Perucetus colossus, Bianucci & Lambert & Urbina & Merella & Collareta & Bennion & Salas-Gismondi & Benites-Palomino & Post & Muizon & Bosio & Celma & Malinverno & Pierantoni & Villa1 & Amson1, 2023

Bianucci, Giovanni, Lambert, Olivier, Urbina, Mario, Merella, Marco, Collareta, Alberto, Bennion, Rebecca, Salas-Gismondi, Rodolfo, Benites-Palomino, Aldo, Post, Klaas, Muizon, Christian de, Bosio, Giulia, Celma, Claudio Di, Malinverno, Elisa, Pierantoni, Pietro Paolo, Villa 1, Igor Maria & Amson 1, Eli, 2023, A heavyweight early whale pushes the boundaries of vertebrate morphology, Nature 620, pp. 824-829 : 2

Perucetus colossus gen.et sp. nov.

Etymology. From Peru,the country of origin of the holotype and Latin cetus (whale). Species epithet from the Ancient Greek kolossós (large statue and by extension any creature of gigantic size and mass).

Holotype. MUSM 3248 (Museo de Historia Natural, Universidad Nacional Mayor de San Marcos, Lima, Peru), a partial skeleton including 13 vertebrae (two tentatively referred to the last thoracics named here Th-a and Th-b and the others to the anterior lumbars named here L-a–L-k), four ribs and right innominate lacking the distal portion of the ilium ( Fig.1 and Extended Data Figs.1–4 ) . The animal was probably approaching skeletal maturity (details of bone histology are provided in the Supplementary Discussion).

Locality and horizon. From the upper part of the Yumaque member of the Paracas Formation exposed in the Ica valley a few kilometres south of the Zamaca locality, Ica Province, southern Peru 12 (Extended Data Fig.5 ). Its geological age is well constrained between 39.8 million and 37.84 million years ago (Bartonian,middle Eocene) through biostratigraphy and 39 Ar– 40 Ar dating on tephra layer. Coordinates for the type locality are available on request from the corresponding author. Further geological and palaeoenvironmental information and detailed justification of the age are provided in the Methods, Extended Data Fig.5 and Supplementary Methods.

Diagnosis. P. colossus differs from all other cetaceans by having an extremely pachyosteosclerotic postcranium. Pachyosteosclerosis is also present in a few other cetaceans 13, 14, including the basilosaurid Pachycetinae 15,but to a substantially lesser degree than observed in this new taxon. P.colossus belongs to Pelagiceti by having a high number of lumbars (at least 11), an extremely reduced innominate, and centra of the last two thoracics and preserved lumbars with a roughly circular cross-section (centrum height/centrum width (CW)> 0.80), not dorsoventrally compressed nor heart-shaped (as observed instead in the more stemward archaeocetes). Within Pelagiceti, P.colossus shares with Basilosaurus , Chrysocetus , Cynthiacetus peruvianus , Mystacodon and Pachycetus wardii a plesiomorphic, well-defined acetabulum on the innominate, but differs from Basilosaurus , Chrysocetus and Mystacodon and probably Pachycetus by the more robust proximal portion of the ilium, and from Basilosaurus by the larger obturator foramen and the overall shape of the innominate, which is triangular in profile view. P.colossus differs from all cetaceans but Basilosaurinae and Pachycetinae by the great elongation of the centra of the lumbars (centrum length (CL)/CW = 1.25–1.56), approaching the values seen in Pachycetus and Antaecetus (CL/CW = 1.30–1.68); more extreme values are observed in Basilosaurus (CL/CW = 169–1.98) (Extended Data Fig.6 ). P.colossus shares with Basilosaurus the giant size (CW> 20 cm) and club-like shape of the distal end of at least some of the ribs. Estimated skeletal length:around 17–20 m(Supplementary Methods and Supplementary Fig. 7 ).

Remarks. The assignation of the vertebrae to the last two thoracic and the first 11 lumbar positions is based on the ventral position of the transverse processes, the large neural canals (although reduced owing to the pachyostosis of the neural arches) and the lack of foveae for the capitula of the ribs.The transverse processes are substantially bent ventrolaterally, similarly to the lumbars of other basilosaurids 16 – 18. Moreover, all of the distal portions of the transverse processes exhibit a peculiar wide, oval, flattened area on their ventral surface ( Fig. 1k and Extended Data Figs. 1–4 ). The two thoracic vertebrae also bear a concavity at the anterolateral tip of their transverse processes, where the last ribs probably articulated.These two vertebrae are also distinguished by their neural spine, which is more slender and with a dorsal edge sloping posteriorly.

For around half of the recovered vertebrae, either one or both of the centrum epiphyses are missing, suggesting partial epiphyseal fusion. This should not be seen as a sign of immaturity for the specimen, as some large, extant cetaceans maintain their thoracic and lumbar centrum epiphyses unfused late into adulthood 19.

The best preserved rib displays a simple proximal end without distinct tuberculum and capitulum and a weak overall curvature in anterior or posterior view ( Fig. 1b, c , Extended Data Fig.6c and Supplementary Data 1). This morphology (also observed in the other three preserved ribs) is consistent with a rib of the posterior region (R17–20), which would entail that the whole rib cage was probably pachyostotic in P.colossus , contrary to other basilosaurids 16, 17, 20.

Figures

Fig.1 | Partial skeleton of the P.colossusMUSM 3248 holotype. a, Schematic body and skeletal reconstruction reporting the preserved bones (red). b,c,Right rib inanterior(b)and lateral(c) views.d–f, Right innominate in medial (d), dorsal (e) andlateral (f) views.g–i, Last preserved lumbar vertebra (L-k) in anterior (g),left lateral(h) anddorsal (i) views.j–l, Articulated sequenceof the 13 collected vertebraeindorsal (j), ventral (k)and rightlateral (l) views.Scale bars,50 cm (b,c andg–l) and10 cm(d–f).Allof the images were generated from 3D surface models.

