contrast, over rock an other elevationdistributed strati rangegraphic over of an 800members elevation m [13 ].largely range Fossil of trackscomprise 800 m occur [13]. steeply-dipping inFossil sandstone tracks occurblocksstrata, in scatteredwhere sandstone bedrock over blocks the exposures scattered scattered show over over bedding a large the part edgessca ofttered the rather slideover than area, a largebedding but originatingpart planes. of the in slide a stratigraphic area, but originatingzoneLocations that contains in a stratigraphicare a tephrashown horizonin zone Table that that 1.contains yieldedReptile a tephra U-Pbtrace agefossils horizon of 53.676have that foundyielded0.023 Maonly a U-Pb [11 at]. agetwo of locations, 53.676 ± ± 0.023WWU-KC-1 Ma [11]. (turtle trackway) and WWU-RCS-1 (Crocodylian tracks). The WWU-KC-1 site has been described in detail [12]. The WWU-RCS-1 site is at a major landslide that occurred 9 January 2009 in the headwaters of Racehorse Creek. The landslide (Figure 3) extends over 1 km in length, with an estimated 5 × 105 m3 of displaced rock distributed over an elevation range of 800 m [13]. Fossil tracks occur in sandstone blocks scattered over the scattered over a large part of the slide area, but originating in a stratigraphic zone that contains a tephra horizon that yielded a U-Pb age of 53.676 ± 0.023 Ma [11].

Figure 3. RacehorseRacehorse Creek landslide, locality WWU-RCS-1.

Figure 3. Racehorse Creek landslide, locality WWU-RCS-1.

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Geosciences 2019, 9, x FOR PEER REVIEW 4 of 18 1.2. Paleoenvironment and Paleoclimate 1.2. Paleoenvironment and Paleoclimate The common occurrence of footprints from a diverse assemblage of birds and animals at a The common occurrence of footprints from a diverse assemblage of birds and animals at a single locality, often on the same bedding plane, shows that these creatures lived contemporaneously. single locality, often on the same bedding plane, shows that these creatures lived Track fossils are typically preserved in sandstone that originated as point bar deposits bordering the contemporaneously. Track fossils are typically preserved in sandstone that originated as point bar ancient meandering river, and the track-makers likely utilized the riverbank as a source of water deposits bordering the ancient meandering river, and the track-makers likely utilized the riverbank and food. The abundance of invertebrate traces in subaqueous strata suggests the availability of a as a source of water and food. The abundance of invertebrate traces in subaqueous strata suggests rich nutrient supply for wading birds, which are represented by several avian ichnogenera. Nearby the availability of a rich nutrient supply for wading birds, which are represented by several avian fine-grained facies originated as overbank deposits that contain abundant sub-tropical rain forest ichnogenera. Nearby fine-grained facies originated as overbank deposits that contain abundant plant fossils. Two of the most common plant fossils are Sabalites palm fronds, and fronds of a tree sub-tropical rain forest plant fossils. Two of the most common plant fossils are Sabalites palm fronds, fern, Cyathea pinnata Pabst (Figure4). More than 30 species of angiosperms are represented by and fronds of a tree fern, Cyathea pinnata Pabst (Figure 4). More than 30 species of angiosperms are leaf and seed fossils. Mustoe and Gannaway [8] used the CLAMP multivariate analysis method of represented by leaf and seed fossils. Mustoe and Gannaway [8] used the CLAMP multivariate Wolfe [14] to calculate a mean annual temperature (MAT) of 16◦, and an estimated annual precipitation analysis method of Wolfe [14] to calculate a mean annual temperature (MAT) of 16°, and an of 150–250 cm for Slide Member strata. Breedlovestrout [11] employed the leaf margin analysis estimated annual precipitation of 150–250 cm for Slide Member strata. Breedlovestrout [11] method [15] to calculate a MAT of 19.8◦ for the same fossil assemblage, and 21.9◦ for specimens from a employed the leaf margin analysis method [15] to calculate a MAT of 19.8° for the same fossil nearby site. assemblage, and 21.9° for specimens from a nearby site.

Figure 4. Semi-tropical plant fossils at Racehorse Creek landslide, WWU locality RCS-1. (A,B) Sabalites Figure 4. Semi-tropical plant fossils at Racehorse Creek landslide, WWU locality RCS-1. (A,B) palm fronds; (C) Cyathea pinnata tree fern frond. Sabalites palm fronds; (C) Cyathea pinnata tree fern frond. 1.3. Previous Work 1.3. Previous Work The first discovery of vertebrate footprints in the Paleocene/early Eocene Chuckanut Formation was madeThe first in 1990,discovery when of logging vertebrate road footprints construction in the in thePaleocene/early Mt. Baker foothills Eocene exposed Chuckanut a trackway Formation of a heron-likewas made birdin 1990, [12]. when In later logging years, road other construction tracks were foundin the Mt. at several Baker nearbyfoothills localities exposed [4 a,6 trackway,8]. Although of a birdheron-like and mammal bird [12]. tracks In later are the years, most other common tracks vertebrate were found trace fossilsat several in the nearby Chuckanut localities Formation, [4,6,8]. reptilesAlthough are bird represented. and mammal The onlytracks known are the vertebrate most common skeletal vertebrate fossil from trace the fossils Chuckanut in the Formation Chuckanut is theFormation, carapace reptiles of a small are pondrepresented. turtle (Figure The only5)[ know16,17].n vertebrate Reptile trace skeletal fossils fossil include from a turtlethe Chuckanut trackway preservedFormation inis thethe slabcarapace found of in a 1990 small that pond contains turtle heron-like (Figure 5) tracks, [16,17]. and ReptileCrocodylian trace fossilstracks thatinclude were a exposedturtle trackway by the 2009 preserved Racehorse in the Creek slab landslide. found in These1990 that trace contains fossils are heron-like described tracks, in detail and in Crocodylian this report. tracks that were exposed by the 2009 Racehorse Creek landslide. These trace fossils are described in detail in this report.

