Lithornis plebius Houde, 1988

Nesbitt, Sterling J. & Clarke, Julia A., 2016, The Anatomy And Taxonomy Of The Exquisitely Preserved Green River Formation (Early Eocene) Lithornithids (Aves) And The Relationships Of Lithornithidae, Bulletin of the American Museum of Natural History 2016 (406), pp. 1-1 : 1-

Lithornis plebius Houde, 1988 Lithornis plebius differs from Lithornis promis- cuus and Calciavis grandei (AMNH 30578) by the

AGE: late Paleocene (Clarkforkian 2 NALMA presence of a distally tapering ischium. Lithornis = Cf2) (Lofgren et al., 2004). plebius differs from Lithornis celetius by the pres- OCCURRENCE: Plesiadapis cookei zone Will- ence of a smaller pectineal process of the pubis. wood Formation, Clark’s Fork Basin, Park The skull of Lithornis plebius is estimated to be County, Wyoming. much smaller than that of Pseudocrypturus cer- HOLOTYPE: USNM 336534, posterior portion canaxius (USNM 336103). of the skull, quadrates, dentary, partial furcula, REMARKS: Lithornis plebius is one of the smallcoracoids, scapulae, right humerus, ulnae, frag- est lithornithids known (Houde, 1988). The wellments of the radii, ulnare, carpometacarpi, parts preserved skeleton is from a single individual and of the presacral column, partial synsacrum, almost every element is represented by at least one pygostyle, partial pelvis, femora, tibiotarsi, fibu- side. The holotype was found among the remains lae, left tarsometatarsus, and other fragments. o f Lithornis promiscuus but is morphologically REFERRED MATERIAL: AMNH 21902; thoracic distinct (see above). A partially skeleton from Isle vertebrae, right distal portion of the tarsometa- of Sheppey in England (NHMUK A 5503) was tarsus, radii, left carpometacarpus, proximal por- assigned to Lithornis plebius based on similarities tion of the femur; AMNH 21903; parts of the in size and morphology. However, the holotype radii, proximal and distal portions of the right and NHMUK A 5503 share no unique character and nearly complete left carpometacarpus proxi- states and, therefore, the remains of Lithornis plemal and distal portions of the tarsometatarsus, bius are known only from Wyoming. left coracoid, right scapula, much of the ulna, KEY REFERENCES: Houde, 1988. much of the right femur, proximal half of the tarsometatarsus, proximal and distal portions of the Pseudocrypturus cercanaxius Houde, 1988 left and right humerus, vertebral fragments, sacrum, pedal elements, skull roof. AGE: Early Eocene, 51.66 ± 0.09 Ma (Smith et REVISED DIAGNOSIS: Lithornis plebius is unique al., 2008). among Aves with the following combination of OCCURRENCE: Thompson Ranch, locality H, characters: quadrate with small fossae on the pos- occurs in F-2 facies of Fossil Lake deposits (of terior side of the body (C38:1); the absence of Grande and Buchheim, 1994) of the Green River small foramina or fossae on the posterior side of Formation, near Kemmerer, Wyoming. the otic process of the quadrate (C43:0); pair of HOLOTYPE: USNM 336103, complete skull distinct furrows on the anterior portion of the (fig. 32) and articulated cervical vertebrae. dentary (C49:1); foramina present on the postero- REFERRED MATERIAL: USNM 424078, cast of ventral surface of the hooked acrocoracoid pro- a nearly complete, articulated skeleton of the pricess of the coracoid (C95:1); acromion of the vately held Siber and Siber specimen of Houde scapula with laterally hooked tip with small (1988). foramina on the posterior side (C104:2); narrow REVISED DIAGNOSIS: Pseudocrypturus cerintermetacarpal space; conspicuous deep pit pos- canaxius differs from all other Aves with the terodistal to pisiform process on the ventral sur- following combination of characters: quadrate face of the proximal end of the carpometacarpus with a small fossa on the posterior portion of (C135:1); distinct tuber present on the ventral side the body (C38:1); the presence of small foramof the proximal portion of metacarpal III (C143:1); ina or fossae on the posterior side of the otic distal end of the ischium tapered and with slight process of the quadrate (C43:1); pair of distinct dorsal expansion; and extra projection on the furrows on the anterior portion of the dentary anterior cnemial crest observed in medial view. and premaxillae (C2:1); lateral exposure of the

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ventral process of the lacrimal broad (C13:1); Paracathartes howardae Harrison, 1979 dorsoventrally compressed posterolateral portion of the nasal (C7:1); posterolaterally directed AGE: Early Eocene (middle Wasatchian processes of the palatine (C25:2); ventral fossa NALMA). of the pterygoid (C23:1); dentary strongly OCCURRENCE: northernmost branch of Elk forked (C55:1); fingerlike process just posterior Creek , southwest of Basin , Wyoming and Willto the glenoid of the mandible (C60:1); vomer- wood Formation , Bighorn Basin , Wyoming. premaxilla contact (C26:1). HOLOTYPE: ROM 22658, distal end of a left

