Dusignathus, Kellogg, 1927

Boessenecker, Robert W., 2013, A new marine vertebrate assemblage from the Late Neogene Purisima Formation in Central California, part II: Pinnipeds and Cetaceans, Geodiversitas 35 (4), pp. 815-940 : 824-826

Dusignathus sp., cf. D. seftoni

REFERRED MATERIAL. — VMS 2, a complete right radius; UCMP 219005, a partial right radius, UCMP 219006, a partial atlas, and UCMP 219001, a partial cervical vertebra, collected by R.W. Boessenecker and C. Dailey from UCMP localities V99834 and V99848 .

STRATIGRAPHIC OCCURRENCE. — Middle and upper parts of the San Gregorio section of the Purisima Formation, Early to Late Pliocene (c. 5-2.5 Ma, Zanclean-Gelasian equivalent; Fig. 2 ).

DESCRIPTION

Ŋese specimens are all relatively large in comparison to the otariid specimens described below. Ŋe radii ( Fig. 4 ) are at least twice the size of Callorhinus sp., cf. C. gilmorei . Ŋe description is primarily based on VMS 2; however, the distal end of VMS 2 is poorly preserved, and the description of the distal end is based instead upon UCMP 219005. Ŋe radii are relatively straight and transversely flattened. Although damaged, the radial head bears an expanded and concave proximal articular surface. Ŋe shaft is proximally cylindrical, and distally becomes anteroposteriorly expanded near the pronator teres process. Ŋe pronator teres process is positioned just slightly proximal to the midpoint of the radius, and occurs as an angle on the anterior margin of the radius ( Fig. 4A ); the lateral surface of the pronator teres process is slightly rugose. Ŋe distal end is massive, transversely thicker, and anteroposteriorly deeper than the proximal shaft. Although the radial process is damaged in VMS 2, in UCMP 219005 it is transversely robust, distally directed, and extends nearly to the level of the distal scapholunar articulation surface ( Fig. 4C, D ). Ŋe supinator longus insertion is present as a small knob. Ŋe scapholunar articulation of UCMP 219005 is deeply concave and oval in distal aspect, with a prominent styloid process. Ŋe medial surface of the shaft is shallowly concave, while the lateral surface is slightly convex.

Nearly the complete left side of the atlas (UCMP 219006) is preserved ( Fig. 5 A-C). Anteriorly, there is a large, concave, and crescentic articular surface for the left occipital condyle. Ŋe neural canal would have been large and circular. Ŋe neural arch is anteroposteriorly broad and dorsally bears a small, transversely oriented foramen. Ŋe posterior articular surface is flat, small, oval, and positioned laterally to the neural canal. Ŋe transverse process is large, positioned at the middle of the centrum, and in dorsal aspect, is triangular and dorsally flat and shelf-like. A large, circular, transverse foramen perforates the transverse process. Ŋe ventral root of the transverse process is cylindrical, and the ventral arch of the centrum is curved and bar-like.

UCMP 219001 is a partial cervical vertebra with a rectangular articular surface of body ( Fig. 5D, E ). In lateral aspect, the anterior and posterior articular surfaces are vertically oriented and parallel, but the body of the centrum is oriented anterodorsally so that the anterior face is dorsally higher than the posterior face. Ŋe ventral surface of the neural canal is nearly flat, and rectangular in dorsal aspect. Ŋe transverse process is broken, but ventrolaterally projecting, tabular, and massive; a circular transverse foramen perforates the transverse process.

COMPARISONS

Ŋese specimens exhibit one odobenid synapomorphy, an expanded distal end of the radius with an enlarged and distally projected radial process ( Deméré 1994a); this feature is damaged in VMS 2, but clearly preserved in UCMP 219005. Ŋese new specimens are similar to radii referred to Dusignathus santacruzensis by Repenning & Tedford (1977), although radii of D. santacruzensis (see Mitchell 1962: fig. 3; and Repenning & Tedford 1977: pl. 16.5) are typically more robust and transversely wider than the San Gregorio specimens and Dusignathus seftoni Deméré, 1994 . Ŋese Purisima Formation specimens share a transversely compressed crosssection with D. seftoni . Both radii can be separated from the larger dusignathine Gomphotaria in their much smaller size and more prominent pronator teres process (on VMS 2). UCMP 219005 and VMS 2 differ from radii of the “toothless” odobenine Valenictus chulavistensis Deméré, 1994 in being transversely flattened and lacking a pachyosteosclerotic bone histology in broken cross-section ( Deméré 1994b). Ŋey further differ from Valenictus and other odobenine walruses such as Aivukus Repenning &Tedford, 1977 , Odobenus Brisson, 1762 , and Pliopedia Kellogg, 1921 in exhibiting a more prominent pronator teres process ( Repenning & Tedford 1977), and having an anteroposteriorly deeper shaft. Furthermore, the similarity of the radii to D. seftoni rather than to D. santacruzensis or any other dusignathine suggests tentative referral to D. seftoni .