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Figure 5. Cast of a carapace of an early Eocene trionychid turtle from the Chuckanut Formation near FigureBellingham, 5. Cast Washington of a carapace USA. of an Adapted early Eocene from [ 17triony]. chid turtle from the Chuckanut Formation near Bellingham, Washington USA. Adapted from [17]. 2. Methods 2. Methods Trace fossils were collected as a collaborative effort by geoscientists and avocational paleontology enthusiasts.Trace fossils Fossils were from collected Chuckanut as Formation a collaborative sites described effort by in thisgeoscientists report are presentlyand avocational housed paleontologyat the Western enthusiasts. Washington Fossils University from GeologyChuckanut Department, Formation and sites at thedescribed University in this of Washington report are presentlyBurke Museum housed of at Historythe Western and Culture.Washington This Univer joint venturesity Geology has caused Department, collecting and locationsat the University to have ofbeen Washington assigned duplicateBurke Museum identifications of History using and both Culture. WWU andThis UWBMjoint venture catalog has numbers. caused Atcollecting present, locationsall Crocodylian to havetrace been fossils assigned are archived duplicate at Western identifications Washington using University both WWU in Bellingham,and UWBM WA catalog USA. numbers.The WWU-KC-1 At present, slab all that Crocodylian preserves trace heron-like fossils tracks are archived and a turtle at Western trackway Washington was quarried University by a team in Bellingham,from the University WA USA. of Washington,The WWU-KC-1 and the slab specimen that preserves is presently heron-like at the Burketracks Museumand a turtle in Seattle, trackway WA wasUSA. quarried At the WWU-RCS-1 by a team from site, trackwaythe University geometries of Washington, were recorded and by the photography specimen is and presently by tracing at the the Burkesurface Museum features in on Seattle, a flexible WA sheet USA. of At transparent the WWU-RCS plastic.-1 site, Footprint trackway molds geometries were made were using recorded latex by or photographysilicone. One and original by tracing reptile trackthe surface slab was features transported on a flexible for display sheet at of the transparent WWU Geology plastic. Department. Footprint moldsOther reptilewere made tracks using are on latex large or blockssilicone. that One remain origin atal the reptile site; track taxonomic slab was descriptions transported utilize for display silicone atmolds the WWU and plaster Geology casts Department. as type specimens. Other reptile Unless tracks otherwise are on noted, large all blocks photographs that remain and illustrations at the site; taxonomicwere created descriptions by the author. utilize silicone molds and plaster casts as type specimens. Unless otherwise noted, all photographs and illustrations were created by the author. 3. Ichonotaxonomy 3. IchonotaxonomyIchnotaxonomic names are traditionally based strictly on morphology. Many articles have beenIchnotaxonomic devoted to this names topic [are18– 23traditiona]. The methodlly based of strictly “pure on ichnotaxonomy” morphology. Many is virtually articles ahave necessity been devotedfor classifying to this invertebratetopic [18–23]. trace The fossils, method which of “p mayure ichnotaxonomy” include burrows, is feeding virtually traces, a necessity marks leftfor classifyinginsect larvae, invertebrate and a host trace of other fossils, features, which some may ofinclude which burrows, are of enigmatic feeding origin.traces, Strategiesmarks left forinsect the larvae,ichnotaxonomy and a host of vertebrate of other footprintsfeatures, some have oftenof which been diarefferent of enigmatic from the classificationorigin. Strategies methods forused the ichnotaxonomyfor invertebrate of trace vertebrate fossils. Thefootprints International have ofte Coden been of Zoological different Nomenclaturefrom the classification [22,23] is methods based on usedthe philosophy for invertebrate that trace trace fossils fossils. are The named International based strictly Code on of morphology, Zoological withoutNomenclature consideration [22,23] ofis basedpossible on track-makers. the philosophy However, that trace vertebrate fossils tracksare named provide based an indication strictly on of morphology, foot anatomy, without and the considerationskeletal structures of possible are known track-makers. for a multitude However, of ancient vertebrate and extant tracks creatures. provide Therefore, an indication it is a common of foot anatomy,practice to and speculate the skeletal on the struct identityures of are the track-maker.known for a Onemultitude approach of hasancient been and to identify extant vertebratecreatures. Therefore,tracks at the it Classis a common or Order practice level, but to speculate to create ichnofamily on the identity and of ichnogenus the track-maker. names toOne classify approach the trace has beenfossils. to The identify result isvertebrate a classification tracks scheme at the that Clas mirrorss or Order conventional level, but zoological to create classification, ichnofamily but usesand ichnogenusichnotaxonomic names categories. to classify the trace fossils. The result is a classification scheme that mirrors conventionalThe foundation zoological of this classification, approach is but the workuses ichnotaxonomic of O.S. Vialov (also categories. spelled “Vyalov” in some citations), whoThe assigned foundation fossil tracks of this as approach ichnospecies is the within work broadly-defined of O.S. Vialov ichnogenera(also spelled [24 “Vyalov”,25]. For example,in some citations),in the Class who Aves, assigned all fossil fossil bird tracks wereas ichnosp givenecies the ichnogenus within broadly-defi name Avipedaned ;ichnogene carnivorera footprints [24,25]. For example, in the Class Aves, all fossil bird tracks were given the ichnogenus name Avipeda; carnivore footprints were given the name Bestiopeda. Vialov’s groundbreaking work is widely cited

Geosciences 2019, 9, 321 6 of 19 Geosciences 2019, 9, x FOR PEER REVIEW 6 of 18 butwere his given monographs the name Bestiopedaare difficult. Vialov’s to access groundbreaking and are published work is widelyin Russian. cited butFortunately, his monographs a recent are Englishdifficult language to access andtranslation are published is readily in Russian. available Fortunately, [26]. English a recent versions English are also language available translation for two is otherreadily articles available [27,28]. [26]. English versions are also available for two other articles [27,28]. Vialov’sVialov’s taxonomy taxonomy was was first first used used to to describe describe tr traceace fossils fossils from from the the Russia Russia and and Eastern Eastern Europe, Europe, butbut later later applied applied to to Miocene Miocene bird bird and and mammal mammal tr tracksacks from from Nevada, Nevada, USA USA [29]. [29 ].Subsequently, Subsequently, a compromisea compromise strategy strategy has has been been introduced, introduced, using using ichnologically-based ichnologically-based families families into into higher-level higher-level “natural” taxonomic categories categories that that are are inferred inferred from from osteological osteological characteristics characteristics [30]. [30]. In In this this report, report, presumedpresumed reptile reptile footprints footprints are are classified classified by by Clas Classs and and Order. Order. Lower Lower level level identifications identifications are are made made usingusing ichnotaxonomic ichnotaxonomic criteria. criteria.

4.4. Chuckanut Chuckanut Formation Formation Turtle Turtle Tracks Tracks ThisThis 62-cm 62-cm long long trackway trackway (Figure (Figure 66)) comprises 14 footprints preserved on a sandstone bedding planeplane that alsoalso containscontains well-preserved well-preserved footprints footprin fromts from a heron-like a heron-like bird. bird The. sharply-preservedThe sharply-preserved tracks tracksindicate indicate the ancient the ancient bird was bird walking was walking on wet on riverbank wet riverbank sediment, sediment, suggesting suggesting that the that nearby the nearby turtle turtletracks tracks were madewere made by an by animal an animal crawling crawling was crawling was crawling on similar on similar terrain. terrain.

Figure 6. (A) Slab found in 1990 at WWU-KC-1 site preserves a turtle trackway and footprints of a Figureheron-like 6. (A bird.) Slab (B )found Turtle in trackway 1990 at WWU-KC-1 marked with site star. preser Arrowsves show a turtle travel trackway directions. and Thefootprints specimen of a is heron-likenow at the bird. University (B) Turtle of Washington trackway marked Burke Museumwith star.of Arrows History show and travel Culture, directions.. Seattle, WA, TheUSA. specimen is now at the University of Washington Burke Museum of History and Culture, Seattle, WA, USA. Distal margins of the impressions show faint claw marks, indicating the direction of travel. TheDistal ~15 mm margins average of the width impressions of the trackway show faint (Figure claw 7marks,) suggests indicating that the the animal direction had of atravel. carapace The ~15width mm of average ~12 cm, width based of onthe thetrackway ambulatory (Figure patterns 7) suggests of modern that the pondanimal turtles had a (Figurecarapace8). width Tracks of ~12alternate cm, based in size. on the Possibly ambulatory the smaller patterns impressions of modern are pond manus turtles imprints, (Figure pes 8). impressions Tracks alternate being in larger. size. PossiblyBased on the this smaller interpretation, impressions the are average manus stride imprin lengthts, pes would impressions be ~17 cm,being yielding larger. aBased width on/stride this interpretation,value of 1.13. Forthe comparison,average stride the length stride would/width be values ~17 cm, for twoyielding Eocene a width/stride turtles from value Texas of are 1.13. 0.55 For for comparison,Chelonipus chadronicus the stride/width, and 1.18 values for C. parvusfor tw[o2 ].Eocene turtles from Texas are 0.55 for Chelonipus chadronicus, and 1.18 for C. parvus [2].

