Exhibit A

Joint Filing Statement

I, the undersigned, hereby express my agreement that the attached Schedule 13D (and any amendments thereto) relating to the beneficial ownership by the undersigned of the equity securities of Aratana Therapeutics, Inc. is filed on behalf of each of the undersigned.

Date: March 13, 2014
MPM BIOVENTURES V GP LLC
By:	MPM BioVentures V LLC,
	its Managing Member
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
Title:	Member
MPM BIOVENTURES V LLC
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
Title:	Member
MPM BIOVENTURES V, L.P.
By:	MPM BioVentures V GP LLC, its General Partner
By:	MPM BioVentures V LLC, its Managing Member
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
Title:	Member
MPM ASSET MANAGEMENT INVESTORS BV5 LLC
By:	MPM BioVentures V LLC
	Its: Manager
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
Title:	Member

Page 20

MPM CAPITAL LLC
By:	Medical Portfolio Management LLC,
	its Managing Member
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
Title:	Member
MEDICAL PORTFOLIO MANAGEMENT LLC
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
Title:	Member
By:	/s/ Ansbert Gadicke
Name:	Ansbert Gadicke
By:	/s/ Luke Evnin
Name:	Luke Evnin
By:	/s/ Todd Foley
Name:	Todd Foley
By:	/s/ James Paul Scopa
Name:	James Paul Scopa
By:	/s/ Vaughn M. Kailian
Name:	Vaughn M. Kailian

Page 21