Pseudocrypturus cercanaxius differs from Lithor- tibiotarsus. nis celetius by the presence of small foramina on the REFERRED MATERIAL: All the referred mateposterior side of the otic process of the quadrate. rial derived from a single locality representing Pseudocrypturus cercanaxius differs from the from at least five individuals preserved together; referred specimens of Paracathartes howardae by USNM 361402-361446, 391984, 404747-404806; the presence of a small foramina on the posterior USNM 361415; partial skull. side of the otic process of the quadrate (C43:1) and REVISED DIAGNOSIS: All the diagnostic features differs from the referred skull of Lithornis promis- derive from the referred specimens and not from cuus (USNM 391983) by the presence of posterolat- the holotype (see below). Paracathartes howardae eral portion of the nasal that is medioventrally thin differs from all other Aves with the following in lateral exposure. Pseudocrypturus cercanaxius combination of characters: absence of a quadrate differs from Calciavis grandei (AMNH 30578) by with a small fossa on the posterior portion of the the presence of a proportionally shorter optic pro- body (C38:0); the presence of small foramina or cess of the quadrate and overall larger size. fossae on the posterior side of the otic process of

The referred skeleton of Pseudocrypturus cer- the quadrate (C43:1); pair of distinct furrows on canaxius (USNM 424078) differs from Calciavis the anterior portion of the dentary and premaxilgrandei (AMNH 30578) in the greater length of lae (C49:1 and C2:1, respectively); posterolaterally the skull than the humerus, the proportionally directed processes of the palatine absent (C25:1); wider shaft of the coracoid, and the slightly con- ventral fossa of the pterygoid (C23:1); fingerlike cave posterior margin of the sternum. process just posterior to the glenoid of the man-

REMARKS: The holotype of Pseudocrypturus dible (C60:1); stout, highly curved scapula; ossicercanaxius is represented by a complete skull fied supratendinal bridge absent (C166:1); that has been prepared on both sides, exposing pygostyle with a foramen through the ventral pordetails of the basicranium and palate. Pseu- tion; and asymmetrical distal end of the tarsodocrypturus cercanaxius was named based on its metatarsus (C180:1). morphological similarity to extant tinamous Paracathartes howardae differs from Lithornis especially in regard to the palate. A second speci- plebius , Lithornis promiscuus , and Lithornis celetius men of a lithornithid (USNM 424078) from the by the presence of a distally expanded scapula Green River Formation was assigned to Pseu- blade that is highly arched, the absence of a tuber docrypturus cercanaxius , but the referral was not on the ventral surface of the proximal portion of explicitly explained in Houde (1988). The holo- metacarpal III, a flattened medial edge of the type and referred specimen skulls are the same acrocoracoid process, a distinct anteriorly curved size and share an identical morphology (see dis- acromion of the scapula, and a more proximally cussion). The referred material from Europe can- expanded and pointed crista tibiofibularis of the not be unambiguously assigned to femur. Paracathartes howardae differs from P. cer- Pseudocrypturus cercanaxius . canaxius by a much more medially expanded por-

KEY REFERENCES: Houde and Olson, 1981; tion of the medial side of the pterygoid and the Houde, 1988. absence of posterolateral processes of the palatine,

2016 NESBITT & CLARKE: ANATOMY, TAXONOMY, AND RELATIONSHIPS OF LITHORNITHIDS 91

and from Calciavis grandei (AMNH 30578) by the and 54.0 Ma (Chambers et al., 2003). broader scapula blade and the asymmetrical distal OCCURRENCE: Fur Formation of Denmark. end of the tarsometatarsus. REFERRED MATERIAL: MGUH 26770, articu- REMARKS: Paracathartes howardae was named lated skeleton consisting of a complete skull, prefor the distal end of a tibiotarsus and assigned to sacral column, left wing, and pelvis. the oldest cathartid vulture by Harrison (1979). REMARKS: Leonard et al. (2005) assigned a Two character states, the absence of an ossified nearly complete articulated skeleton of a lithorsupratendinal bridge and the medial location of nithid from the Fur Formation to Lithornis vulthe tendinal groove, led Houde (1988) to assign turinus, the type species of Lithornis . In a later Paracathartes howardae to Lithornithiformes . paper, Bourdon and Lindow (2015) followed However, these two character states are plesio- this assignment and rediagnosed Lithornis vulmorphic for Aves in many analyses (Clarke, 2004) turinus. Their assignment of MGUH 26770 to and found in a variety of basal Aves (e.g., lithorni- Lithornis vulturinus is based only on “consistent thids, tinamous) and taxa just outside Aves (e.g., morphology” and not a combination of charac- Ichthyornis dispar ) (Clarke, 2004). Other than ter states or autapomorphies. We do not dispute similar size, we have not found any unambiguous their assignment, but given that there seem to apomorphies linking the holotype of Paracathar- be a diversity of lithornithids, this assignment tes howardae to the referred specimens assigned is presently based on weak evidence and it likely by Houde (1988). Therefore, we score only the will always be based on the fragmentary neoreferred material from a calcareous nodule from

type (NHMUK A5204). From our observations,

the Willwood Formation as described by Houde

MGUH 26770 is unique among lithornithids

(1988).

based on the following characters: hooked

KEY REFERENCES: Harrison, 1979; Houde,

medial processes of the pterygoids, rodlike pha-

1988.

lanx III-1, and a larger maxillopalatine pocket

formed just anterior to the ventral portion of

MGUH 26770 Lithornis vulturinus

the lacrimal.

AGE: Lowermost Eocene, two volcanic ash KEY REFERENCES: Owen, 1841; Leonard et al., layers within the Fur Formation dated to 54.5 2005; Bourdon and Lindow, 2015.

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O N T H E C O V E R: A GREEN RIVER FORMATION LITHORNITHID (CALXAVIS GRANDEI) STANDING BY THE SHORES OF THE EOCENE FOSSIL LAKE (~ 50 MA) WITH A SMALL RALLID IN THE BACKGROUND. ART BY VELIZAR SIMEONOVSKI.