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Figure 7. Chuckanut Formation turtle trackway and drawing showing travel direction and dimensions. Figure 7.7.Chuckanut Chuckanut Formation Formation turtle turtle trackway trackway and drawing and drawing showing showing travel direction travel and direction dimensions. and dimensions.

Figure 8. Pond turtle (Family Emydidae: Terrapenecarolina), showing sprawling gait typical of Figure 8. Pond turtle (Family Emydidae: Terrapenecarolina), showing sprawling gait typical of amphibious turtles. amphibious turtles. 4.1. SystematicFigure 8. Pond Ichnology turtle (Family Emydidae: Terrapenecarolina), showing sprawling gait typical of 4.1. Systematic Ichnology amphibiousClass Reptilia turtles. ClassOrder Reptilia Testudines Linnaeus 1758. 4.1. Systematic Ichnology OrderMorphofamily Testudines ChelonipedidaeLinnaeus 1758. Sarjeant and Langston 1994 [2]. ClassMorphofamilyIchnogenus Reptilia Chelonipus ChelonipedidaeR˝uhlevon Sarjeant Vilienstern and Langston 1939 [ 311994], emended[2]. by Sarjeant and Langston 1994OrderIchnogenus [2]. Testudines Chelonipus Linnaeus R 1758.űhle von Vilienstern 1939 [31], emended by Sarjeant and Langston 1994 Morphofamily[2].Diagnosis: Quadripedal Chelonipedidae footprints Sarjeant with and trackways Langston very 1994 broad [2]. and stride short. Semiplantigrade to digitigrade;IchnogenusDiagnosis: footprints QuadripedalChelonipus broader Rű footprintshle than von long Vilienstern with and digitstrackways 1939 stiff and[31], very very emended broad short, and by bearing stride Sarjeant short,short. and often Semiplantigrade Langston blunt claws. 1994 [2].to digitigrade; footprints broader than long and digits stiff and very short, bearing short, often 4.2.Diagnosis:blunt Comments claws. Quadripedal footprints with trackways very broad and stride short. Semiplantigrade to digitigrade; footprints broader than long and digits stiff and very short, bearing short, often Turtles have a long evolutionary history, with a body plan that has remained relatively constant 4.2.blunt Comments claws. since the Mesozoic. For this reason, their footprints provide little information about the taxonomic identityTurtles of the have track a long maker; evolutionary tracks are history, described with asa body a single plan ichnogenus,that has remainedChelonipus relatively. The constant oldest 4.2. Comments examplessince the dateMesozoic. from theFor late this Early reason, Triassic theirof footprints Wyoming provide and Utah, little USA, information and the earlyabout Middle the taxonomic Triassic ofidentity GermanyTurtles of havethe [32 ].track a long The maker; evolutionary Chuckanut tracks Formation history,are described with turtle a bodyas trackwaya singleplan that ichnogenus, is has preserved remained Chelonipus in relatively fluvial. sandstone,The constant oldest sinceanexamples environment the Mesozoic. date from that For the suggests thislate Earlyreason, the Triassic tracktheir footprints makerof Wyoming may provide haveand Utah, little been USA,information a member and the ofabout early the Middlethe Superfamilies taxonomic Triassic Trionychoideaidentityof Germany of the [32]. ortrack Testidinoidea.The maker; Chuckanut tracks The Formation are former described group turtle as istrackway representeda single isichnogenus, preserved in the Chuckanut inChelonipus fluvial Formationsandstone,. The oldest byan examplesskeletalenvironment remains date fromthat [17 ].thesuggests Trioncychid late Early the Triassic turtlestrack maker areof Wyoming common may haveand Eocene Utah, been fossils USA, a member in and Europe the earlyof and the NorthMiddle Superfamilies America; Triassic ofextantTrionychoidea Germany members [32]. or have The Testidinoidea. worldwideChuckanut Therange.Formation former However, turtlegroup thetrackway is semitropicalrepresented is preserved in Eocene the Chuckanutin paleoenvironment fluvial sandstone, Formation may anby environmenthaveskeletal provided remains that a favorable[17]. suggests Trioncychid habitat the track forturtles some maker are members common may have ofEocene Testudinoidea, been fossils a member in Europe e.g., Familyof andthe North Geoemydidae,Superfamilies America; whoseTrionychoideaextant membersmembers or presently haveTestidinoidea. worldwide inhabit The aquaticrange. former However, environments group isthe represented semitropical in warm areasin Eocene the of Chuckanut Asia, paleoenvironment Europe, Formation and North may by Africa.skeletalhave provided Alternately,remains a[17]. favorable the Trioncychid Chuckanut habitat turtles trackwayfor some are commonmember may represents Eocene of Testudinoidea, the fossils Family in Europe Emydidae, e.g., Family and aNorth groupGeoemydidae, America; of pond extantandwhose marsh-dwelling members members have presently turtles worldwide inhabit that have range. aq auatic long However, environments history the in Northsemitropical in warm America, areas Eocene including of Asia,paleoenvironment examplesEurope, and from North may the haveUpperAfrica. provided Cretaceous, Alternately, a favorable Paleocene, the Chuckanut habitat and Eocenefor trackway some [33 member]. may represents of Testudinoidea, the Family Emydidae,e.g., Family a Geoemydidae,group of pond whoseand marsh-dwelling members presently turtles inhabit that ha aqveuatic a long environments history in North in warm America, areas includingof Asia, Europe, examples and from North the Africa.Upper Alternately,Cretaceous, thePaleocene, Chuckanut and Eocenetrackway [33]. may represent the Family Emydidae, a group of pond and marsh-dwelling turtles that have a long history in North America, including examples from the Upper Cretaceous, Paleocene, and Eocene [33].

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5.5. CrocodylianCrocodylian TraceTrace FossilsFossils CrocodylianCrocodylian tracetracetrace fossils fossilsfossils from fromfrom the thethe Chuckanut ChuckanutChuckanut Formation FormatFormat haveionion havehave been been observedbeen observedobserved on sandstone onon sandstonesandstone blocks wereblocksblocks exposed werewere exposedexposed by the 2003 byby Racehorse thethe 20032003 CreekRacehorseRacehorse landslide. CreekCreek The landslide.landslide. first specimen TheThe firstfirst to be specimenspecimen found preserves toto bebe found afound pair ofpreservespreserves plantigrade aa pairpair manus ofof plantigradeplantigrade imprints that manusmanus show imprintsimprints acuminate thatthat terminations showshow acuminateacuminate of the terminationsterminations five digits (Figure ofof thethe 9 fivefive). digitsdigits (Figure(Figure 9).9).