Fig.2 | Pachyostosis inthe P.colossus MUSM 3248 holotype. a–c, Skeleton (a, preserved bonesindark grey), life reconstruction(b; artist, A.Gennari)and 3D modelsof a rib (c) and vertebra (L-k) (d)of P.colossus. e–j, Skeletons (e,h), posteriorrib (f,i) andlast lumbar vertebra (g,j) of the Wexford blue whale (B.musculus; e–g) andanother basilosaurid,C.peruvianus(h–j).Scale bars, 50 cm (isolated bonesinc, d, f, g, i and j) and 2 m (skeletonsand reconstruction in a, b, e and h). Thedagger symbols (†) indicate extinct species.

Fig.3 | Bone histology ofthe P.colossus MUSM 3248 holotype. a, Half rib cross-section(in three parts),featuringvery sparse porosity andmultiple growthmarks (arrows).b, Detail of the above,showing ahighly convoluted, woven-parallel complex,devoid of remodelling (left, natural light; right, cross-polarized light).c, Brokenpeduncle of alumbar vertebra in ventral view (notethe fracturesfollowing growthlines),made of compactbone throughout. Thesmall inset(vertebraparts inanterior view,notto scale)indicates theposition of theillustrated break surface.Scale bars,1 cm (a),0.5 mm (b) and 5 cm(c).

Fig.4 |Scaling of the skeletal massacross amniotes’ bodymass range. Regressionsbased on extantterrestrial mammals (grey line) andextant cetaceans(blueline).The estimated valuesfor extreme terrestrial amniotes, sirenians (either measured or estimated) as well as theresults of our estimationsfor P.colossusarealso shown.Eachdata point corresponds toa distinctspecies (specific meanfor thelargest extantcetaceans,B.musculus and Physeter macrocephalus), except for extantsirenians (represented by two species).Thehorizontal andvertical barsindicate the maximum andminimum estimates (someare too small to bevisible).Scales arelogarithmic.Human height,1.8 m(scale forlarger silhouettes); pennydiameter,2 cm.See the Supplementary Methods and SupplementaryData 2.

Extended DataFig.5 | Location and stratigraphicposition of Perucetus colossus MUSM3248. a. Mapshowing theposition of the Coastal Batholith andmajor trench-parallelstructural highs along thecoast of Peru (redrawnand modified afterTraviset al.82. andThornburg & Kulm83).b. Stratigraphic column of theCenozoic succession exposed inthe EastPisco Basin (redrawnand modified after Malinvernoet al.84, Bianucci & Collareta85, andBosio et al.86). c.Measured stratigraphic sectionof thetype localityof P.colossus, indicating the stratigraphic height of the holotypeMUSM 3248,the dated tephra layer, and the identifiedbioevents.

Extended Data Fig.6 | Vertebral andcostal morphologyof Perucetus colossus (MUSM 3248, holotype)compared tothat ofother cetaceans.a,mean and maximumratioofvertebralcentrumanteroposteriorlength(CL)tomediolateral width(CW),computed with thethoracic and lumbarvertebrae of other cetaceans(Ba =Basilosaurinae;Dor=Dorudontinae;FM=fossilMysticeti; FO =fossil Odontoceti;P =Pakicetidae+Ambulocetidae;Pa=Pachycetinae; Pr= Protocetidae;R =Remingtonocetidae).b,profile ofvertebral centrum length inP.colossus (Pc) compared with theprofilesof otherbasilosaurids (Aa = Antaecetus aithai; Bc = Basilosaurus cetoides; Bi = Basilosaurus isis; Cf = Chrysocetus fouadassiiCp =Cynthiacetus peruvianus; Da =Dorudon atrox; data of A.aithai andB.isis fromGingerich etal.15, fig.11).c, curvature (length/chord)of thebest preserved rib,R-a(red horizontal line) compared tothose of Cynthiacetusperuvianus (MNHN.F.PRU10).

ExtendedData Fig.7 | Vertebral microanatomy illustrated with binarized sections(black=bone)obtainedwithphysicalcoredrills(a) orµCTdata(b–f). a, Perucetus colossus (MUSM 3248,holotype,vertebra L-e forthe centrum and transverseprocess and L-cfor the neuralspine).Theglobal compactness(Cg)was measured inten areas(orange outlines) toassess overall centrum compactness (see Supplementary Methods).b, common minke whale (Balaenoptera acutorostrata, LRM 523).c, Amazon river dolphin (Inia geoffrensis, ZMB_ Mam_41500).d, common dolphin (Delphinus sp.ZMB_Mam_697.59).e,dugong (Dugong dugon, ZMB_Mam_69340).f, manatee (Trichechus manatus, ZMB_ Mam_17377).(1) Centrum(anteroposterior mid-length,from itsdorsal edge to itscentre),external towardstop;(2)Neural spine (dorsoventralmid-height), external towards right); (3)Transverse process (mediolateral mid-width), external towards top.Widthof the virtual cross-sections were defined as representing 7.2%of the centrum dorsoventral height; thisis the meanratio between thecentrum heightof P.colossusand thewidth of thephysical core drills.The core drillsof the neural spine andtransverse process do not reach the middle of the correspondingvertebral parts (but break surfacesindicate uniform structure throughout).See core drills’ location in Supplementary Fig.S6c,d.Scalebars:a= 13.9 mm,b =23.6 mm,c =2.2 mm,d =2.7 mm,e =3.2 mm, f = 3.0 mm.