Ŋe cervical vertebra can be referred to Dusignathus due to the similarity with those of SDNHM 905000, a partial skeleton of D. seftoni from the San Diego Formation. Ŋe atlas (UCMP 219006) exhibits pointed and triangular transverse processes that are laterally (not dorsolaterally) oriented, unlike the atlas of V. chulavistensis , the only other odobenid from the San Diego Formation ( Deméré 1994b). Neither vertebra is pachyosteosclerotic as in Valenictus . Although these vertebrae cannot be compared to the latest Miocene to Early Pliocene odobenine Pliopedia pacifica Kellogg, 1921 (only known from a weathered braincase and appendicular elements), Pliopedia has not been found in Piacenzian or Gelasian equivalent strata ( Repenning & Tedford 1977), and the similarity to D. seftoni and clear separation from Valenictus suggests tentative referral of these vertebrae to Dusignathus .

REMARKS

Ŋe only postcranial element originally referred to D. seftoni was an isolated humerus ( Demere 1994b). New cranial and postcranial fossils of this taxon have emerged and are present in SDNHM collections. Ŋe radii described here compare best with SDNHM 92134, an isolated undescribed radius of D. seftoni from the San Diego Formation. Ŋese specimens tentatively extend the geographic range of D. seftoni into Northern California, although more diagnostic remains of Pliocene walruses from this region are necessary for further evaluation. Although fossils of odobenine walruses are absent from the San Gregorio section of the Purisima Formation, odobenine specimens are known from the Pliocene part of the Santa Cruz section including partial tusks (Odobeninae indet., UCMP 190024), a partial femur (cf. Valenictus sp., SCMNH 21366), and a complete skull (cf. Valenictus sp., UCMP 219091) Although fragmentary, these specimens (in concert with those of Callorhinus sp., cf. C. gilmorei ) document a change from the Late Miocene pinniped assemblage from slightly older horizons of the Purisima Formation (e.g., UCMP localities V99875 , V99876 , V99877 , V90042 , V6875, lowermost 75 m of the Santa Cruz section of the Purisima Formation, Messinian equivalent) to a Pliocene assemblage similar to that of the San Diego Formation.

Figures

FIG. 2. — Stratigraphy of the Purisima Formation: A, Stratigraphic column of the San Gregorio section of the Purisima Formation (in meters), showing stratigraphic position of UCMP vertebrate localities, dated ash beds, diatom floras, and species of Carcharodon; thickness measurements start over above missing section in uppermost part due lack of overlapping strata; epoch and stage boundaries marked as "?" are estimated; modified from Boessenecker (2011a); Pliocene-Pleistocene boundary after Hilgen et al. (2012); B, photograph of 3-4 m thick ash within Purisima Formation at mouth of San Gregorio Creek correlated with 2.5 Ma Ishi Tuff (Powell et al.2007); C, trace fossils in pervasively bioturbated sandstone,Purisima Formation,mouth of San Gregorio Creek; D, swaley cross-stratified sandstone, mouth of San Gregorio Creek. Abbreviations: Qt, Pleistocene terrace deposits; Tp, Purisima Formation; C, claystone; St, siltstone; Ss, sandstone. Scale bars: A, 2 m; B, C, 10 cm.

FIG. 4. — Radii of Dusignathus sp., cf. D. seftoni: A, VMS 2, right radius in lateral view; B, VMS 2, medial view;C, UCMP 219005, fragment of right radius, in lateral view; D, UCMP 219005 in medial view; E, UCMP 219005 in distal view. Scale bar: 3 cm.

FIG. 5. — Vertebrae of Dusignathus sp., cf. D. seftoni: A, UCMP 219006, fragment of left side of atlas in posterior view; B, UCMP 219006 in anterior view; C, UCMP 219006 in dorsal view; D, UCMP 219001,partial cervical vertebra in anterior view;E, UCMP 219001 in lateral view. Scale bar: 3 cm.