Figure 9. Manus imprints. Specimen WWU-TR-063, excavated November, 2010. A. trackways; B. Pes ofFigureFigure modern 9.9. ManusManus crocodile, imprints.imprints. showing SpecimSpecim threeenen clawed WWU-TR-063,WWU-TR-063, digits; C. Manusexcavatedexcavated of modern November,November, crocodile, 2010.2010. showingAA.. trackways;trackways; four clawed BB.. PesPes digits;ofof modernmodernD. Dimensions crocodile,crocodile, ofshowshow Chuckanutinging threethree manus clawedclawed impression. digits;digits; CC.. ManusManus ofof modernmodern crcrocodile,ocodile, showingshowing fourfour clawedclawed digits;digits; DD.. DimensionsDimensions ofof ChuckanutChuckanut manusmanus impression.impression. Three other trackway shows smaller footprints associated with tail drag marks. On one landslide block,ThreeThree the upper otherother surface trackwaytrackway is a shows beddingshows smallersmaller plane thatfootprintsfootprints has a prominent associatedassociated sinuous withwith tailtail drag dragdrag mark, marks.marks. with On shallowOn oneone impressionslandslidelandslide block,block, of several thethe upperupper bilateral surfacesurface footprints isis aa beddingbedding bordering planeplane the thatthat central hashas aa groove prominentprominent (Figure sinuoussinuous 10). dragdrag mark,mark, withwith shallowshallow impressionsimpressions ofof severalseveral bilateralbilateral footprintsfootprints borderingbordering thethe centralcentral groovegroove (Figure(Figure 10).10).

FigureFigure 10.10.10. Crocodylian CrocodylianCrocodyliantrackway trackwaytrackway preserving preservingpreserving multiple multiplemultiple footprints footprintsfootprints on eitheronon eithereither side side ofside a prominentofof aa prominentprominent drag mark. dragdrag Tracksmark.mark. TracksTracks are highlighted areare highlightedhighlighted with chalk. withwith chalk.chalk.

Geosciences 2019, 9, 321 9 of 19 Geosciences 2019, 9, x FOR PEER REVIEW 9 of 18 Geosciences 2019, 9, x FOR PEER REVIEW 9 of 18 TwoTwo adjacent adjacent sandstone sandstone blocks blocks discovered discovered in 2015in 2015 preserve preserve trackways trackways where where small small footprints footprints are Two adjacent sandstone blocks discovered in 2015 preserve trackways where small footprints positionedare positioned in association in association with a with linear a draglinear mark drag (Figures mark (Figures 11 and 1112 ).and Lithologic 12). Lithologic similarities similarities in the twoin are positioned in association with a linear drag mark (Figures 11 and 12). Lithologic similarities in blocksthe two suggest blocks the suggest possibility the possibility that both that trackways both trackways may have may originally have originally been contiguous. been contiguous. the two blocks suggest the possibility that both trackways may have originally been contiguous.

FigureFigure 11. 11.Trackways Trackways showing showing multiplemultiple footprints and and tail tail drag drag marks. marks. Specimen Specimen WWU-TR-073. WWU-TR-073. A. Figure 11. Trackways showing multiple footprints and tail drag marks. Specimen WWU-TR-073. A. A. leftleft manus;manus; BB.. rightright pes.pes. Both Both footprints footprints are are in in proximity proximity to to a a central central tail tail drag drag mark. mark. CC. right. right manus manus left manus; B. right pes. Both footprints are in proximity to a central tail drag mark. C. right manus fromfrom a larger a largerCrocodylian Crocodyliantraveling traveling in in a a di differentfferent direction. direction.Footprints Footprints are highlighted with with chalk. chalk. from a larger Crocodylian traveling in a different direction. Footprints are highlighted with chalk.

FigureFigure 12. 12.Specimen Specimen WWU-TR-074. WWU-TR-074. Prominent Prominent dragdrag markmark withwith digitigrades footprints. footprints. AA, B, .B right. right Figure 12. Specimen WWU-TR-074. Prominent drag mark with digitigrades footprints. A, B. right manus;manus;C. rightC. right pes. pes. manus; C. right pes. InterpretationInterpretation of of these these trackways trackways is is based based on on locomotionlocomotion patterns that that have have been been observed observed for for Interpretation of these trackways is based on locomotion patterns that have been observed for modernmodernCrocodylians Crocodylians. For. For terrestrial terrestrial travel, travel, these these reptilesreptiles may use use “low “low walking” walking” and and “high “high walking” walking” modern Crocodylians. For terrestrial travel, these reptiles may use “low walking” and “high walking” styles,styles, depending depending on on the the position position of of the the body body relative relative toto thethe substrate.substrate. In In both both modes, modes, footprints footprints styles, depending on the position of the body relative to the substrate. In both modes, footprints typically occur in regular patterns, with a prominent central drag mark (Figure 13)[34–36]. This type

Geosciences 2019, 9, 321 10 of 19 Geosciences 2019, 9, x FOR PEER REVIEW 10 of 18 Geosciences 2019, 9, x FOR PEER REVIEW 10 of 18 oftypically trackway occur may in also regular be made patterns, by Crocodylianswith a prominentwading central in shallowdrag mark water, (Figure where 13) [34–36]. much ofThis the type body typically occur in regular patterns, with a prominent central drag mark (Figure 13) [34–36]. This type weightof trackway is carried may by also the feetbe made [37]. by Crocodylians wading in shallow water, where much of the body weightof trackway is carried may byalso the be feet made [37]. by Crocodylians wading in shallow water, where much of the body weight is carried by the feet [37].

FigureFigure 13. 13.Left Left by by modern modern American American AlligatorAlligator (Alligator mississippiensis mississippiensis) )from from Florida, Florida, USA, USA, showing showing manusmanusFigure and and13. pes Left pes imprints byimprints modern and and American central central tailtail Alligator dragdrag mark.mark. (Alligator Photo mississippiensis courtesy courtesy of of Kim) Kim from Trebatowski. Trebatowski. Florida, USA, showing manus and pes imprints and central tail drag mark. Photo courtesy of Kim Trebatowski. VeryVery di ffdifferenterent trackways trackways are are produced produced when whenCrocodylians Crocodyliansare submerged,are submerged, when when buoyancy buoyancy causes theircauses feetVery their to beardifferent feet very to bear trackways little very weight little are weight [ 38produced]. Webbed [38]. Webbedwhen hind Crocodylians feethindmay feet ma be arey used be submerged, used to push to push o ffwhen offthe the bottom,buoyancy bottom, and toandcauses paddle to theirpaddle the animalfeet the to bearanimal forward. very forward. li Frontttle weight feetFront can[38]. feet be Webbed can used be to hindused probe feet to for probema surfacey be for used surface obstacles, to push obstacles, off and the to bottom, and facilitate to changesfacilitateand to in paddle travelchanges the direction. in animal travel The forward.direction. use of Front frontThe usefeet feet ofcan for front detectingbe usedfeet forto substrate probedetecting for irregularities substratesurface obstacles, irregularities is valuable and tois as a sourcevaluablefacilitate of protection aschanges a source in to travel avoidof protection direction. accidentally to Theavoid injury use accide of to front thentally tender feet injury for underbelly. detectingto the tender substrate The tailunderbelly. extends irregularities distallyThe tail is as theextendsvaluable crocodyle distallyas glidesa source as forward. theof protectioncrocodyle During toglides forward avoid forwar accide travel,d.ntally theDuring feetinjury mayforward to onlythe tendertravel, intermittently underbelly.the feet makemay The contactonly tail withintermittentlyextends the substrate, distally make causingas contactthe footprintcrocodyle with the sequences substrate,glides forwar to ca beusingd. irregular During footprint compared forward sequences totravel, terrestrialto be theirregular feet or shallow maycompared only water intermittently make contact with the substrate, causing footprint sequences to be irregular compared locomotion.to terrestrial The or distances shallow between water locomotion. successive tracks The maydistances be lengthened between whensuccessive water tracks currents may or wavebe to terrestrial or shallow water locomotion. The distances between successive tracks may be actionlengthened facilitate when forward water propulsion. currents or This wave type action of submerged facilitate locomotion,forward prop describedulsion. This as “punting”,type of lengthened when water currents or wave action facilitate forward propulsion. This type of hassubmerged previously locomotion, been reported described for crustacea as “punting”, [39,40 has] and previously skates (Condricthyes) been reported [for41 ].crustacea The transition [39,40] to submerged locomotion, described as “punting”, has previously been reported for crustacea [39,40] swimmingand skates occurs (Condricthyes) when foot [41]. contact The ceases,transition and to the swimming crocodyle occurs moves when forward foot contact primarily ceases, by oscillations and the crocodyleand skates moves(Condricthyes) forward [41]. primarily The transition by oscillatio to swimmingns of the occurs powerful when foottail. contactA common ceases, form and theof of thecrocodyle powerful moves tail. Aforward common primarily form of locomotionby oscillatio onns wetof the substrates powerful is bytail. belly A sliding.common Swim form tracks of arelocomotion typically strongly on wet digitigradesubstrates is manus by belly impressions, sliding. Swim sometimes tracks are being typically only scrape strongly marks digitigrade left when manuslocomotion impressions, on wet substratessometimes is beingby belly only sliding. scrape Swim marks tracks left whenare typically clawed stronglydigits gouged digitigrade soft clawed digits gouged soft sediment [42]. sedimentmanus impressions, [42]. sometimes being only scrape marks left when clawed digits gouged soft The tracks shown in Figure9 can be interpreted as manus imprints produced by a “punting” sedimentThe tracks[42]. shown in Figure 9 can be interpreted as manus imprints produced by a “punting” Crocodylian. The discordant angles of the two imprints is consistent with tracks made by modern CrocodylianThe tracks. The shown discordant in Figure angles 9 canof the be twointerpreted imprints as ismanus consistent imprints with produced tracks made by aby “punting” modern crocodyles, where the splayed gait causes manus imprints too occur at broad angles relative the crocodyles,Crocodylian. whereThe discordant the splayed angles gait ofcauses the two manus imprints imprints is consistent too occur with at broad tracks angles made relativeby modern the directiondirectioncrocodyles, of travelof wheretravel (Figure (Figurethe splayed 14). 14). The Thegait absence absencecauses of manus pesof pes imprints imimprintsprints and tooand central occur central tail at dragtailbroad drag suggest angles suggest that relative thethat animal the wasanimaldirection swimming was of swimmingtravel rather (Figure than rather bottom 14). than The walkingbottom absence walking (Figure of pes (Figure 15imprints). 15). and central tail drag suggest that the animal was swimming rather than bottom walking (Figure 15).

Figure 14. Trackway of modern American Crocodile (Crocodylus acutus) showing angular Figure 14. Crocodylus acutus relationshipsFigure 14.Trackway Trackwayof manus of modern imprintsof modern American (shaded American tracks) Crocodile toCrocodile direction ( of(Crocodylus travel (arrow).) showing acutus Adapted) angularshowing from relationships angular[43]. of manusrelationships imprints of manus (shaded imprints tracks) (shaded to direction tracks) of to travel direction (arrow). of travel Adapted (arrow). from Adapted [43]. from [43].

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Figure 15.15.Locomotion Locomotion patterns patterns of crocodilesof crocodiles travelling travelling in water. in Awater.. Bottom A walking,. Bottom with walking, intermittent with footintermittent contact andfoot linear contact tail drag,and Blinear. Swimming tail drag, in shallow B. Swimming water, with in possibleshallow intermittent water, with foot possible contact andintermittent tail undulations foot contact for propulsion.and tail undulations Drawing for by Jessiepropulsion. Thoreson, Drawing 2019. by Jessie Thoreson, 2019.

For the other trackways (Figures 10–12), the long distances between tracks, the irregular distribution, For the other trackways (Figures 10–12), the long distances between tracks, the irregular and the shallowness of the impressions are suggestive of punting rather than bottom walking. distribution, and the shallowness of the impressions are suggestive of punting rather than bottom Ichnotaxonomy of Crocodylian Tracks walking. Crocodylian trace fossils found at the Chuckanut Formation Racehorse Creek landslide location Ichnotaxonomy of Crocodylian Tracks include manus and pes impressions, and associated tail drag marks, but in two specimens these Crocodylian trace fossils found at the Chuckanut Formation Racehorse Creek landslide location trace fossils lack sufficient information for reliable ichnotaxonomic assignment (Figures9 and 10). include manus and pes impressions, and associated tail drag marks, but in two specimens these Two trackways preserved on adjacent landslide blocks (Figures 11 and 12) provide better detail. trace fossils lack sufficient information for reliable ichnotaxonomic assignment (Figures 9 and 10). The irregular pattern of the footprints is presumably evidence of punting-style locomotion of a Two trackways preserved on adjacent landslide blocks (Figures 11 and 12) provide better detail. The small submerged crocodyle, resulting in trackways that are dissimilar to the more regular footprint irregular pattern of the footprints is presumably evidence of punting-style locomotion of a small patterns that occur during terrestrial or shallow water walking. However, Crocodylian “swim tracks” submerged crocodyle, resulting in trackways that are dissimilar to the more regular footprint (e.g., punting tracks) are common in the geologic record, suggesting that these irregular footprint patterns that occur during terrestrial or shallow water walking. However, Crocodylian “swim tracks” patterns(e.g., punting are a typicaltracks) phenomenon.are common in the geologic record, suggesting that these irregular footprint Class Reptilia patterns are a typical phenomenon. OrderClass Reptilia Crocodylia Wermuth 1953 [44] Crocodilia Owen 1842 [45] Anticusuchipes amnis IchnogenusOrder Crocodylia Wermuth 1953ichnogen [44] Crocodilia et ichnosp. Owen nov. 1842 [45] IncludedIchnogenus ichnospecies: Anticusuchipes The amnis ichnogenus ichnogen is presently et ichnosp. known nov. to include the type ichnospecies. Anticusuchipes amnis TypeIncluded ichnospecies: ichnospecies: The ichnogenusMustoe, is presen herein.tly known Early to Eocene, include Slide the type Stratigraphic ichnospecies. Member, ChuckanutType ichnospecies: Formation, western Anticusuchipes Whatcom amnis County, Mustoe, Washington herein. USA. Early Eocene, Slide Stratigraphic Anticus suchi Crocodylians Member,Etymology: Chuckanut Latin: Formation,, ancient;western Whatcom, referring County, to Eosuchia, Washington the basalUSA. clade for ; pes amnis , foot;Etymology:, river. Latin: The Anticus name, ancient; thus roughly suchi, translates referring asto “ancientEosuchia, river the basal crocodyle clade footprint”. for Crocodylians; pes, foot;Diagnosis: amnis, river. Tetradactyl The name tracks thus with roughly slender translates elongate as digits “ancient with river apices crocodyle that range footprint”. from rounded to sharplyDiagnosis: pointed. Tetradactyl Claws tracks are not with distinctly slender evident elongate at digits the digital with apices apices. that Heel range pad from are rounded.rounded Tracksto sharply are plantigradepointed. Claws to digitigrades. are not distinctly Manus evid impressionsent at the are digital much apices. smaller Heel than pad pes are impressions. rounded. DetailedTracks are measurements plantigrade to are digitigrades. shown in Figure Manus 16. impr The Chuckanutessions are Formationmuch smaller footprints than pes are impressions. distinctively Crocodylian Crocodylopodus diDetailedfferent frommeasurements previously-described are shown in Figuretracks 16. (FigureThe Chuckanut 17). Footprints Formation of footprints [are46] anddistinctivelyBatrachopus different[47] are from much previously-described smaller than Anticusichipes Crocodylian; Mehliella tracks[48 ],(FigureAlbertasuchipes 17). Footprints[3], and anof unnamedCrocodylopodus track from[46] USAand [Batrachopus49] are much [47] larger. are Themuch shapes smaller of all than of these Anticusichipes ichnogenera; areMehliella unlike [48], that ofAlbertasuchipesAnticusichipes [3]. Mesozoic, and an unnamedCrocodylian tracktracks from that USA are [49] comparable are much inlarger. size are TheHatcherichnus shapes of all [of50 ]these and Kuanguanpusichnogenera are[51 ],unlike but these that footprints of Anticusichipes are much. Mesozoic broader andCrocodylian have shorter, tracks wider that are digit comparable impressions in thansize Indosuchipesare Hatcherichnusor Anticusuchipes [50] and Kuanguanpus. The most [51], notable but dithesefference footprints between areIndosuchipes much broaderand andAnticusuchipes have shorter,is wider digit impressions than Indosuchipes or Anticusuchipes. The most notable difference between

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Geosciences 2019, 9, x FOR PEER REVIEW 12 of 18 the shape of the heel pad, which is broadly rounded and prominent in the former ichnogenus, and Indosuchipes and Anticusuchipes is the shape of the heel pad, which is broadly rounded and smaller and narrow for Anticusichipes. An unnamed track from Portugal [52] is more similar in shape, prominent in the former ichnogenus, and smaller and narrow for Anticusichipes. An unnamed track but digit impressions of the latter fossil are short and triangular, compared to the slender elongated from Portugal [52] is more similar in shape, but digit impressions of the latter fossil are short and digits characteristic of Indosuchipes and Anticusuchipes. triangular, compared to the slender elongated digits characteristic of Indosuchipes and Anticusuchipes.

Figure 16. Sketches of Anticusuchipes amnis footprints A–C, showing interdigital angles and track dimensions,Figure 16. Sketches specimen of WWU-TR-073.Anticusuchipes Trackamnis imagesfootprints have A– beenC, showing rotated tointerdigital show parallel angles orientations. and track Thedimensions, original positionsspecimen areWWU-TR-073. shown in Figure Track8. images have been rotated to show parallel orientations. The original positions are shown in Figure 8. Anticusuchipes amnis Holotype: Western Washington University specimen WWW-TR-073, represented by a plaster cast made from a silicone mold archived at the W.W.U Geology Department archives, Bellingham, WA USA. Diagnosis: Same as for the ichnogenus Horizon and locality: Chuckanut Formation, Slide Stratigraphic Member (Early Eocene), Racehorse Creek landslide, WWU-RCS-1, Whatcom County, Washington USA. Remarks: Tracks A and B are positioned on either side of a central drag mark. Footprint A is a left manus, and the larger footprint B is a right pes. Track C is not part of this trackway, being the right manus print of a larger individual travelling in a different direction. For Crocodylians, footprint size is not a reliable parameter for ichotaxonomy, because even the largest individuals began their life as tiny hatchlings. Also, apparent track sizes and shapes may be affected by the sedimentary characteristics of the substrate. Trace fossils in specimen WWU-TR-074 are not as well preserved as WWU-TR-073. In the former specimen, three footprints are located to the left of the drag mark and appear to represent and two incomplete manus imprints (Figure 12A,B) one pes impression (Figure 12C). Possibly these trace fossils represent the same track-maker as the holotype specimen WWU-TR-073; both trackways appear to occur on a single bedding plane that is now represented by two adjacent landslide blocks. Alternately, the two trackways may have been made by two individuals of the same approximate size. The small sizes of these tracks are suggestive of juveniles, consistent with the common tendency for extant Crocodylians to live in proximity [55].

Figure 17. Outlines of crocdylian tracks of Mesozoic and Cenozoic age. A. Early Cretaceous Laiyangpus, China [51]; B. Late Jurassic Crocodylopodus [46]; C. Early Cretaceous, Thailand [53]; D. Early Jurassic Batrachopus [47]; E. Late Jurassic, Portugal [52]; F. Miocene, Characichnos, Czech

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Indosuchipes and Anticusuchipes is the shape of the heel pad, which is broadly rounded and prominent in the former ichnogenus, and smaller and narrow for Anticusichipes. An unnamed track from Portugal [52] is more similar in shape, but digit impressions of the latter fossil are short and triangular, compared to the slender elongated digits characteristic of Indosuchipes and Anticusuchipes.

Figure 16. Sketches of Anticusuchipes amnis footprints A–C, showing interdigital angles and track dimensions, specimen WWU-TR-073. Track images have been rotated to show parallel orientations. GeosciencesThe 2019original, 9, 321 positions are shown in Figure 8. 13 of 19

Figure 17. Outlines of crocdylian tracks of Mesozoic and Cenozoic age. A. Early Cretaceous Laiyangpus, Figure 17. Outlines of crocdylian tracks of Mesozoic and Cenozoic age. A. Early Cretaceous China [51]; B. Late Jurassic Crocodylopodus [46]; C. Early Cretaceous, Thailand [53]; D. Early Jurassic Laiyangpus, China [51]; B. Late Jurassic Crocodylopodus [46]; C. Early Cretaceous, Thailand [53]; D. Batrachopus [47]; E. Late Jurassic, Portugal [52]; F. Miocene, Characichnos, Czech Republic [42]; Early Jurassic Batrachopus [47]; E. Late Jurassic, Portugal [52]; F. Miocene, Characichnos, Czech G. mid-Cretaceous, Mehiella, USA [48]; H. Paleocene Albertasuchipes, Canada [3]; I. Late Jurassic Hatcherichnus, USA [50]; J. Late Paleocene Borealosuchipes, USA [4]; K. Late Cretaceous, USA [49]; L. Middle Jurassic, Kuanguanopus China [51]; M. Oligocene, Indosuchipes,[54]; N. Anticusuchipes amnis Mustoe, ichnogen. and ichnosp. nov., Eocene, northwest Washington.

6. Discussion The Crocodylian crown group dates to the Cretaceous, but the earliest known crocodylomorph footprints are Triassic. Lockley et al. [56] reviewed the evolutionary history of Crocodylians and described modern members. Crocodylomorph tracks have broad geographic distribution in Triassic and Jurassic strata. The most abundant footprints have been found in Cretaceous formations, most notably the Albian-Cenomania Dakota Group of Colorado, Kansas, and New Mexico, USA, where Crocodylian tracks are known from 70 sites. Unlike non-avian dinosaurs, Crocodylians survived the great K/T extinction. These reptiles clearly existed throughout the Cenozoic, as evidenced by extant members that comprise three groups: Alligatoridae (8 species), Crocodylidae (14 species), and Gavailidae (1 species). However, Cenozoic tracks have rarely been reported. McCrea et al. [3] described Paleocene Crocodylian tracks from the Paleocene of Alberta, Canada, and Erickson [4] illustrated similar footprints from North Dakota, USA. Miocene scrape marks from Czech Republic were interpreted to have been made by a Crocodylian [42]. Modern Crocodylians inhabit tropical regions, where they dwell in rivers, lakes, swamps, and coastal estuaries. They are endemic to southern North America, Central America, South America, Africa, Australia, and Southern Asia; the fossil record is reviewed by Brochu [57]. Scarcity of North American Cenozoic Crocodylian fossils is probably related to the worldwide decline in temperatures that began in the late Eocene. Because Crocodylians require subtropical or tropical climate, this climatic decline would have greatly reduced the geographic extent of suitable habitat. At low latitudes, tropical environments allowed Crocodylians to remain successful throughout the Cenozoic. In Asia, saltwater crocodiles (Crocodylus porosus) are endemic to coastal areas of the Indo-Pacific region, and the freshwater gharial (Gavialis gangeticus) inhabits rivers in Northern India. The presence of Oligocene Crocodylian trace fossils in Manipur, Northeast India [54] is evidence that these reptiles have a long history in a region Geosciences 2019, 9, 321 14 of 19

whereGeosciences the 2019 equatorial, 9, x FOR latitude PEER REVIEW provided continued warm temperatures. In South America, Crocodylians14 of 18 reached a zenith in the late Cenozoic, with 14 species present in the late Miocene, compared to 10 extant taxapresent [58]. in Africa the late is todayMiocene, inhabited compared by one to 10 native extant crocodile, taxa [58].Crocodylus Africa is (Niletoday Crocodile), inhabited by with one known native ancestorscrocodile, that Crocodylus date to the(Nile late Crocodile), Miocene [with59]. known ancestors that date to the late Miocene [59]. InIn North North America, America, Cretaceous Cretaceous and Paleogeneand Paleogene crocodyllian crocodyllian fossils occurfossils as occur far north as asfar Wyoming, north as butWyoming, the only but modern the only representatives modern representatives are the American are the alligator American (Alligator alligator mississippiensis (Alligator mississippiensis), whose range), extendswhose range from Floridaextends to from Texas, Florida and the to AmericanTexas, and crocodile the American (Crocodylus crocodile acutus (Crocodylus), which inhabits acutus), Florida. which Ininhabits Northwest Florida. Washington, In Northwest USA, Washington, the subtropical USA, climate the subtropical of the coastal climate lowlands of the during coastal the lowlands Eocene wouldduring have the providedEocene would a favorable have provided environment a favorable for Crocodylians environment. Their for presence Crocodylians in fluvial. Their Chuckanut presence Fm. in stratafluvial indicates Chuckanut that theseFm. strataCrocodylians indicateswere that river these dwellers. Crocodylians Early Eocenewere riverCrocodylians dwellers.from Early the Eocene Green RiverCrocodylians in Wyoming, from USAthe Green provide River possible in Wyoming, analogs for USA the provide Chuckanut possible Formation analogs track-makers. for the Chuckanut In both occurrences,Formation track-makers.Crocodylians inhabited In both occurrences, fluvial/lacustrine Crocodylians habitats, inhabited and the fluvial/lacu formationsstrine are age habitats, correlative and (Figurethe formations 18). Teeth, are schutes,age correlative and coprolites (Figure 18). are theTeeth, most schutes, common and Crocodylian coprolites are fossils the most in the common Green RiverCrocodylian basin sediments, fossils in butthe articulated Green River skeletons basin sedime have beennts, foundbut articulated [60,61]. Plant skeletons fossils have indicate been that found the early[60,61]. Eocene Plant paleoclimate fossils indicate of central that Wyomingthe early wasEocene warm paleoclimate temperate toof subtropical,central Wyoming comparable was towarm the Chuckanuttemperate Formation.to subtropical, The ancestralcomparable history to the of theseChuckanut Cenozoic Formation.Crocodylians Theis ancestral enigmatic history because of of these the scarcityCenozoic of PaleoceneCrocodylians sedimentary is enigmatic formations. because However,of the scarcityCrocodylians of Paleoceneare represented sedimentary at the formations. Wannagan CreekHowever, fossil Crocodylian quarry in Norths are represented Dakota, USA at bythe skeletal Wannagan remains Creek of atfossil least quarry 80 individuals in North [4Dakota,,62–64]. USA by skeletal remains of at least 80 individuals [4,62–64].

. Figure 18. Location of Paleogene Crocodylian fossil localities in Western USA. Figure 18. Location of Paleogene Crocodylian fossil localities in Western USA. Favorable habitat conditions for crocodiles depends on local environmental factors, e.g., the presenceFavorable of suitablehabitat bodiesconditions of water for crocodiles and adequate depends food. on From local a geographicenvironmental standpoint, factors, tectonice.g., the forcespresence may of be suitable involved, bodies because of marinewater coastsand adequa and lowte elevationfood. From inland a geographic lakes and rivers standpoint, are required tectonic by Crocodyliansforces may. However,be involved, the dominantbecause marine factor iscoasts temperature. and low Beginning elevation in inland the late lakes Eocene, and progressive rivers are climaticrequired cooling by Crocodylians beginning. However, in the late the Eocene domina explainsnt factor the is demisetemperature. of Crocodylians Beginningat in high the latitudeslate Eocene, in Northprogressive America climatic (Figure cooling 19). Extant beginning members in the are late limited Eocene to explains the warm the climate demise of of the Crocodylians Southeastern at high US. Forlatitudes perspective, in North modern AmericaAlligator (Figure misssissipiensis 19). Extant stopsmembers eating are at limited temperatures to the belowwarm 16climate◦C, but of can the surviveSoutheastern freezing US. conditions. For perspective, In contrast, modernCrocodylus Alligator acutus misssissipiensissuccumbs to hypothermiastops eating atat 7temperatures◦C[65]. belowCompared 16 °C, but to Crocodylianscan survive, turtlesfreezing have conditions. broader climaticIn contrast, tolerance; Crocodylus extant acutus species succumbs native toto Washingtonhypothermia and at 7 Oregon, °C [65]. USA, are Actinemys marmarota (Western Pond Turtle) and Chrysemys picta (PaintedCompared Turtle). to A Crocodylians non-native, taxon,turtles Trachemyshave broader scripta climatic(Pond tolerance; Slider) has extant appeared species as native a result to ofWashington the release and of turtlesOregon, that USA, had are originally Actinemys been marmarota soldas (Western pets.Other Pond non-nativeTurtle) and formsChrysemys include picta (Painted Turtle). A non-native taxon, Trachemys scripta (Pond Slider) has appeared as a result of the release of turtles that had originally been sold as pets. Other non-native forms include Cheleydra serpentina (Snapping Turtle) and Apalone spinifera (Spiny Softshell). Leatherback Turtles (Dermochelys coriacea) are regular off the Washington coast. Other sea turtles, including Green (Chelonia mydas),

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Cheleydra serpentina (Snapping Turtle) and Apalone spinifera (Spiny Softshell). Leatherback Turtles (GeosciencesDermochelys 2019, coriacea 9, x FOR) PEER are regular REVIEW o ff the Washington coast. Other sea turtles, including Green (Chelonia15 of 18 mydas), Loggerhead (Caretta caretta) and Pacific Ridley (Lepidochelys olivacea) are much rarer, but a few Loggerhead (Caretta caretta) and Pacific Ridley (Lepidochelys olivacea) are much rarer, but a few have have been found washed up on northwest beaches [66]. Warm Paleogene climate likely provided been found washed up on northwest beaches [66]. Warm Paleogene climate likely provided favorable habitat, but turtles have a well-documented record as evolutionary survivors. favorable habitat, but turtles have a well-documented record as evolutionary survivors.

Figure 19. Mean global surface temperatures during the Cenozoic, and age of Paleogene Crocodylian Figure 19. Mean global surface temperatures during the Cenozoic, and age of Paleogene Crocodylian fossils from North America. Paleoclimate data adapted from [67,68]. fossils from North America. Paleoclimate data adapted from [67,68].

Funding: This research received no external funding. Funding. This research received no external funding. Acknowledgments: Spencer Lucas provided helpful advice during manuscript preparation. Brennan Martens andAcknowledgments: Emmet Martens assistedSpencer withLucas field provided work at helpful the WWU-RCS-1 advice duri locality.ng manuscript Brennan Martenspreparation. deserves Brennan special Martens credit forand discovering Emmet Martens Crocodylian assisted trackways with field (Figures work at11 the and WWU-RCS-1 12) at a site that locality. had previously Brennan Martens received deserves numerous special visits bycredit geoscientists. for discovering At age 14Crocodylian at the time trackways of the 2015 (Figures discovery, 11 Brennan and 12) Martens at a site displayed that had admirable previously observational received skills.numerous Following visits theby originalgeoscientists. 2009 visitAt age to the 14 WWU-RCS-1at the time of site, the dozens 2015 discovery, of volunteers Brennan have assisted Martens in displayed searching the extensive landslide in pursuit of fossils, resulting in recovery of many specimens, and preparation of molds admirable observational skills. Following the original 2009 visit to the WWU-RCS-1 site, dozens of volunteers of specimens on blocks that are too big to transport. Particular credit goes to Tadd Dillhoff, Wes Gannaway, Donhave Hopkins,assisted in Keith searching Kemplin, the extensive Dave Tucker, landslide and Jared in pursui Watson,t of whofossils, all resulting made many in recovery visits to theof many site. specimens, and preparation of molds of specimens on blocks that are too big to transport. Particular credit goes to Tadd Conflicts of Interest: The authors declare no conflict of interest. Dillhoff, Wes Gannaway, Don Hopkins, Keith Kemplin, Dave Tucker, and Jared Watson, who all made many visits to the site. References Conflicts of Interest: The authors declare no conflict of interest. 1. McDonald, H.G.; White, R.S.; Lockley, M.G.; Mustoe, G.E. An indexed bibliography of Cenezoic vertebrate Referencestracks. In Cenozoic Vertebrate Tracks and Traces; Milàn, J., Lucas, S.G., Spielmann, J.A., Lockkley, M.G., Eds.; New Mexico Museum of Natural History and Science Bulletin: Albuquerque, NM, USA, 2007; Volume 42, 1. McDonald,pp. 275–302. H.G.; White, R.S.; Lockley, M.G.; Mustoe, G.E. An indexed bibliography of Cenezoic vertebrate 2. tracks.Sarjeant, In W.A.S.;Cenozoic Langston, Vertebrate W., Tracks Jr. Vertebrate and Traces footprints; Milàn, J.,and Lucas, invertebrate S.G., Spielmann, traces from J.A., the Lockkley, Chardronian M.G., (LateEds.; NewEocene) Mexico of Trans-Pecos, Museum of Texas. Natural In TexasHistory Memorial and Science Museum Bulletin: Bulletin Albuquerque, 36; University NM of, Texas: USA, 2007; Austin, Volume TX, USA, 42, 1994;pp. 275–302. p. 86. 3.2. Sarjeant,McCrea, R.T.;W.A.S.; Pemberton, Langston, S.G.; W., Currie,Jr. Vertebrate P.J. New footpr ichnotaxaints and of mammalinvertebrate and trac reptilees from tracks the from Chardronian the upper (LatePaleocene Eocene) of Alberta. of Trans-Pecos,Ichnos 2004 Texas., 11 ,In 323–339. Texas Memorial [CrossRef Museum] Bulletin 36; University of Texas: Austin, TX, 4. USA,Erickson, 1994; B.R. p. 86. Crocodile and arthropod tracks from the Late Paleocene Wannagan Creek fauna of North 3. McCrea,Dakota, USA.R.T.; IchnosPemberton,2005, 12S.G.;, 303–308. Currie, [CrossRefP.J. New ]ichnotaxa of mammal and reptile tracks from the upper 5. PaleoceneJohnson, S.Y.of Alberta. Stratigraphy, Ichnos 2004 age,, and11, 323–339. paleogeography of the Eocene Chuckanut Formation, northwest 4. Erickson,Washington. B.R.Can. Crocodile J. Earth and Sci. arthro1984, 21pod, 92–106. tracks [fromCrossRef the Late] Paleocene Wannagan Creek fauna of North 6. Dakota,Johnson, USA. S.Y. CyclicIchnos fluvial2005 12 sedimentation, 303–308. in a rapidly subsiding basin. Sediment. Geol. 1984, 38, 361–391. 5. Johnson,[CrossRef S.Y.] Stratigraphy, age, and paleogeography of the Eocene Chuckanut Formation, northwest Washington. Can. J. Earth Sci. 1984, 21, 92–106. 6. Johnson, S.Y. Cyclic fluvial sedimentation in a rapidly subsiding basin. Sediment. Geol. 1984, 38, 361–391. 7. Mustard, P.S.; Rouse, G.E. Stratigraphy and evolution of Tertiary Georgia Basin and subadjacent Upper Cretaceous sedimentary rocks, southwestern British Columbia and northwestern Washington. In Geology and Geological Hazards of the Vancouver Region, Southwestern British Columbia; Monger, J.W.H., Ed.; Geological Survey of Canada Bulletin: Ottawa, ON, Canada, 1994; Volume 481, pp. 97–169.

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